Buffer tank and secondary pumps. Do I need them?

[joth]

On 25/12/2025 at 17:56, JohnMo said:

How do you do your schedule?

It's controlled by Loxone with load and weather compensation, but during periods of very cheap energy i push right down the turn on threshold, bump up target temperature very slightly, and boost the curve based on the difference between the floor slab temperature and target temperature 

Means i can squeeze out a day's heating demand in about 4 hours operation. The DHW schedule is a bit more hard coded as I want that to happen last thing during the cheap rate block

 

[joth]

5 hours ago, jimseng said:

I don't have to run the UHF at 35, I can set any level and I intend to collect lots of data and learn how to get the most out all my energy demands and sources

To get best energy efficiency you'll want to use weather compensation, so you set a curve of flow temperature for given outdoor temperature. Perhaps 35 or more on vary coldest days tapering down to around 20 when the outdoor temperature approaches 20.

The only downside of running a very low flow temperature is risk of the heat pump short cycling, which reduces its efficiency greatly, but a large ufh area makes this very unlikely so long as most looops remain open. 

[JohnMo]

42 minutes ago, joth said:

squeeze out a day's heating demand in about 4 hours operation.

So is your heat pump oversized to allow this?

[joth]

On 26/12/2025 at 20:55, JohnMo said:

So is your heat pump oversized to allow this?

No, it was oversized to facilitate fast DHW reheat times, and because the MCS installer was skeptical our enerphit would succeed. PHP said 4kW demand but we installed 8kW.

This supporting the fast cheap rate heating cycle is a happy coincidence, as in 2019 I really wasn't specifying this based on the existence of such deals. 

 

[JohnMo]

7 hours ago, joth said:

No, it was oversized to facilitate fast DHW reheat times, and because the MCS installer was skeptical our enerphit would succeed. PHP said 4kW demand but we installed 8kW.

This supporting the fast cheap rate heating cycle is a happy coincidence, as in 2019 I really wasn't specifying this based on the existence of such deals. 

 

I'm in the process, of really considering replacing my 6kW ASHP, that doesn't modulate well with a 4kW one, that does modulate well. 

 

Will then either repurpose the 6kW ASHP, as a hot tub heater (via a plate exchanger), or sell it on to someone with a suitable heat loss.