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Renewable Hydronic Heating: Page 6 of 6
Last Updated:
11/19/12
Beginner
- Beginner
The Big Picture
The main component in the schematic (above) is a well-insulated drainback/storage tank equipped with an electrical element or integrated gas burner for backup, with an internal heat exchanger. The element/burner keeps the water at the top of the tank warm enough to provide domestic hot water (typically 120°F to 130°F).
A drainback-protected solar collector feeds the hydronic heat distribution system. The collector circulator runs when the collectors are a few degrees warmer than the water near the bottom of the storage tank. No antifreeze is required in this system, and no heat exchanger is needed between the collectors and the storage tank. These features reduce cost and increase collector efficiency. The same water that flows through the collectors also flows through the heating distribution system. The system is completely “closed” from the atmosphere.
The captive air at the top of the tank is under slight positive pressure. This airspace provides a drainback reservoir, and acts as an expansion tank. The water in the tank provides thermal storage for the solar collectors, and it provides thermal mass to buffer the zoned space-heating distribution system. The latter function protects the burner against short operating cycles, which would otherwise decrease efficiency and increase maintenance. Short cycle protection is very important in a hydronic system with extensive zoning.
A flow switch detects whenever domestic hot water is being drawn at a flow rate of 0.5 gpm or higher, activating a small circulator that moves hot water from the top of the thermal storage tank through a plate heat exchanger. Cold domestic water is heated as it passes through the other side of this heat exchanger, and sent to the taps.
An antiscald thermostatic mixing valve protects against high domestic water temperature when the tank is very hot, like at the end of a sunny, warm day. For the fastest possible response, the piping between the thermal storage tank and heat exchanger should be short and insulated. Combination isolation/flushing valves should be installed on the domestic water inlet and outlet of this heat exchanger. They allow the heat exchanger to be isolated and flushed if necessary to remove scale.
A single variable-speed pressure-regulated circulator feeds the home run distribution system for space heating. One circulator can supply the entire distribution system in a typical 2,500-square-foot house using no more than 40 watts under maximum heating load.
Each panel radiator has an adjustable thermostatic valve that monitors room temperature, and adjusts the flow rate to maintain that temperature. No thermostats, batteries, transformers, or programming—just simple, effective, and reliable room-by-room temperature control.
The mixing valve upstream of the manifold station protects the distribution system from what could be a very hot storage tank following a sunny spring or fall day. It also adjusts the water temperature supplied to the panels based on outdoor temperature, known as “outdoor reset,” and stabilizes room temperature for optimum comfort.
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John Siegenthaler is a mechanical engineering graduate of Rensselaer Polytechnic Institute, a licensed professional engineer, and professor emeritus of Engineering Technology at Mohawk Valley Community College. “Siggy” has more than 32 years of experience designing hydronic heating systems. The third edition of his textbook, Modern Hydronic Heating, will be released in January 2013.
Hydronic Heating System Products:
Caleffi • caleffi.com • Solar collectors, storage tanks, controls, thermostatic valves
Grundfos • grundfos.com • Hydronic circulators
Heatlines • heatlines.com • Panel radiators
HTP • htproducts.com • Hydronic heat sources, including solar integration
Jaga • jaga-usa.com • Low-temperature hydronic heat emitters
Smith’s Environmental Products • smithsenvironmental.com • Low-temperature baseboard
Wilo • wilo.com • Hydronic circulators
Xylem/Bell & Gossett • completewatersystems.com • Hydronic circulators
Comments (8)
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I was looking at a small room heating idea using that under floor electric heat Mat stuff instead of a liquid based solution. How do these compare to one another in terms of cost, installation and efficiency?
The space is a small 12X13 bedroom I plan on building this summer in my walk out basement. The location is on the north side where it's going to get chilly.
Wait a minute... are you two saying that we should forget about thermal solar and go straight to insulation/PV? That thermal solar is a waste of time and energy?
Hi Doug,
I think Suzan's latest comments are spot on, but it depends on the specific situation and climate. It's almost always a balance between cost and benefit, resources and results.
In many climates (like mine), there isn't a lot of sun when space heat is needed, so focusing on solar space heating doesn't make a lot of sense. And when it does, passive solar design is usually a simpler and more cost effective option.
