ᐅ Sizing of Air-to-Water Heat Pumps for New Construction

Reconsidered. If operated with 30/22, the flow rate for the heat pump will probably be too low, with all the negative consequences...

Is it really uncommon to request a heating load calculation? Our general contractor claims that I am the very first client out of more than 25 houses per year to ask for this.

I brought up that we had always discussed the calculation and the supply temperature of 30°C (86°F). Then he said they have never needed that before and that the heating installer usually does a good and correct job. Temperatures of 30-35°C (86-95°F) would be quite normal. If he meant 30°C (86°F) supply temperature, then that would mean the floor would be that warm. But what goes into the heating pipes cannot be controlled. He apparently completely misunderstood everything we have discussed so far and documented in writing.

Well, I then suggested that I could run the calculation myself, for example, using online tools (Google + Heckmann), and plan the heating layout and select the heat pump accordingly. He did not agree to this even 30 minutes after our phone conversation. Right now, I am not so sure that was a wise decision. Would I still be covered under warranty?

On the other hand, the worst that can happen is that I have pipe spacing that is too close or that the heat pump is undersized, right? Then, for example, if the bathroom stays too cold, I would have to operate at a higher supply temperature (=less efficient) and/or, in the worst case, install a larger heat pump, resulting in additional costs and inefficiencies.

What about the areas beneath furniture like the kitchen, shower, bathtub, etc.? Do these areas need to be kept clear? That was the contractor’s point – which sounds reasonable at first glance. However, in the kitchen, for example, only a minor thermal short circuit should occur under the furniture. Eventually, the furniture will warm up from below to around 30°C (86°F), and then the water continues circulating in the heating loop, allowing energy to be distributed further.

A temperature of 30°C (86°F) under the furniture is quite different from the old days of full-on gas heating, right? And generally, there is no guarantee that my kitchen layout will remain the same forever as planned when moving in. It could change, meaning different areas might be covered by furniture, while the areas originally kept clear become exposed and colder.

OWLer schrieb:

Is it really that uncommon to do a heat load calculation? Our general contractor claims that I am the very first of his clients to request this – out of 25+ houses per year.

That’s because the vast majority of homeowners spend more time and thought on choosing the toilet bowl—which is vital for survival—than on considering the heating system, which is only slightly more expensive. After all, you spend more time sitting on the toilet than on the heater. However, when it comes to underfloor heating, everyone abandons that logic and hardly thinks about it at all (maybe a bit about the selection of the fault detection device), since you mostly only touch the floor with your feet and sit in a chair...

OWLer schrieb:

I brought up that we had always talked about the calculation and a supply temperature of 30°C (86°F). Then the conversation went in the direction that they never needed that before, and the heating technician does a good and proper job. 30–35°C (86–95°F) is quite normal. If he meant 30°C (86°F) supply temperature, that’s supposed to mean the floor would be that warm. But what goes into the heating pipes can’t be controlled. He completely misunderstood everything we had discussed and documented so far.

Well, if the system is designed and built so that 30°C (86°F) is to be delivered to the supply line, then 30°C goes into the supply line. What is there not to control? What are the temperature sensors and control electronics for, then?

OWLer schrieb:

Well, now I suggested running the calculation, for example, online (Google+Heckmann), and planning the heating layout and also selecting the heat pump accordingly.

Very, very smart decision, do it that way! The general contractor and plumber will only guarantee the house will be warm, not that it happens efficiently and without putting unnecessary strain on the materials. You wouldn’t get that kind of guarantee in any contract.

OWLer schrieb:

On the other hand, the worst that can happen is that I have pipe spacing that’s too narrow or the heat pump is undersized, right? Then, for example if the bathroom stays too cold, I’d have to run at a higher supply temperature (= less efficient) and/or in the worst case install a larger heat pump, incurring extra costs and inefficiencies.

To avoid that, you do a heat load calculation and design the underfloor heating based on room-by-room heat loads with the room temperature and supply temperature you specify. You also need to consider the total flow rate to be able to avoid buffer tanks and bypass valves in the hydraulic setup. But you’re already on the right path.

OWLer schrieb:

What about the floor areas under furniture like kitchens, showers, bathtubs, etc.? Do these areas need to be left free? That’s what the general contractor said – which sounds reasonable at first. However, under kitchen units, for example, there should only be a thermal short circuit. Eventually, the furniture underneath will reach about 30°C (86°F), and then the water continues through the heating circuit, releasing energy further.

30°C (86°F) under the furniture is completely different from the old gas heating systems running at full power, right? And generally, there’s no guarantee that my kitchen layout will stay the same forever after moving in. It could change, meaning different floor areas would be covered by furniture, and the areas originally left free might end up cold?

You’ve basically answered your own question. If the room realistically only allows one possible furniture layout, then leave those areas free. If there are variations possible, don’t leave any floor areas uncovered.
Keep in mind that the screed (floor leveling layer) without heating pipes is not thermally separated from the rest of the floor and will warm up as well, just more slowly. So, in doubt, it’s better not to leave any free areas and cover the entire floor.

Cover the entire area, including under the shower if there is enough space.

The floor surface temperature does not reach 30 degrees Celsius (86°F); the normal range is 24-27 degrees Celsius (75-81°F).

Of course, it is possible to check what goes into the heating pipes. Your general contractor (GC) does not seem to have much expertise in this area.