ᐅ Connection of a new underfloor heating system to an existing gas boiler with a planned future switch to a heat pump

Hello everyone,

This is meant as a general question, as I currently cannot provide specific details about the installation. Of course, any heating technician on site could answer this for me, but my goal here in the forum is simply to gain knowledge about the topic and possibly be alerted to any potential pitfalls.

In a planned future renovation of a single-family house with a basement, ground floor, and first floor, underfloor heating is to be installed on the ground floor. (New screed or milling in is initially irrelevant/unclear.) The radiators in the basement and on the first floor will likely remain.

In the longer term, the gas boiler (not a condensing boiler, it dates back to 1989) is to be replaced by a heat pump. However, the heating system replacement is not planned as the first step – the gas boiler will remain for now. The reason for this is to keep the renovation period short initially.

So, the underfloor heating would have to be connected to the old boiler. Is this in principle possible, and what factors or requirements of the boiler would be relevant? As mentioned, I cannot provide specifics, but it is an old gas boiler. The hot water tank was replaced a few years ago, if that is relevant.

The other way around works, as I understand it (heat pump with both underfloor heating and radiators, where the radiators would then need to be low-temperature radiators, correct?).

I understand that the water in underfloor heating is much cooler than the water for radiators. How is this managed in a heating system? Naively, I imagine some kind of mixing valve that blends a portion of cooler water with the hot water before it flows into the underfloor heating manifold, or something similar?

If such a setup is possible and is implemented, then a heat pump would be introduced at a later stage. I imagine that this type of mixing valve would then no longer be necessary, but the radiators in the basement and on the first floor would have to be replaced (because, as I have read, they need to be low-temperature radiators). Is this correct so far?

Thanks for your opinions. I’m just trying to get an introduction to the topic, so please don’t be harsh if my questions cause eye rolls.

Best regards,
Tristan

Hi Tristan,

In short – yes, that’s possible. An old gas boiler can easily supply both radiators and underfloor heating at the same time, as long as you install a mixing circuit for the underfloor heating. This mixer lowers the high boiler temperature to about 30–40°C (86–104°F). The boiler itself doesn’t require any special technology for this; what matters is having enough space and a proper hydraulic connection.

Later on, when switching to a heat pump, the mixer is usually no longer needed, because all heating surfaces are intended to operate at low temperatures. Whether the old radiators are suitable for that depends on the supply temperatures your house currently needs in winter. Perhaps you happen to know that?

Good luck!

The underfloor heating is then supplied with a fixed-value control, which is designed for the higher flow temperature. Clear.

trissedp schrieb:

So radiators need to be the corresponding low-temperature radiators, right?

No, modern low-temperature radiators are not necessarily required for a heat pump.
The combination of heat pump, radiator, and room size or heating demand must be appropriate. If this is the case, old cast-iron radiators or other standard radiators can also work. A room-by-room heat load calculation is important to determine where radiators need to be replaced.

Great, thanks everyone, that helps me a lot!
It was just important for me to know that the HVAC/trades sector has the necessary materials/components in their range.
Since this is probably common practice in this form, I probably could have guessed that as well.

Thanks to you all!