@duncan-mac

if your max heat loss is 8kW and you have a 7kW ASHP, what happens when it's really cold?  Can the ASHP keep up?

Kev M

I have been very happy with both the Hot Water temperature and the heat in the house. I have the Passive Living monitoring fitted and the Internal temperatures I quoted come straight from that monitoring kit.

Posted by: @kev-m

@duncan-mac

if your max heat loss is 8kW and you have a 7kW ASHP, what happens when it's really cold?  Can the ASHP keep up?

Forgive me if I’m being dumb, but I thought a 7kW ASHP would draw a maximum of 7kW, not deliver. If that’s correct, even when really cold and only delivering a COP of 2, that’d mean the ASHP being able to chuck out about 14kW in heat energy.

Or have I got my understanding completely skewed?

Posted by: @majordennisbloodnok

Posted by: @kev-m

@duncan-mac

if your max heat loss is 8kW and you have a 7kW ASHP, what happens when it's really cold?  Can the ASHP keep up?

Forgive me if I’m being dumb, but I thought a 7kW ASHP would draw a maximum of 7kW, not deliver. If that’s correct, even when really cold and only delivering a COP of 2, that’d mean the ASHP being able to chuck out about 14kW in heat energy.

Or have I got my understanding completely skewed?

Yes 😉 

The rated kW figure is delivered energy under nominal conditions.  Most ASHPs will be able to deliver a bit more and in extreme conditions may not be able to deliver as much as the the rated amount.  I don't think the way manufacturers do the ratings is very consistent though. 

If you think about it, domestic ASHPs go up to 16kW or so and you couldn't draw 16kW on a normal domestic supply. It would also cost rather a lot to run. 

@kev-m , thanks. That makes sense. Good to be corrected.

Posted by: @majordennisbloodnok

Posted by: @kev-m

@duncan-mac

if your max heat loss is 8kW and you have a 7kW ASHP, what happens when it's really cold?  Can the ASHP keep up?

Forgive me if I’m being dumb, but I thought a 7kW ASHP would draw a maximum of 7kW, not deliver. If that’s correct, even when really cold and only delivering a COP of 2, that’d mean the ASHP being able to chuck out about 14kW in heat energy.

Or have I got my understanding completely skewed?

I am no Heating engineer but I think there are a number of things to consider here

1) the MCS Heat Loss calculation is for the entire house. I have zoned my living space and Bedrooms seperately so my maximum heat demand is likely to be well less. Living during the day - bedrooms through the night with very little overlap.

2) Vailliant give output on range of performance for my unit based on working Temp and flow temperature.

at -5degrees and a flow temp of 55 degrees output is 7kW

at 2 degrees and a flow temp of 35 degrees output is 10kW

certainly we are cosey and have plenty of Hot Water.

@duncan-mac you are correct. Each unit has its own curve. You can see some examples in the sheet in my signature. You can select a unit and then it'll show you output at a range of temperatures. You should have a suitable unit for your location and that looks at the maximum lows. 

Posted by: @batalto

@duncan-mac you are correct. Each unit has its own curve. You can see some examples in the sheet in my signature. You can select a unit and then it'll show you output at a range of temperatures. You should have a suitable unit for your location and that looks at the maximum lows. 

Thanks for the info I will research further. From the information I am getting my system has never drawn more than 4kW at any time since the install.

I have confidence that between Vaillant and my Heating Engineer supplier I have a system that works but it is worth doing a significant amount of research so that you can ask the correct questions and ensure that your not just be talking to a salesman.

I have seen (Heat Geek on You Tube) that its not a good idea to oversize your Heat Pump as this can lead to Short Cycling which in turn will lead to a drop in efficiency. He also recommends that we keep out zones to a minimum.

Posted by: @duncan-mac

Posted by: @batalto

@duncan-mac you are correct. Each unit has its own curve. You can see some examples in the sheet in my signature. You can select a unit and then it'll show you output at a range of temperatures. You should have a suitable unit for your location and that looks at the maximum lows. 

Thanks for the info I will research further. From the information I am getting my system has never drawn more than 4kW at any time since the install.

I have confidence that between Vaillant and my Heating Engineer supplier I have a system that works but it is worth doing a significant amount of research so that you can ask the correct questions and ensure that your not just be talking to a salesman.

I have seen (Heat Geek on You Tube) that its not a good idea to oversize your Heat Pump as this can lead to Short Cycling which in turn will lead to a drop in efficiency. He also recommends that we keep out zones to a minimum.

 

Hi Duncan,

From the information that you supplied it would appear that your system is adequately sized for most eventualities, though like most heat pumps in the UK, it would probably struggle if we had a prolonged period with an outdoor temperature of -15C.

Do you know if your system is operating in weather compensation mode? If not, I would suggest that you consider changing your system to do so, which should improve operation and overall efficiency.

@derek-m thanks for that.

I'm pretty sure it is in "weather compensation mode "as it cuts out when outside temperature goes above 21degrees and its has a heat curve set, although it has is Adaptive Heat Curve is "deactivated" because, so I was told, Ofgem don't like it cause it can affect my RHI, but I'll ask my installer just to make sure

thanks

Posted by: @duncan-mac

@derek-m thanks for that.

I'm pretty sure it is in "weather compensation mode "as it cuts out when outside temperature goes above 21degrees and its has a heat curve set, although it has is Adaptive Heat Curve is "deactivated" because, so I was told, Ofgem don't like it cause it can affect my RHI, but I'll ask my installer just to make sure

 

thanks

Do you have thermostats and/or TRV's installed and are they switching your heat pump on and off? If that is the case then your system may not be operating in weather compensation mode. Also Auto Adaptive control is a combination of room temperature and weather compensation control, which will possibly provide the most accurate and efficient control of the indoor temperature.

Do you know what the water flow temperature is, and does it vary with changes in outdoor air temperature?

What was the outdoor air temperature when your ASHP was drawing 4kW from the power supply?