Can passive solar and hydronic radiant heat live together?

February 7, 2010 · Posted in Construction, Lessons Learned from a Green Building Project

We’ve now lived with our radiant floor heat for most of six heating seasons, so I think it’s time to draw some conclusions. The house was built in 2004. Insulation is better than average. The air leakage is impressively low at 1.8 air changes per hour at 50 Pascals. It’s also a true passive solar design with 60 percent of the glazing facing south, high solar heat gain glazing and concrete floors for thermal mass.

Being our first experience with radiant floor heat, we anticipated trotting around barefoot all winter on warm floors. It was immediately apparent that toasty toes were not a given. Because the house is so tight and well insulated, the heating system doesn’t really operate that much of the time. The house requires no heat at all until several days have passed with temperatures in the mid-40s and little sunshine. With the heat system sitting idle, the concrete floor stabilizes at about the same temperature as the room air. That means the floors are considerably cooler than we are. Instead of heat flowing from the floor to us, it’s the other way around. This situation holds true much of the heating season. However, during the coldest, darkest periods, the heat kicks in and our toes rejoice.

Our hydronic system is typical of many radiant floors. Cross-linked polyethylene (PEX) tubes are imbedded in a concrete slab. This slab is only three inches thick and sits atop a typical wood-framed floor. Each of the three zones is controlled by a standard wall mounted thermostat, which measures air temperature. Each zone has a small manifold that feeds three circuits. The flow of water to each circuit can be adjusted up to a maximum of 3/4 gallons per minute (gpm). Over the years, I’ve experimented with different flow rates, and I think I’ve come up with an optimum distribution within each zone.

Zone 1 is the top floor great room with living, dining and kitchen areas. It’s about 600 sq. ft. and contains three large south-facing windows – the source of considerable solar gain. The three-inch-thick concrete floor serves as thermal mass, absorbing heat when the sun shines on it and then slowly releasing the heat throughout the evening. Energy experts often say that this combination of passive solar and radiant floor heat doesn’t work. If the concrete is already warm from the hydronic heat, it will be unable to absorb additional solar heat. That’s certainly correct, but the design of the circuits allows us to have our solar heat and hydronic, too.

Hydronic heat cirucuits are laid out parallel to the solar windows to the left. The short wall will be a kitchen island.

The three circuits of Zone 1 are arranged parallel to the south facing windows dividing the room into three sections. The section adjacent to the south windows is turned off entirely because it receives direct solar gain for the entire heating season. We also keep this section largely free of furniture that would shade the floor. The middle section runs at about 50 percent of maximum flow – between 1/3 and ½ gpm. This section receives solar gain only during mid winter when the sun angles are low and light penetrates deeply into the room. The third section never sees direct gain, so the water flow is set to full. This arrangement allows solar heat to be stored while keeping other sections comfortable.

Hydronic radiant floor doubles as solar thermal mass, if carefully laid out.

Another view of the floor shows the kitchen too far from the solar windows to receive direct sunlight.

Zone 2 is much different. This zone contains the master bedroom, master bath and the home office. The office has a large south-facing window, but sun can’t strike the floor directly because of desks. The flow rate for this room is about 1/3 gpm or 30 percent of maximum. The bedroom has it’s own circuit also set to about 1/3 gpm. The last circuit in this zone comprises the bathroom and a small walk in closet. The flow is set to the full ¾ gpm. Because the thermostat sits in the bedroom, heat concentrates in the bathroom floors where it stays warm, while the bedroom is nice and cool for sleeping.

Zone 3 is the ground floor containing two bedrooms and a bath for the kids. This zone gets no solar gain, so the flow rate is set to full for all circuits.

With thoughtful design of the hydronic system, I think solar and radiant floors can work together. Credit for this design goes to Jim Chauncey at Sunterra Homes.

That leads us to another question. Is hydronic heat more efficient? Let’s break it down into two parts: distribution and conversion.

Hydronic is a far more efficient way to distribute heat than the most common method, namely forced air. Ductwork is notoriously leaky. Typical duct systems, which are located outside the conditioned envelope, can lose 20 to 30 percent of the heat before reaching the register. Hydronic systems lose virtually nothing. Actual leakage isn’t tolerable as it is with forced air. Plus, the pipes are commonly located inside the building’s conditioned space. What little heat does escape the pipes still flows into the house.

The conversion from fuel to distribution medium is also more efficient. With hard floors, the delivery temperature of the water can be as low as 85°F. It takes less energy to raise the water to this lower operating temperature. Because our hot water is supplied by a heat pump, there is an extra operating advantage to this lower delivery temperature with reduced wear and tear on the mechanical equipment.

