Heat-Pump Water Heaters


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Heat-pump water heater
Heat-pump water heaters may have a slightly different footprint than conventional water heaters. Since they draw heat from the surrounding air, it is important to consider their location. With its louvered door, this closet essentially shares the same airspace with the living room.
Energy guide rating
With their high COPs, HPWHs can significantly reduce the costs and energy needed for water heating.
Heat-pump water heater
Energy guide rating

In the right situation and location, heat-pump water heaters offer improved efficiency and increased energy savings compared to standard tank-style units. Here’s how to tell if your household would benefit from this technology.

Using electricity to run a compressor, a heat-pump water heater (HPWH) extracts heat from air, often in the basement, and transfers that energy to heat water in a tank. It’s like a refrigerator, only running backward: A refrigerator moves heat from its interior to the room it’s located in. An HPWH transfers heat from the room to a storage tank, and does it with remarkable efficiency. In fact, an HPWH is able to transfer more energy (up to 2.5 times) than the electricity it consumes. Today’s HPWHs are much less expensive and more efficient than their predecessors.

Inside an HPWH

A heat pump has three basic components: an evaporator, a compressor, and a condenser. Liquid refrigerant is exposed to room-temperature air in the heat pump’s evaporator coil. As it picks up energy from the room air, the refrigerant changes from a liquid to a gas within the evaporator coil. The refrigerant passes into the compressor pump, which compresses the gas, increasing its temperature. The heated refrigerant then runs through a condenser, which is either a coil that is wrapped around the integral tank, or a pumped heat exchanger that feeds a separate water storage tank. As the hot refrigerant gives up its heat to the colder water, the refrigerant cools and condenses back into a liquid and passes through an expansion valve, where its pressure is reduced and the cycle starts over.

By taking advantage of the unique characteristics of the refrigerant, the heat pump can extract energy from the surrounding room air (slightly chilling that air). How much energy is extracted depends on the ambient air temperature and the tank temperature.

This transformation of one unit of electrical energy into three units of heat is the heating advantage that is unique to HPWHs. This increase in the heating effect is known as the coefficient of performance (COP). Most HPWHs have COPs between 2 and 4, depending on room temperature and humidity, and water temperature. By comparison, a conventional electric water heater is considered to have a COP of 1.

The U.S. Department of Energy considers the COP of HPWHs to be about 2, which accounts for storage losses as heat is lost through the walls of the storage tank. The temperature of the room in which the HPWH is housed, the insulation of the storage tank, and hot water usage frequency all impact the HPWH’s COP. For our purposes, we are only considering the actual heating performance. However, it is critical that, with any heat source, the heat storage tank be well-insulated to minimize heat loss.

The energy to feed the heat pump can come from passive solar gain, heat from a conventional heating system, the warming effect of soil surrounding a basement wall, or any other heat source. The resulting cooling of the room by the HPWH is advantageous in the summer, but a disadvantage in the winter. If the unit is installed in a basement, the larger the basement, the less impact it will have on the living area. Additionally, in basement installations in humid regions, a significant benefit of an HPWH is that it also removes some moisture from the air, reducing and usually eliminating the need for a separate dehumidifier. Since HPWHs operate as dehumidifiers, a condensate drain must be provided.

Comments (26)

TomGocze's picture

The CCHP units that Fred mentions made by Nyle are rather expensive, being about $8-9000. They are primarily used for space heating and DHW in motels in the south. We have one unit in use for about three years. They have reworked these Cold Climate air source units that are specifically for making hot water and are currently rolling out the improved unit.

I would suggest that if someone is looking at an $800 repair bill for a HPWH, you are better served to replace the unit. I can tell you that the Nyle units are pretty solid on the refrigeration side. The control side is all hand wired and is simple to repair.

We have also been testing some imported space heating heat pumps that are used for radiant floor heating. Into our second winter, they are working reasonably well. The lower operating temperature is about +5F. We use wood pellets for back up.
The major manufacturers of mini split heat pumps are supposed to be introducing their units for hot water space heating soon.

There are some new older stock stand alone heat pump water heaters here:

Fred Golden's picture

You can find a list of qualified HPWH on this website.


I found this information at GE website. http://www.geappliances.com/ge/heat... Click on 'Get Rebate Information' and it will ask for your zip code.

