Is a tighter installation spacing for underfloor heating not possible with gas heating?

Hello dear forum,

while we are waiting for the next draft of our house, I am continuing to focus on the heating. In an earlier thread, I already reported that we decided on gas condensing and 15% better insulation, so standard new construction. Now we asked our general contractor if the underfloor heating could be designed with a heat pump compatible low flow temperature and thus a greater pipe spacing density (for a later possible switch to such). He said that less than 40°C flow temperature does not make sense with gas condensing. I can't believe that, I have read a lot about it online, among other things that the condensing effect would also come more into play if the return temperature were particularly low. And that can only be achieved with a correspondingly lower flow temperature, right, or am I on the wrong track here? Otherwise, I had imagined fixing a future-oriented flow temperature for the heating planning as a binding parameter. I would be happy about your tips!

Best regards, Mecc

Why should less than 40° flow temperature make no sense for gas condensing boilers? The lower the flow temperature, the more efficiently a condensing boiler operates. The underfloor heating must be designed for the low flow temperature as you requested. This can certainly mean a flow temperature of 32°. The underfloor heating will then correspondingly be more expensive.

....I can't believe that, I've read a lot about it on the internet, including that the condensing effect would come more into play if the return temperature were particularly low.

That is correct. Cooling is best achieved with the lowest possible return temperature, provided that the supply temperature is already below the condensation temperature of the respective energy carrier. In most low-temperature systems this should be the case (supply < ~ 55 °C). On the other hand, the condensingbenefit is overestimated! Theoretically, about ~11% can be realized. If you exclude hot water, realistically only about 7% remain at best! A heat pump-optimized underfloor heating system offers relatively little potential here. I have sometimes foregone this in the optimization of existing systems (radiators). Result: energy savings of up to 30..35%, despite reduced condensing effect! ;-) Besides purely energetic considerations, safe operation must be ensured. This is only safely possible with gas condensing boilers at a supply temperature < 40°C (design case) with some special measures.

Hi Humi, yes, that's how I thought it too. But that is probably not so easily possible. @€uro, first of all, thanks for the explanation!

The condensing valueuse is overestimated on the other hand! Theoretically, about 11% can be realized. If you exclude hot water, realistically at best 7% remain! A heat pump-optimized underfloor heating offers relatively little potential here. ... Besides purely energetic considerations, safe operation must be ensured. This is only safely possible with gas condensing value with supply temperature < 40°C (design case) with some special features. regards

I understood that a heat pump-optimized underfloor heating makes little sense with gas condensing value, since it is inefficient for gas, right? That's a pity! We had seen underfloor heating as an option to be able to switch to a heat pump later. So that's not so easily possible? Or what special features need to be considered? Best regards, Mecc

...I understood it to mean that a heat pump-optimized underfloor heating system makes little sense with gas condensing boilers because it is inefficient for gas, right? That’s a shame! We had considered the underfloor heating as an option to later switch to a heat pump. So that’s not easily possible?

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

...Or what special considerations need to be taken into account?

With heat pumps, every degree of avoidable flow temperature counts, whereas with gas condensing boilers, as long as it is below the condensation limit, it is rather less significant. Differentiated requirements arise from this alone. If you want to size the heating surfaces for current gas condensing boiler supply to meet future heat pump demand, appropriate precautions are necessary! The commissioned MEP planner will certainly be able to provide information on this! Best regards

Hi €uro,

I understand that you have to pay for planning tasks. We are not opposed to that at all! But I would just be interested beforehand whether it is possible or not, before I make a huge fuss. Or whether the general contractor is just claiming something that suits him better.
Is a heat pump-optimized underfloor heating system with a gas condensing boiler significantly more expensive (= several thousand €uro), or does it just need to be well thought out with a few technical additions?
Best regards, Mecc