We turned the heating on 29th November last year, a couple of weeks earlier than last winter, but close to the normal time. It had snowed the week before, and there had been rather too many of the kind of November days you get in England.
The underfloor heating goes on from 8 to 8:30 then from 9 to 9:30, and a week later, according to the T and D thermometers we have in the slab, it's now about a degree warmer at the top than the bottom.
Underfloor heating, called radiant heating in the US, seems to elicit strong emotions, both for and against. In the building pantheon, it is a god for some, and a devil for others.
There seems to be a strong sentiment among Passivhaus proponents against underfloor heating. I don't think it's completely logical.
Certainly badly implemented underfloor heating has problems. Typical mistakes are:
1. Only heating part of the floor
2. Not insulating the other sides
3. Setting the temperature too high
4. Using the wrong floor materials
5. Not having a thick enough floor
6. Not distributing the heating elements evenly
7. Incorrect dimensioning
8. Using electricity
If only part of the floor is heated, the heat will obey the second law of thermodynamics and go to the part that is not heated. Our neighbour has underfloor heating in the living room, but it is in the slab which also runs under their garage. A significant amount of the heat they put in will be trying to heating their garage.
In a Passive House the first two problems won't happen, as long as the heated floor is within the thermal envelope. Since the heating demand is low, there should never be a need to set the temperature high.
The advantages of underfloor heating are:
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| Underfloor heating visible before the walls went up |
2. The heat is evenly distributed, and will radiate through the whole building.
3. Underfloor heating may increasing the thermal mass of the building and store heat.
Here's a picture of our underfloor heating, but it had to be taken before the screed floor was poured. There is usually nothing to see, unless you have a thermograph. You can see in the picture below with a temperature difference less than 5 degrees.
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| 26 degrees enough to heat a Passive House |
So why do the Passivhaus people seem to be so against underfloor heating? It may be partly sticking to the Passivhaus ideal that the house can be heated only by warming the incoming air. In that case why is there no similar disparaging of radiators? All you need for underfloor heating is a boiler, and in fact most houses have one of those for the domestic hot water. Where do you think we get our hot water from... the kettle?
Since the temperature for underfloor heating doesn't really need to be above 40 degrees centigrade, there are lots of other options for a heat source, such as ground source heat pumps or solar thermal. With the low heat requirement of Passivhaus, a slightly oversized domestic hot water supply should be able to cope with underfloor heating in its stride. Typical boilers use power in kilowatts. The heating requirement of a passive house is of the order of 10 Watts per square metre.
There may be more technical arguments against underfloor heating, regarding the efficiency of the heat transfer. All heat losses are within the thermal envelope and will end up eventually heating the house anyway. If you're in a poorly insulated house where you want to quickly make a room warm while you're there, and not waste too much energy keeping it warm when you leave, then you may not want underfloor heating and its inherent thermal mass. Passive houses are designed to stay at an optimum temperature, and heating is not a quick fix for comfort on demand, but keeping the thermal envelope topped up.
Compared to radiators, underfloor heating probably requires a more powerful pump to circulate the heating fluid since there is a greater resistance. But pumps draw negligible power compared to the heat they deliver.
This article from Buildinggreen.com gives four reasons for underfloor heating being the wrong choice in low energy homes. If you're ripping up the floors of an old house to put underfloor heating in, it's going to be expensive. But if you're putting some pipes in while you pour a screed for a tile floor it may be no more expensive than radiators. The piping may even be cheaper than for radiators since all the pipes can go into the floor in the same place, while each radiator will need its own pipe.
The argument against overheating in houses with high solar gain may be more pertinent. However, passive houses don't need the heating on for twenty-four hours, so the heating could be switched on after the sun goes down. There should be no problem with solar gain then. And it can still be switched on for those winter days when there is no solar gain.
Perhaps some powerful character within the Passivhaus movement had a bad experience with underfloor heating. Maybe he tried it once and got burnt, sitting on an overheated floor with his pants off.