Yes the 80 gallon tank has a higher energy efficiency ratio, due in part to it's ability to draw 2 GPM for 8 minutes several times before the electric heater will cycle on. If you only have 3 showers per hour for a total of about 6 showers, then the back up heat will not need to come on at all on a warm day. The new GE 80 gallon system is rated for 4 showers before the water dips below 105F, and also 2.7 Northern Climate energy factor.

So while a 1,000 watt resistance heater will put about 3,400 Btu's into the water heater, (per KW used), this HPWH will transfer about 9,180 Btu's per KW of power consumed, or 2.7 times as much heat per KW. It will cool the room it is located in by about 8,000 Btu's and recover some heat from the compressor windings while it is heating the water.

Fred Golden's picture


You mentioned a $800 repair estimate for your HPWH. Did this come from a air conditioner company? You might try a refrigerator repair company, and see if you could get a lower rate. It might also help to wait for better weather, and not when it is blazing hot out either, so the companies are a bit slower time of the year.

You might also consider a Nyle water heater, they seem to perform much better in rooms that are less than 30F in the winter. Nyle makes a CCHP Cold Climate heat pump, rated at 30,000 Btu's and will work with 0F inlet air, is mounted outside, with the freon lines piped inside to the water heater tank. You could use any tank, so your HPWH tank could be the tank with the back up heater only used when it is really cold outside.


You should be able to find a refrigerator mechanic that can come out and look at the unit for a nominal fee (say $80 for the first hour) and if there is a port to recharge the system, they could add a pound of freon for another $10 or so. This might restore the system for another 2 years. Fixing a leak might cost an additional $100 or more, depending on how easy it is to fix and if they can even find it. Losing all the freon over 35 months indicates a really tiny leak, less than 1 ounce per month. Look up all the information on the data plate, especially the charge freon type (R-134A or R-410 or perhaps R-417?) So that they bring the right type of freon, and do not need to charge you another hour of time (at $80 - 120 per hour typical shop time rates) to go get the right stuff.

Good luck!

Fred Golden's picture

To ouachitaoffgrid2009:

You can disconnect one wire to the electric resistance heater of your HPWH, and make sure it will never use up a large amount of your electrical power.

I like the idea of installing more solar panels and not increasing the battery size. At 600 AH, you have plenty of storage for one or two nights. Remember that the storage battery is always 'Consuming' about 10% of the power that you put into it. So a 600 AH battery system might lose 60 AH monthly to efficiency losses, a 1,800 AH battery will lose three times as much power monthly.

Solar panels are now less expensive so it makes sense to buy more!

Arthur Perrea's picture

Arthur Perrea,
I got my 2 GEO Spring HPWH at Lowes in NY on sale $700 each and $99 for 10 year Lowes warranty I using them to supply my potable water and to heat my home on a open radiant floor system

TomGocze's picture

I alluded to the fact that all-in-one units are throw away once the tank fails. There is no way to re-use the heat pump unless you are a skilled refrigeration mechanic and want an adventure.
The folks at Nyle tell me that they have re-worked the Geyser for slightly higher output and they are going to get an Energy Star rating on their unit with a separate tank. Energy Star, which is required for most rebates, would not certify the stand alone unit, claiming that the efficiency could suffer with a poorly insulated tank. Most electric tanks in 2015 are well insulated, but I suppose they have a point.

Any heat pump with PV's seems like a no-brainer, but as you note, the details make the difference.
Tom Gocze

ouachitaoffgrid2009's picture

This is another update on the GeoSpring HPWH that we installed in our off-grid cabin in the Ouachita Mountains in July 2013.

It is my understanding that this unit was manufactured in the USA after GE moved their production back stateside from China.

It continues to work fine (knock on wood!) and provides plenty of hot water in the heat pump only mode. I don't think that it has ever operated in resistance mode so far. If it has, the Trace SW4024 with a 220 volt Trace transformer has handled the load.

In the past year, we have enlarged the battery storage capacity to a total 16 AGM batteries at 105 ah each and added 2,565 watts of PV for a total of 3,725 watts in my combined PV array.

In a very helpful June 2015 Ask the Experts response, Ian Woofenden at Home Power suggested suggested that I add more PV panels to my array instead of increasing the size of the battery bank. He noted that the more PV that I add, the less I'll need a generator.

So, I took his advice and enlarged the array. So far, so good. At this point though, I'm still reluctant to add a generator - because it seems to be at odds with the our original intention of creating our own electrical power, but may ultimately have to do so.

