ᐅ Underfloor Heating and Air-to-Water Heat Pump in New Construction: Am I Inviting Problems?

Hello everyone,

I have posted here several times before and have received a lot of really helpful advice. I have learned a lot. But reality is catching up with me: we are currently building a new house, and because of the builder, we are tied to a specific heating installer (alternatively, we could choose our own, but then we would have to find all the other tradespeople ourselves, including electrical, plumbing, and everything that follows). Since this is already contractually agreed, the matter is settled.

In my opinion, based on what I have learned here, he is doing everything "wrong":
1.) Viessmann air-to-water heat pump Vitocal 222-S with 7.2 kW, although the calculation shows only 5.5 kW needed

2.) Buffer tank to enable flow rate and prevent short cycling

3.) ERRs (electronic room regulators) in the main rooms (he wanted ERRs in all rooms, I limited it to the important ones)

4.) I saw on site today: heating circuits of varying lengths—from 30 m (98 feet) up to 120 m (394 feet). At least the spacing of the underfloor heating pipes is adjusted according to the room requirements, from 5 cm (2 inches) in the bathroom to 30 cm (12 inches) in the bedroom

Now my questions: Am I setting myself up for real problems, or is all this actually irrelevant (as he claims) and the system will still run properly? By properly I mean that a) it heats well and b) electricity consumption stays within reasonable limits (seasonal performance factor > 4.5, calculated at 4.9).

Best regards, Andreas

Sounds pretty reasonable to me. Not perfect, but what in construction ever is. ;-)

1) Whether the heat pump could actually be smaller is hard to say without numbers.
2) It depends on the size and whether it’s just in the return flow.
3) I don’t find ERR that bad either if you understand how it works and how to use it. You can request a normally open (NO) valve, so you can simply unplug it if you manage without it.
4) From 5 to 30cm (2 to 12 inches) is quite a range, which you can only estimate.

a) quite certain
b) we’ll see. Honestly, I don’t really believe that you can save significant electricity, and therefore money, by using a smaller heat pump or an even smaller buffer. There’s a lot of theory in forums, some of which is true, but actual comparisons, studies, and so on are missing. Because of this, I also tried hard to optimize these things, but you have to be realistic and understand that you can’t fine-tune everything to decimal points when building a house.

I’m just an informed layperson, but I can still share how things are with me and what I discussed with my heating technician.

Regarding point 1)
That’s not ideal at first, since it’s likely more expensive than a perfectly matching equivalent. However, with a modulating heat pump, it’s not necessarily a big issue as long as the minimum output is low enough. Sometimes you have to choose the larger model because of the required pump capacity (this is the case with my system, among other reasons). I don’t know how it works with your model.

Regarding point 2)
It’s also not a problem as long as the buffer tank is very small (around 50 liters (13 gallons)) and installed on the return flow. In this case, it actually prevents the pump from short cycling when all the thermostatic radiator valves are closed. The additional energy consumption is limited due to the small size. Also, if you have a tenant (for example, on a sabbatical abroad or for some other reason), they won’t immediately damage the heating system just because they don’t know how to operate it.

Regarding point 3)
This is required according to the energy saving regulations (building permit / planning permission rules). Exemptions are theoretically possible, but as far as I know, no one in this forum has reported actually managing to get one. However, you can effectively bypass this by removing the adjustment knobs, disconnecting the actuators from power, or uninstalling them. When selling the house, nobody will make a fuss about that.

Ah, too late (my wife had some tasks for me), so some points are repeated. But it’s good to see we’re not saying exactly the opposite...

Regarding point 4) in my design, I have heating circuit lengths ranging from 40m (hallway and small children's room) to 120m (bathroom). Since my design planning was specifically calculated by an engineering firm, I assume this is not a major issue. The heating circuits are also kept as equal in length as possible for each room. Theoretically, I understand that having uneven heating circuit lengths can cause problems with the hydraulic balancing and may lead to a thermal short circuit. Maybe @Daniel-Sp can provide more insight on this...

Overall, it’s not ideal, but the installation of the buffer tank is really nonsense. More details can be found in the other forum.

I wouldn’t generalize it that way. What’s important in the return flow is that the buffer tank is not heated first, and then the underfloor heating is heated from the buffer, and that the buffer tank isn’t too large.

If the buffer tank is several hundred liters large and connected in the supply line, that’s really problematic.

Regarding not being able to change it: If the buffer tank is connected in the supply line and you can’t convince the heating installer to do it differently, you could try what I unintentionally achieved: ask so many questions and insist so much that the installer eventually gives up. Then it’s not your fault, you can find your own installer (or the general contractor must find an alternative), but actually, the general contractor shouldn’t require you to handle all the following trades yourself.

But be aware, this can lead to significant cost increases and delays. So this is only a last-resort recommendation.