ᐅ Is a Tight Pipe Spacing for Underfloor Heating Not Possible with a Gas Heating System?

Hello dear forum,

while we are waiting for the next draft of our house, I am continuing to research the heating system. In an earlier thread, I mentioned that we decided on a gas condensing boiler and 15% better insulation, so a standard new build.

Now we asked our general contractor whether the underfloor heating could be designed with a heat pump-compatible low flow temperature and thus a higher pipe spacing density (for a possible switch to a heat pump later on). He said that a flow temperature below 40°C (104°F) doesn’t make sense with a gas condensing boiler. I find that hard to believe—I’ve read a lot online suggesting that the condensing effect is even more effective if the return temperature is especially low. And that can only be achieved with a correspondingly lower flow temperature, right? Or am I mistaken here? I was thinking it would be possible to set a future-proof flow temperature as a fixed parameter in the heating design. I’d appreciate your advice!

Best regards,
Mecc

Why is a flow temperature below 40°C (104°F) supposedly pointless for a gas condensing boiler? The lower the flow temperature, the more efficiently a condensing boiler operates. The underfloor heating system simply needs to be designed according to the low flow temperature you want. This can easily mean a flow temperature of 32°C (90°F). The underfloor heating system will then just be correspondingly more expensive.

Mecc schrieb:
....I can't believe that, I've read a lot about it online, including that the condensing effect would play a bigger role if the return temperature were particularly low.

That is correct.
Cooling is most effective with the lowest possible return temperature, provided that the supply temperature is already below the condensation temperature of the respective energy carrier.
For most low-temperature (LT) systems, this should be the case (supply temperature < ~ 55°C (131°F)).
On the other hand, the condensing effect benefit is often overestimated! Theoretically, about 11% can be achieved. If domestic hot water is excluded, realistically only up to 7% remains!
A heat pump-optimized underfloor heating system offers relatively little potential here.
In my experience optimizing existing systems (radiators), I sometimes omitted this consideration.
Result: energy savings of up to 30–35%, despite a reduced condensing effect! ;-)
Besides purely energetic considerations, safe operation must be ensured.
This can only be reliably achieved with gas condensing boilers when the supply temperature is below 40°C (104°F) (design case) by implementing some special measures.

Best regards.

Hi Humi,
yes, that’s how I had imagined it too. But it seems that it’s not so easily feasible.
@€uro,
first of all, thank you for the explanation!

€uro schrieb:

On the other hand, the condensing efficiency benefit is often overestimated! In theory, about 11% can be achieved in reality. If you exclude hot water, realistically only about 7% remain at best!
A floor heating system optimized for heat pumps offers relatively little potential here.
...
Besides purely energy-related considerations, a reliable operation must be ensured.
This is only safely possible with gas condensing boilers at supply temperatures below 40°C (104°F) (design case) under certain special conditions.

best regards

As I understand it, this means that a floor heating system optimized for heat pumps makes little sense with gas condensing boilers because it is inefficient for gas, right? That’s a pity! We had considered floor heating as an option in order to switch to a heat pump later. So this is not easily possible? Or what special conditions need to be taken into account?

Good luck, Mecc

Mecc schrieb:
...From what I understand, a heat pump-optimized underfloor heating system makes little sense with a condensing gas boiler, since it’s inefficient for gas, right? That’s a pity! We had considered underfloor heating as an option to later switch to a heat pump. So that isn’t easily possible?

Not easily! Unfortunately, you can’t have everything for free in life due to competing demands! ;-)

Mecc schrieb:
...Or are there any special considerations to keep in mind?

With a heat pump, every degree of avoidable supply temperature matters, whereas with a condensing gas boiler, as long as it’s below the condensation temperature, it’s less critical. This alone leads to different requirements. If you want to size the heating surfaces with the current condensing gas heating system to meet future heat pump demand, appropriate precautions are necessary!
The commissioned MEP (mechanical, electrical, plumbing) engineer will surely be able to provide detailed information!

Best regards

Hi €uro,

I understand that you have to pay for planning services. We are not opposed to that at all! But I would simply like to know in advance whether it is possible or not before I make a huge effort. Or whether the general contractor is just saying something that suits him better.

Is a heat pump-optimized underfloor heating system with a gas condensing boiler significantly more expensive (meaning several thousand €uro), or does it just need to be well designed with a few technical additions?

Good luck, Mecc