We may have hit the "sweet spot" point of having the appropriate array size, battery bank size and electrical consumption. I'll find out for sure in the next few weeks when the sun is at its lowest point on the first day of winter.

So far, we still have power and the battery bank recharges quickly when there is sun. Thanks for the help, Ian at Home Power!

nospam's picture

Long term comment on the GeoSpring HPWH:

Worked great for the first year with significant reduction in energy costs. Saved $300 in electricity over standard electric water heater. After 13 months started getting F-C error code and found that the coil was freezing up. Contacted GE and found out my "10 year warranty" was only 1 year warranty in Canada. Received a quote to repair was $800 for an $1100 unit. I continued using the HPWH as is and performance declined. After 35 months in service the heat pump unit failed completely and now only operates in electric resistance mode only.

Bottom Line: GE is unreliable and doesn't stand by their warranty. Will never purchase GE products again.

Does anyone have any long term experience with the Stiebel-Eltron Accelerate 300 units? I'm interested in hearing about reliability and if they honour their 10 year warranty.

ouachitaoffgrid2009's picture

This is another update on the GeoSpring HPWH that we installed in our of-grid cabin in the Ouachita Mountains. The HPWH has now been in place over one year and is still working fine. I haven't yet run the condensation line out from under the crawl space and it is currently draining into a 5 gallon plastic bucket, which I empty regularly. In the past year, there have been several occasions where the PV system shut down due to cloudy conditions. In each case, I either re-started the HPWH manually by re-setting the breaker in the electrical panel or it re-started itself once the batteries were charged.

Overall, it has been great and I plan on installing a 4,000 to 5,000 watt generator soon with an auto start to charge the batteries when needed, so that I can have power in the less sunny times of the year.

Additionally, after installing the generator, it's my intention to add to the size of the solar array and batteries. I suspect that the generator may run only about 5% of the time, but I'll find out when I install it. Overall, the GeoSpring HPWH keeps the water hot and works well for us.

Fred Golden's picture


Thanks for the 4-24-14 update on your failed HPWH. I guess GE did not design the evaporator right? A small leak can leak out the pound or so of freon pretty fast in such a small system. Moisture in the system can cause the oil to become very acidic, causing leaks too. I hope that your repairman installed a 8 cubic inch filter dryer, it is pretty standard in such repairs with R-410.

Someone mentioned that tubing was worn thin by the system being acidic, and this can happen just because the original manufacture employee did not pull a deep enough vacuum before adding the R-410. Moisture in the system will turn the oil acidic, and that can eat away at the inside of all the tubing. The heat exchanger to the water tank is expected to hold back 400 - 600 PSI, and I suspect will be thicker wall than the evaporator, with it's 125 PSI running pressure and 200 PSI pressure when the unit is off.

Also you mention that the poor mechanic did not have space to work on the system. (I am 6'4" and would not take that job either). If the room is less than say 400 cubic feet, you will need to duct in air to the unit, or duct out the cold discharge air to someplace else. A 12,000 Btu A/C unit will move about 400 cubic feet per minute, or more air than is located in that closet, and cool it by about 20F each time it passes through the unit. You would be wasting energy if you are cooling the output air to below say 45F.

For those with a wood stove, while it is heating, you could be using a hot water loop and small pump to heat the water in your tank. A solar hot water heater controller can run the pump automatically, and shut it off at say 150F tank temp. You would need a water mixing valve if you are collecting water above 125F for safety of the people in the home.

James Royston's picture

Read my addendum below. You have to click on "Show all comments" below in order to see it. It is an important HPWH cautionary tale.

Fred Golden's picture

I am about to install a heat pump to warm my home in Portland Oregon. It will be 48000 Btu and 14 SEER or collect about 14,000 Btu's per KW in the heating mode.

For water heating, I will recover the freon, then cut the hot gas line, install a heat exchanger to "De-Superheat" the hot gas from the compressor. This will be a 1/2" copper line for the freon inside a 1-1/8" copper line for the water, insulated with 3/4" foam pipe insulation. I will use a standard 3 GPM water pump with solar controller to turn on the pump when my hot water tank is cooler than the outlet line on the desuperheater. This will run upwards of 120F on a moderately warm day in the cooling cycle, or in the heating cycle I can easily warm my water tank to 105F.

I estimate I will collect about 8,500 Btu's per KW in the heating mode (while heating my house while running the water pump) and collect upwards of 12,000 Btu's per KW while in the cooling mode, with a outdoor fan cycling switch that will keep the freon pressures higher to make this system run warmer.

