Two circuits for cooling?

[joth]

On 10/08/2025 at 08:56, -rick- said:

 

Oh now I'm confused. Am I thinking of someone else?

 

I thought you had a heat gain problem in the bedrooms even during heating season so wanted the ability to cool the bedroom while the downstairs wanted heat. After I read that I looked back at some old threads and thought I read about how to do that you had to install a second zone with a big buffer tank to give the zone enough volume to avoid cycling. Have I misunderstood something?

Haha well remembered - yes that is a problem we have in our house, but not one I've tried to solve through simultaneous ASHP heating and cooling. (Although a w2w heatpump would indeed allow pumping excess heat from the FCUs upstairs back down to the UFH downstairs)

My solution is during the heating season, the ASHP is in heating mode only (for the UFH) and the bedrooms have additional fan bringing in cool air from outdoors as needed (essentially an MVHR mega-boost mode). This air intake fan shares duct work and electronic dampers with the FCU network.

 

[ectoplasmosis]

On 07/08/2025 at 23:25, JohnMo said:

So including DHW if 9kW is correct you need a 10kW Vaillant, but if 9kW is rounded up you could use a 7kW. Your flow temp shouldn't exceed 35, possibly lower and a 7kW puts out 8.8kW.

 

 

We need to stop quoting/using Vaillant datasheet numbers in relation to sizing; they are peak output figures which cannot be achieved over any length of time in UK winters with frequent defrosting. 
 

There’s a reason the units are named ‘7kW’, ‘10kW’; these are roughly the sustained outputs they’re capable of under prolonged defrost conditions (providing there’s enough water volume to facilitate the defrosts). 

 

[JohnMo]

4 minutes ago, ectoplasmosis said:

There’s a reason the units are named ‘7kW’, ‘10kW’; these are roughly the sustained outputs

What is your source for said information? So we can use that instead, and what do we use for other manufacturers data?

[ectoplasmosis]

2 minutes ago, JohnMo said:

What is your source for said information? So we can use that instead, and what do we use for other manufacturers data?

Lots of data here: 

 

https://community.openenergymonitor.org/t/vaillant-maximum-output-capacity-testing/27221

[JohnMo]

16 minutes ago, ectoplasmosis said:

Lots of data here: 

 

https://community.openenergymonitor.org/t/vaillant-maximum-output-capacity-testing/27221

At least that is some good testing to show where Vaillant get the name plate data from, which has always been unclear. 

 

So a general in the round assumption for heat pumps - not just Vaillant.

 

ASHP are tested to European standards. Europe (not UK) is less coastal than the UK, so has a lower humidity in winter than the UK. As a result, the ASHP has less need to complete defrosts. 

 

To compensate for the difference in humidity levels at or close to design point, an ASHP designed to be installed in the UK, should be over sized by 20% (above datasheet) at design point conditions, to allow a clear margin for defrosting. This is especially important in coastal and near coastal locations.