In general (again, specific homeowner goals and the specifics of the site, climate, and house will affect this greatly), I would focus first and foremost on efficiency, thermal and otherwise. Then I would look at efficiency of the heating system, with mini-split heat pumps being the current star performer in the cost/benefit arena. Then I would go after domestic water heating, using a solar hot water system. And then PV.
That said, recent drops in the cost of PV, and its simplicity compared to SHW, lead many people to go for PV sooner in their priority list. And each person has different goals, budget, patience for complexity, and attention span for RE and efficiency work. Many of my students and clients choose only to invest in PV, 'cause it's easy and effective, even after I've advised efficiency work first.
I think all work towards RE and efficiency is not a waste of time, and we each have different goals and situations. Active solar thermal _space heating_ is often of questionable cost effectiveness in my experience, in my moderate, cloudy-winter climate.
Thanks for reading and discussing!
Ian Woofenden, Home Power senior editor
Hi Ian,
"Then I would look at efficiency of the heating system, with mini-split heat pumps being the current star performer in the cost/benefit arena."
Do you have some numbers to show this so that an apples to apples comparison can be done?? Can you point me at some spec sheets for some heat pumps and I can do my own analysis? (I'm an engineer)
I agree with the thermal efficiency statement, we still build lousy houses but the house you have is the house you have. A lot of articles (like this one) don't seem to take into account the retrofit market. I'm looking at the heating for my own home (presently baseboard electric) as my last electric bill was $600 (of course most of the charges were not for the actual electricity but that's another story).
"That said, recent drops in the cost of PV, and its simplicity compared to SHW, lead many people to go for PV sooner in their priority list."
In my area (Ontario, Canada), I would say this is because our government has made a large investment in solar programs (paying $0.83/kWH for solar when rate from utility is $0.08-0.10) and people jump on the bandwagon. From and engineering efficiency point of view solar still doesn't make a lot of sense (IMO). By the time the system is paid off (20 yrs), it's time for a new one (20 yr expected lifetime).
Hi Tom,
I don't have cost comparisons handy for heating systems, and I'm out of the country with poor connectivity at the moment too. But I encourage you to do more research on mini-split air-source heat pumps. The COPs claimed are in the 2-5+ range, which means 2 to 5+ times the heat for your kWh. And the cost is very modest -- in the $4-10K range installed complete, depending on home size and number of indoor units.
When I referred to the recent drops in PV, I was not talking about incentives at all, but the actual installed costs of systems, which has come down dramatically in the last several years, due to lower prices on PVs primarily. PV _very_ often makes purely financial sense with incentives, and it makes even more environmental sense. PV modules have _warranties_ of 20-25 years, and will be producing for 40+ years if well installed and maintained. I have modules on my roof that were installed in 1984 and are still going strong. Even if your very low prediction (20 years) were true (it is not), what else can you buy that is _productive_ and lasts that long? PVs are an amazing product that is very underrated.
Best,
Ian Woofenden, Home Power senior editor
I orginally got interested in solar because I live in Maine where the vast majority of people have a boiler fired by oil. It seems intellegent to use solar hot water. What I learned was regular baseboard was designed for 180 degree which is not realistic from solar in the winter. So I went about recommending everyone build a new home with radiant floors. I have now changed my mind. There is no logical reason to build an inefficient home. Proper air sealing and insulation is not very expensive. If you build right a simple air source heat pump, our favorite is Mistubishi mini split, is enough to heat the home. For really long term cold you may need a simple electric space heater, but those can be had for $30. Since energy efficient homes dont loose heat fast a day of cold temps will have little impact on the indoor temp even if the heat pump cannot produce. These days heat pumps run down to -17 degrees, that covers 99.9% of the days in Maine. Heat pumps are MUCH less expensive than solar powered radiant heat or low temp baseboard & radiators. The savings can go into air sealing and insulation. A small solar hot water system can meet domestic needs.
Suzan, You have come to the same conclusion that I often do -- thermal energy efficiency coupled with mini-split air-source heat pumps is the best option. In addition to the excellent reasons you cite, shifting the heating load to electricity also means that you may be able to power your heating system renewably, either through an on-site PV/wind/hydro system, through the renewable electricity your utility already sells, or through renewable energy credits.
Is there any benefit to using a water-to-water "geothermal" heat extraction device to remove heat from the fluid entering the solar thermal circuit? Would this, in your opinion, contribute to efficiency if the heat obtained via "geothermal" was used to augment heat gains obtained through the solar thermal loop?