The 4 KW additional power to run my heat pump each day while heating water will be more than offset by the daily savings of running a normal 4.5 KW per hour water heater that has only about 14,000 Btu's per hour output.

I install air conditioners for a living, and desuperheaters where popular in the 80's in commercial applications like a restaurant, where 10 ton A/C systems would warm the dozens of gallons of city water to 105F before going to the hot water heater for further heating. This saved the restaurant about $100 a month in gas or electric charges. Most such systems have long ago been disconnected.

A side note, I installed a Airtemp heat pump water heater at my sister's house several years ago. It was manufactured by the Chrysler Corp before 1979. So they have been around a long time.

ouachitaoffgrid2009's picture

This is a follow-up to my GeoSpring HPWH comments in the off-grid cabin referred to below (in November 2013).

I have since added four more AGM Deka 105 ah 12 volt batteries to the power house battery bank and used foil-backed single bubble wrap roll insulation to insulate the entire crawl space (where the HPWH is located) of this off-grid cabin.

In addition, I have since found out that I had a serious loose connection from the battery bank to the inverter circuit breaker/cut-off switch - which my electrician-brother helped me isolate.

As a result of fixing the loose connection noted above and, at his suggestion, I tightened all of the other connections (many of which were loose) in the power house (breakers, a 220 volt transformer, inverter connections etc.) and re-started the entire system.

The GeoSpring went right back into heat pump mode and everything else worked fine.

A few days later, we were warned about and experienced a very severe ice storm in the Ouachita Mountains and were without power in our grid-tied rural home for days.

In preparation for the ice storm, I shut off the HPWH at the off-grid cabin in order to save battery power.

Even though our nearby house (about one mile from the cabin) was without power for days, I was able to use the cabin to charge phones, listen to local news, and even get cleaned up as the water remained warm for several days in the HPWH. Once the sun came back out and we switched the HPWH back on, it went through the usual diagnostics and began heating water again.

It's my understanding that when the air temperature surrounding the GeoSpring HPWH drops to 45 degrees or below, the resistance heating elements will begin operating.

So far this winter, the temperature in the crawl space has not dropped below 45 degrees. Insulating the crawl space has helped reduce the power draw on the HPWH as the temperatures in the crawl space have remained relatively warm during the coldest times. Several nights so far, the outside air temperature has been in the mid-teens, so the insulation appears to be helping.

As my 220 volt system is not designed to handle any more than a surge load for a short time, the HPWH would likely shut down while in resistance element heating mode and that has not yet happened.

However, even though the solar panels and power house that holds all the balance of system components were covered with a heavy coat of ice, the off-grid system continued to operate and power the cabin.

As a result of the ice storm, there were many trees and limbs damaged near the cabin and on the farm and the benefit of running underground conduit from the power house to the cabin cannot be overemphasized!

This continues to be a challenging and fun project and I've also learned the importance of making sure all of the connections are tight!

Fred Golden's picture

The heat pump water heater needs to collect heat from someplace.

By insulating the basement that it is located in, and making that space air tight - I am guessing that the heat pump runs a few minutes to cool the basement to 45F and then might be changing over to electric heat mode? The problem with many heat pump water heaters is lack of a defrost mode. Just running the evaporator fan in a 40F area will defrost the coil, however in a situation where the temps are below 25, a defrost mode would be required, such as a hot gas solenoid to send warm gas into the evaporator to thaw it out once in a while This is what is used in walk in freezers. (Air defrost is used in 39F walk in refrigerators).

What might have happened is you are getting more heat from the ground by insulating the basement walls? If your average ground temp is say 50F, then the basement might warm to that temp if you are not taking to much heat from the basement to warm the water. But if your outside temp is say 50F and your discharge air is less than that temperature, then discharging the air outside would make sense. Yes if the outside is 10F and the discharge air from your 55F basement is say 32F, you would not want to blow 32F outside and have 10F follow that air back in.

I am installing a desuperheater on my new heat pump for my home soon. It will use outside air to warm the water, and take advantage of the 48,000 BtuH compressor to heat the water for free in the summer! It will collect about 8,500 Btu's per KW in the winter while running the heat pump to warm my house. In the summer, it will collect about 12,000 Btu's per KW, yet is considered "Free" because the 1 KW is being used to cool the home, the hot water is a by-product of cooling the home, and thus the system runs for only the 38 watt pump running energy and 5 watt solar hot water heater controller energy.