Water has a higher specific heat than air so heat transfer within the heating plant (furnace, boiler, etc) is probably a little more efficient than the heat transfer to air. Is this enough to make a difference? I rather doubt it, but I think it deserves mention in the interest of completeness.

The typical advantages of hydronic floor heat obviously still apply. It’s quiet and doesn’t affect furniture placement.

Is high-mass hydronic heat like ours the best choice for a super-efficient house? I will agree with critics to some degree. My current vision of a “perfect” green house is so small and so simple that any type of central heating system is overkill. I would rather spend money on R40 walls, R60 ceilings and the best windows I could afford. The complexity and expense of a hydronic system would be a waste of money and resources.

However, not everyone shares my vision of the perfect house. Many new home designs include a mix of solar-heated space and those with no solar gain. Depending on the design, it may be that hydronic heat is the best approach.

The distribution efficiency might also make it an excellent choice for larger commercial buildings, multifamily buildings and district heating. In these cases, a highly efficient central plant can supply a lot of floor space.

Another advantage to hydronic heat, in general, is adaptability. There are many ways to heat water, including biomass boilers, other biofuels and active solar. With uncertainty about the future, this kind of flexibility may be useful.

So, is hydronic heat appropriate in green buildings? I will avoid making any sweeping proclamations. Instead, I’ll say that “it depends” on the requirements of each project and the care with which the design addresses those requirements.

–Bruce Sullivan

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Comments

6 Responses to “Can passive solar and hydronic radiant heat live together?”

Tampile on February 8th, 2010 4:02 pm

I would go for heat provided by electrical devices. Not a central system as you said, but isolated devices.
Bruce Sullivan on February 9th, 2010 2:48 am

In this house, the design heat load — the amount of heat needed to maintain 68 degrees inside when it’s 15 degrees outside — is about 20,000 btu. That would be about 6 kilowatts of electric heat. That is still a bit high for electric heaters. When the design heat load gets down around 2 kW, electric resistance would be viable. This is “net-zero” house territory.
vegetarian update « green keen on February 16th, 2010 8:13 pm

[...] floor heat, vegan, vegetarian Before I get to the meat of the post (haha) I wanted to point out this blog post on radiant floor heating that I came across in my Google alerts from work this weekend. I liked it [...]
Ryan Shanahan on March 1st, 2010 10:49 pm

There is a similar article echoing a lot of this on the last page of the Feb Environmental Building News. I’d link it up, but it’s a paid service!
Gary Winters on September 26th, 2010 9:35 pm

We are designing a similar system to yours except single floor slab. We don’t anticipate needing to turn off or reduce the flow to individual circuits in the same zone. Do your pumps cycle on and off? We anticipate keeping our largest zone pump running continuously throughout the heating season to equalize the circuit temperatures. This zone has large south facing windows that should warm the water in the circuit nearest them. This water will continuously return to the water heater and mix with the water in other circuits of this zone. If there is sufficient solar heat for the entire zone the water heater will remain off and the circuits will all be the same temperature. If there is insufficient solar gain the water heater will come on and make up the difference. Would this work for you?
Bruce Sullivan on October 3rd, 2010 10:51 am

Yes, this system is set to cycle the circulators (pumps). You’re correct that you can allow the water to circulate continuously, but then you have to change the temperature of the deliver water to maintain a the proper room temperature. Changing water temperature is possible with a boiler, but not the heat pump that we have. Also, the water temperature is already set to about 85 degrees, so we couldn’t go much lower. The house is so tight and well-insulated that the heating system just doesn’t need to run that much.

I’ve tried to make the solar zone (where the sun strikes the floor) pick up heat and carry it back to the storage tank. I haven’t been able to make that happen. It seems to me that the solar zone would have to be in cooling mode (calling for pumping when the temperature exceeds a minimum temperature) while other zones are in heating mode ( calling for pumping when temperature declines below a upper limit). Also, the heat source (heat pump, in my case) would have to be turned off, so that the circulation fluid is cooler than the floor slab in the solar zone. I hope that makes sense. My controls don’t allow zones to be in different modes, so I haven’t been able to test this idea.

If the water runs continuously then the room would get very hot before any heat would enter the circulation loop which is already at 85 or 90 degrees. Again, you might be able to get this to work by turning off the heat source. Otherwise, the room will get too hot for most people. However, energy nerds will endure a lot to prove a point!

Thanks for your thoughts.