ᐅ Underfloor Heating Heat Load Calculation – Explained Simply

If you do not specify the desired temperatures, standard values of 20°C (68°F) for living areas and 24°C (75°F) for bathrooms will be used.

Thank you for the feedback. I just found out that when applying for the BAFA grant, you can theoretically specify any heat pump, and later during implementation, you can still receive funding if a different heat pump is installed, as long as it is eligible. This reduces some of the time pressure for us, and we will then require a room-by-room heating load calculation.

Thanks again!

Just as a quick heads-up:

We have a KFW 55 house ourselves, which, due to the ventilation system (which we absolutely wanted and is definitely a good choice), almost meets the KFW 40 standard. The house is located in the Westerwald region, where it used to get quite cold in winter (-12°C to -15°C / 10°F to 5°F). The calculated heating load for 100m2 (1,076 sq ft) of living space is just under 3.5 kW (which is still a bit too high in reality), and our air-to-water heat pump with 3.5 kW only starts operating efficiently at around 5°C (41°F) outdoor temperature, where it doesn’t short-cycle.

What I want to say is
(put bluntly and assuming you want normal indoor temperatures with a maximum of 24°C (75°F) in the bathroom and 20°C-22°C (68°F-72°F) in the other rooms):

Choose a heat pump that can modulate its output as far down as possible...

Go for the smallest possible heat pump – less is more...

Avoid all-in-one heat pumps that even integrate the ventilation system...

Don’t trust the general contractor (GC). They often use a mixed calculation method and will install whatever just to keep the house warm, no matter if the system ends up heating directly with electricity at a 1:1 ratio. The performance factor (COP) or efficiency guarantee is usually not contractually defined. They just need to provide something that heats the house. The monthly electricity bill is your problem, not the GC’s.

Forget about having 24°C (75°F) in the bathroom and only 18°C (64°F) in the bedroom right next to it. You will get a maximum of about a 2°C (4°F) temperature difference between rooms, if that.

Note on storage rooms or pantries: According to the energy saving regulations, rooms under 6m2 (65 sq ft) do not need individual room controls or underfloor heating.

No buffer tank.

Design the underfloor heating system for a maximum flow temperature of 30°C (86°F) at the heat pump, not 35°C (95°F).

Keep the spacing between underfloor heating pipes as narrow as possible—especially in bathrooms. Don’t just opt for 20cm or 25cm (8 inches or 10 inches) because “that’s how it’s always done.”

Make sure the minimum or nominal flow rate of the heat pump is reached by the underfloor heating without any bypass valves.

Keep heating circuits of the underfloor heating as equal in length as possible and use pipes with a large diameter—at least 16mm (5/8 inches), preferably 17mm (about 11/16 inches).

Use 28mm (1 1/8 inch) copper pipes for the distributor feed lines—not 20mm or 25mm (3/4 inch or 1 inch) aluminum composite pipes or similar cheap alternatives.

Insulate the supply pipes for the underfloor heating that are often laid centrally in hallways. Otherwise, they will heat the hallway and lose heat on the way to the actual rooms. Also, no matter how much you turn down the hallway’s room thermostat, it won’t help because those uninsulated supply lines keep releasing heat. Our GC didn’t know this either until I pointed it out, so one day before screeding, the pipes were insulated. That was painful for him because it meant an extra trip, and the heating installer had to come 100 km (62 miles) one-way for just one hour of work.

Well, if you throw all of this at the GC, either they or their heating installer will get fed up with you and suggest you upgrade to a stage where you can hire your heating company yourself.

And yes, I am satisfied with our heating installer and our GC, even though planning the heating system was a real struggle...

It should also be added that in such houses, mechanical ventilation with heat recovery (MVHR) is as unnecessary as a third wheel. It creates more problems than one would like, and the supposed savings, of course, do not materialize—in fact, the opposite happens.

You really can’t avoid the ERR, especially when working with a general contractor, because they are mandatory according to the energy saving regulations. There is an exemption request that can be submitted (before or along with the building permit / planning permission application), but by the time clients write here, the building permit / planning permission has usually already been applied for or approved.

lesmue79 schrieb:

Correct about the ERR, but especially with a general contractor, you can't really avoid it because it is mandatory according to the energy saving regulations. There is an exemption application that can be submitted (before or with the building permit / planning permission application), but usually it’s already too late by the time builders are asking about this here, as the application has mostly already been submitted or approved.

Yes, that’s also the case here. I was already "aware" of that beforehand. But the wife acceptance factor (WAF) for the ERR is very high 😉