This 4 KW heat pump will replace the existing pair of 10 KW electric heaters that are installed now, while producing about the same amount of heat. The water heating ability will be a bonus allowing me to disconnect the electric water heater and it's pair of 4500 watt elements! and also collect up to about 24,000 BTUH from the heat pump.

ouachitaoffgrid2009's picture

My friend and I installed an American-made GeoSpring water heater in our off-grid cabin in July 2013. It is powered only by my modest 1.16 kw PV system and a TRACE SW4024 with a 220 Volt Trace transformer. I currently have modest battery storage with 4 AGM Deka 105 ah 12 volt batteries configured for 24 volts.

The water heater is installed in the crawl space of the cabin (which I am in the process of insulating) and is set to run on heat pump only mode.

This was an experiment to see if it would work in heat pump only mode, as I couldn't find any evidence of other similar installations at the time. I have since found out that similar installations are not uncommon.

Anyway, my net price for the GeoSpring was a little more than $700 at Lowes. I got it on sale and received an additional 10% veteran's discount.

At this point, it works fine; but I am not living in the cabin full-time. My battery storage capacity is woefully inadequate and I am in the process of doubling it - so that I will have a usable capacity of approximately 5 kwh.

Currently, when days are sunny (and they aren't this time of year!), the system will run the HPWH in heat pump only mode and power my small refrigerator, an energy-sucking coffee pot (necessary for life), a submersible well pump (1/2 hp 220 v, 90' deep) and all my fans, lights etc.

I credit my friend, and the system designer and the Trace SW4024 inverter for being able to handle the load - when there is enough sun or when the batteries are fully charged.

As a bonus, the GeoSpring is really reducing the humidity level in the cabin crawl space.

This time of year (November), the Trace SW4024 inverter automatically shuts the system down due to low batteries. On sunny days though, when there is enough power from the batteries and/or the PV panels, the GeoSpring (so far, at least) automatically turns back on and goes back into heat pump only mode. As the tank is well-insulated, the water stays quite warm for days.

Future plans and experiments include more batteries and more PV panels as well the possibility of operating a mini-split heat pump A/C unit off of the inverter in the summer.

The cabin is very well insulated and is less that 700 sf in size.
Other than for the source of power (PV) it is completely wired to code (mostly!) just like a conventional home but with only 30 amp main breaker from the power house to the cabin main power panel.

I'd love to hear from others who have done similar things. It's really been a fun project - and I am pleased (knock on wood) so far. Murphy's Law always applies to my projects, by the way - always, always, always!

Thanks for letting me inform Home Power readers about this fun project!

TomGocze's picture

It seems to me that solar thermal is hard to beat in terms of heat output and cost of operation. My sense is that HPWH are a great backup for solar thermal. My current system is a HPWH with a wood fired system.
We live near the ocean so the HP is wonderful for keeping household humidity under control.
I think there are some very exciting changes coming with variable speed units which will be quieter and more efficient.
Nyle Systems is developing a air source water heating space heater HP that delivers hot water at 100F over ambient for any ambient temperature.

Fred Golden's picture

Carrier built a 3 piece heat pump many years ago (95 - 99?) and it had a outdoor coil, indoor coil, and compressor unit with de-superheater to warm the hot water. In operation, it could cool with the indoor coil, or heat with it, or run just the outdoor unit and heat hot water.

My sister has a 1970's Airtemp heat pump water heater. It is designed to run from a existing tank, so you remove the water drain for the cold water supply to the HP, and remove the pressure relief valve to install a tee for hot water return to a standard 40 - 80 gallon electric water heater tank. Heat pumps are not new.

I am in the process of installing a desuperheater to my central heat pump in my house. It will use a solar hot water controller to run a pump when the hot water coil is warmer than the tank is.

Anyone with a wood stove heater can basically do the same thing. Mount a 50' long coil of un-insulated 3/4" tubing behind your stove, and then install a solar hot water pump and controller. Anytime the hot water tank is cooler than the coil of tubing, the pump comes on to warm the hot water heater. Of course insulate the water lines between the stove and water heater to prevent heat loss on the way.

