Renewable Hydronic Heating: Page 2 of 6

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Hydronic heating in a floor slab
Hydronic heating systems can provide comfort for your home, like this one in the floor.
Hydronic heating wall unit
Hydronic heating systems can provide comfort for your home, like this one on the wall.
The Heating Edge fin-tube baseboard heating (detail)
The Heating Edge is a recent development in low-temperature fin-tube baseboard heating (detail).
The Heating Edge fin-tube baseboard heating
The Heating Edge is a recent development in low-temperature fin-tube baseboard heating.
Tubing embedded in a concrete floor slab
Tubing embedded in a concrete floor slab is the most common form of radiant floor heating.
A thin-slab radiant panel installation
A thin-slab radiant panel installation awaits the concrete pour. The 1/2-inch PEX-AL-PEX tubing has been carefully fastened using a special stapler. A layer of 6-mil polyethylene film provides a bond break between the slab and the plywood subfloor underneath.
A radiant ceiling system
A radiant ceiling system is installed in much the same way as a radiant wall system.
An infrared thermograph
An infrared thermograph of a hydronic radiant ceiling as it is warming up. The water flow is from left to right, as shown by the red and orange areas.
JAGA North America’s Low H20 panels
JAGA North America’s Low H20 panels provide the latest in contemporary looks and thermal performance. Small fans move air past the radiator fins, increasing low-temperature output up to 250%.
Radiant panel from Vasco Heating Concepts
Contemporary radiant panel designs, like this one from Vasco Heating Concepts, can look like a work of art in themselves. Be sure to verify low-temperature performance when choosing any radiant wall panel.
Radiator from Runtal
This radiator from Runtal is part baseboard, part wall panel. Radiant panels come in many shapes and sizes to fit almost any application.
Home run distribution system
This home run distribution system is simple and effective, with a manifold accessible through a wall panel.
A panel radiator
A panel radiator with an integrated thermostatic radiator valve
Thermostatic radiator valve
A panel radiator with an integrated thermostatic radiator valve (detail).
Circulation pumps
Pressure-regulated, variable-speed circulation pumps (from left): Grundfos, Wilo, and Bell & Gossett.
Hydronic heating in a floor slab
Hydronic heating wall unit
The Heating Edge fin-tube baseboard heating (detail)
The Heating Edge fin-tube baseboard heating
Tubing embedded in a concrete floor slab
A thin-slab radiant panel installation
A radiant ceiling system
An infrared thermograph
JAGA North America’s Low H20 panels
Radiant panel from Vasco Heating Concepts
Radiator from Runtal
Home run distribution system
A panel radiator
Thermostatic radiator valve
Circulation pumps

The coefficient of performance (COP) is the heat pump equivalent of efficiency: the ratio of the heat output divided by the electrical input. A COP of 4.0 means that the heat output is four times greater than the electrical energy required to operate it. The higher a heat pump’s COP, the lower its operating cost.

The graph shows the heat pump’s COP dropping rapidly as the hydronic heating system’s water temperature increases. Thus, for the highest possible COP, the water temperature supplied to the hydronic heat emitters should be kept as low as possible.

Wood-fired boilers can produce higher water temperatures, even up to 200°F, but that doesn’t negate the benefits of matching a wood-fired boiler to a low-temperature distribution system. These heat sources are best used with a thermal storage tank. They add heat to the storage tank, and the heating distribution system draws heat out. The lower the tank temperature can go and still supply sufficient heat to the building, the less often the boiler has to be stoked.

For the best efficiency, design hydronic heating systems supplied by either solar collectors or hydronic heat pumps so that the supply water temperature to the load (under maximum load conditions) doesn’t exceed 120°F. This temperature is a reasonable compromise between maintaining good heat source performance, while not overly increasing the cost and space requirements of the heat emitter.

Making It Happen

It’s critical to understand what determines the supply water temperature (the temperature being supplied to the load via heat emitters) in a hydronic heating system. Some designers think that it is determined by the heat source—because many boilers come with a dial or digital control that “sets” the temperature produced. Unfortunately, that’s not how it works. That setting is only a high-temperature limit on the heat source output. It does not guarantee that the set water temperature will ever be reached. The water temperature in any operating hydronic heating system climbs only high enough for thermal equilibrium—where the rate of heat release from the heat emitter balances the rate of heat input from the source. Once thermal equilibrium is reached, there is no thermodynamic incentive for the water temperature to climb higher, and it won’t!

It’s the hydronic distribution system’s design, rather than the high-limit setting, that determines the system’s operating water temperature. Almost everyone who designs heating systems wants to maximize thermal efficiency. For hydronic systems, this means moving away from high water temperatures by using heat emitters with larger active surfaces, or other details, such as internal microfans, that increase both convective and radiant heat transfer. This allows thermal equilibrium to occur at relatively low water temperatures during both maximum and partial load conditions.

Emitter Evolution

There are several ways to design modern hydronic distribution systems around RE’s lower water temperatures, starting with the heat emitters—any device that removes heat from water flowing through it, and releases it into the room. 

Many homeowners are used to the look of fin-tube baseboard heaters. From the outside, modern baseboard heaters look similar to older ones—but what’s inside is very different. Fins can be about three times larger than traditional baseboard heaters, with multiple copper tubes running through those fins. Tubes can be piped for either parallel or series flow. In the latter case, the hottest water flows in the upper tube, makes a U-turn at the end of the heater, and flows back in the lower tube.

Assuming an average water temperature of 110°F, this baseboard releases about 290 Btu/hr./ft.2 at 1 gpm when the tubes are configured for parallel flow. With 4 gpm, the output increases to about 345 Btu/hr./ft.2 If the two tubes are configured for series flow, the output drops about 10%.

Consider a 12- by 16-foot room in a well-insulated home, with a maximum heating load of 2,880 Btu per hour (e.g., 15 Btu/hr./ft.2). This load could be met using a 10-foot length of Heating Edge baseboard operating at an average water temperature of 110°F at 1 gpm. A 10-foot length of conventional residential baseboard heater would require an average water temperature of about 150°F. This temperature is well above what a typical geothermal heat pump can produce, and would significantly lower the efficiency of solar thermal collectors. An additional 14 feet of conventional baseboard would be required (24 feet total) to deliver the same output at an average water temperature of 110°F.

Comments (9)

Tom M's picture

With any renewable type system the rule should be make as much as you can whenever you can then use what you gain wisely, eg. do laundry when the sun is shining. The simple solution for moderating high temperatures especially with water systems is a mixing valve to only use what you need and leave the rest for other uses.Same with electricity.

mkogrady's picture

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.

Doug Jones's picture

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?

Ian Woofenden's picture

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

Tom Bednarchuk's picture

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).

Ian Woofenden's picture

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

Suzan Elichaa's picture

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.

Ian Woofenden's picture

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.

benny_terry's picture

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?

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