James Royston's picture

I have two GE Geospring HPWH's and they are using 1kW or less per day as per my dedicated kWh meter that feeds one of them. In the winter I have to switch them to conventional heating element mode due to low indoor room temps. In the conventional mode they use 3kW per day each which is a little less than the 10 yr old water heaters they replaced. Luckily, I don't have to switch them to conventional very often here in San Antonio, TX. I am very pleased with the dehumidification function and room cooling as well. They do make some noise but they are in smallish utility closets so the noise is about the same as the air handlers for our central air conditioners. Our utility closets are smaller than the recommended room size so I just crack open the door when they run if I'm home. I'm pretty sure these units are reducing my central A/C usage. I looked at SHW but due to our house being set up for two separate heaters it would have been difficult and expensive (our house is long). We do have a 6.5kW GT solar array to power the two HPWH.

Update 4/24/14: Well, after 14 months of use one of my Geospring water heaters stopped heating water in heat pump mode. I had GE send a repair person but he only told me what I already knew: it wasn't working. The conventional electric heating elements were working fine so we still had hot water. After three visits, the heat pump has a new evaporator and seems to be working fine. The drain nipple on the cheap plastic condensate drain pan also broke and was replaced at the same time.

GE was not initially willing to cover anything but parts on this unit. I had to pay $98 for the first service visit which only confirmed what I already knew. I did convince them to cover the evaporator and drain pan replacement labor. So far, I had saved about $90 in electricity with this unit so I'm not saving anything really yet (saving money was not my reason for buying it though). GE was less than satisfying to work with considering that I had bought two of their water heaters and was barely out of the full warranty period.

Another very important thing I want to mention is that when you install any heat pump water heater you should think about how you are going to make it accessible for a repair person should repairs become necessary. Initially, my repair person told me there was no way on earth he was going to be able to fix the evaporator without the whole unit removed from the smallish utility closet. This would have cost me $400 to R&R the unit if I had a plumber come out and do it since it is so heavy and awkward and I'm not a big guy:) What I did as a workaround for access was to partially drain the tank and rotate the whole unit to where the evaporator was accessible. I also had to strip all of the sheet metal shroud from the top of the unit in order to take and then text pictures to the repair guy to convince him it was worth a shot in fixing it in situ. He went for it and commented he had OK access after the repairs were made. SO, to make a long story even longer, be thinking about access in case of repairs, read reviews before you buy and think twice about extended warranties. The Geospring water heater has a record of failed evaporators just out of warranty. You'd think they would have figured out how to spec a reliable 5000btu air conditioner to sit atop their hpwh but I guess not...

All in all, I don't regret anything for this project. Parts fail. I am still saving a bunch of electricity. I like the "free" air conditioning, too!

Fred Golden's picture

James, You might consider mounting a grill in the door to let in more air to the utility closet that the water heater is located in. Then also blow the discharge air out of that room. It will allow running the HPWH more efficiently and extend the time it can be run in the heat pump only mode.

Several supply houses sell room to room fans. They look like a bathroom fan, but have a 10" square grill on both sides of a 4" thick wall. You will need to run a power cord down the inside wall on the utility room side of the wall to power it. I would leave it on a timer, or else a thermostat to run it when it is below say 60F in the utility room. But would not need it running at all if the A/C system takes air from the same utility room to heat and cool the house.

wroscello's picture

Same here. I had one sensor of my energy monitor on the HWH for a while and during that time it averaged only 1.7 kWh per day (so about 10% of our non-heating usage). I have the HWH set to only 115 degrees and it is in a partially-conditioned (basement) space.

We also switched from gas to electric HW knowing that we were getting PV solar panels. I didn't want to have two different solar systems.

Marc Fontana's picture

While I like the idea of preheating water using a solar thermal collector, lower PV module costs are making PV + HPWH a more attractive solution. Isn't the cost of installing solar thermal, when you include collectors, pumps, storage tanks comparable to installing a few more PV modules and a Heat Pump water heater? I would also expect PV modules to be more durable and reliable than a solar thermal installation. What do you think?

Ben Root's picture

Hi Marc,

Michael is right, the question is coming up a lot recently, and the jury is still out. Besides the fact that results will depend on a lot of variables, including site and climate as well as water use habits, it's also good to remember that there may be other advantages of SHW over a grid-tied PV system of the same cost. E.G. it's pretty easy to create a SHW system that will continue to provide hot water even in the event of a grid outage (assuming that other parts of your water supply aren't grid dependent). But the cost and complexity of having a GT PV system provide power for hot water when the grid is down is much greater. It's not always about cost. SHW systems are simple and reliable, and that direct thermal energy path has a grace to it. Personally, I want both.

Ben Root's picture

Hi Marc,

Michael is right, the question is coming up a lot recently, and the jury is still out. Besides the fact that results will depend on a lot of variables, including site and climate as well as water use habits, it's also good to remember that there may be other advantages of SHW over a grid-tied PV system of the same cost. E.G. it's pretty easy to create a SHW system that will continue to provide hot water even in the event of a grid outage (assuming that other parts of your water supply aren't grid dependent). But the cost and complexity of having a GT PV system provide power for hot water when the grid is down is much greater. It's not always about cost. SHW systems are simple and reliable, and that direct thermal energy path has a grace to it. Personally, I want both.

Michael Welch's picture

Hi Marc. We have been hearing the same rumor: that PV has become as cheap as solar thermal for heating water, though as of yet the jury is still out. I talked to a SHW friend who said he did quick calcs showing that PV will cost double what SHW system would cost for heating water. Toward that end, we hope to have an article in the works which will take a very close look at the comparison.

One concept that is interesting is that once your water is hot, a solar thermal system sits there doing nothing, but if it was grid-tied PV instead, it would keep contributing to the grid.

Fred Golden's picture


I will look forward to reading a comparison between solar PV heated water and solar thermal collectors. Using grid tied and a inverter is one way, and once the water is hot, then excess is used to lower the grid costs, so it will keep the PV working full time. By using HPWH and grid power, you get a lot of Btu's per KW.

Yet a evacuated tube system is also simple, can heat water on a freezing day, and will work well when tilted correctly, and can prevent overheating in the summer if almost upright. I would be installing a 80- 100 gallon solar hot water storage tank, then feed a 30-50 gallon HPWH in this case, to act as the back up heater. Or if I would be getting nearly 100% from the solar system, then a tiny 10 or 15 gallon 240 volt water heater to lessen any heat loss, and provide the increase from the 100F pre-heated solar water to what is needed in the home at the time.

I have been considering installing a SHW system at a rest home, that uses hundreds of gallons of hot water per hour during the day. It comes in at around 50F, and pre-heating it to just 85F will lessen the demand on the gas water heaters. However capitol outlay is kinda expensive when you have to pull permits, call in union plumbers, and can not save labor costs by doing it yourself. In that case, the PV system and 48,000 Btu heat pump dedicated to warming the 100 gallon pre-heat water tank might work very efficiently, to preheat the water to 85F before going to the gas tanks.

One of your long time advertisers is selling PV panels near $1 per rated watt. This will make a simple system, such as a 120 volt 4,000 watt heating element connected to a series of solar panels without a controller work well to pre-heat water in a tank. Then it is only shut off when the tank temp is over 155F or something. One could install any number of panels in series as long as they did not exceed either the 120 volt or 4,000 watt limits, and run the heater to pre-heat the water. If the PV system is large enough, you could run a standard 240 volt heater with PV power, say 7 panels in series at 30 volts output each at 7 to 10 amps. This will not exceed the electric heating element's ability, and will not go over it's maximum voltage rating. Once the heating element is at 145F, the system will shut off automatically. Using a simple solar controller that will accept 220 volts input and will be switched on when the hot water tank is warm can cause it to either feed a existing onsite grid tied inverter, or charge some batteries, or do something else useful with the power once the water is hot. One suggestion is charge 12 volt battery bank to power LED exterior lighting.

De-superheaters have been installed on air conditioners for a long time. I saw some that had been disconnected on the roof of a restaurant back in 2006, because the system started leaking due to age. Basically the air conditioner is running 12 hours a day while they are open (it was in Scottsdale AZ) and dumping 60,000 Btu's into the outside air (that can by 110F during the day in summer). By preheating the incoming 65F fresh water to 120F they saved money by running the compressors cooler, and also pre-heated the domestic water for free.

My sister's Airtemp heat pump water heater was built by Chrysler Corp before 1979, so they are not new either.

Marc Fontana's picture

Very informative article - I like the chart comparing daily average energy usage of different heaters. I've read somewhere that the 80-gal Heat Pump water heater is more efficient than 50 or 60-gal version from the same manufacturer. Why is that? Could it be that the same Heat Pump is used on both sizes, but the larger water tank, somehow, makes the 80-gal more efficient than the smaller tank? Another question: If changing the anode rod is recommended for longer life, how much space above the tank is needed to replace the anode rod without having to move the tank?

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