@batpred Have a read of this if you haven’t read it already:

https://energy-stats.uk/heat-pump-cycling/

Heat Geeks told me that another factor to take into account is de-icing when it comes to heat pump size.

Posted by: @l8again

↑

Heat Geeks told me that another factor to take into account is de-icing when it comes to heat pump size.

 

That's true.  Some heat pumps quote capacity including defrost, others don't, installers need to take this into account.

UK is more or less worst case because it's damp and we generally design for just below zero.  So defrost occurs very close to the point where demand is highest.  In colder countries where the design oat is lower, defrost occurs when the demand is quite a bit less than max so doesn't really matter in terms of pump selection.

Posted by: @l8again

↑

@batpred Have a read of this if you haven’t read it already:

https://energy-stats.uk/heat-pump-cycling/

thanks, that was very useful reading. 

It seems grant pumps can modulate more than most.

I am still wondering whether a 5kw Vaillant would do, which would offset the slightly higher Vaillant cost, when considering the sensocomfort controls and a 7kw model. 

Posted by: @batpred

↑

I am still wondering whether a 5kw Vaillant would do, which would offset the slightly higher Vaillant cost, when considering the sensocomfort controls and a 7kw model. 

Based on what I remember of what you have said I think quite possibly yes. I took a similar risk with the 7kW and dont for a moment regret it.  If it had turned out wrong I would just have used some supplementary heating on the few days it matters (until the Gulf Stream collapses!). 

Do you by any chance have half hourly (or even daily) meter readings.  They would give a relatively high level of confidence than the crude calculation based on annual consumption.

Posted by: @jamespa

↑

Do you by any chance have half hourly (or even daily) meter readings.  They would give a relatively high level of confidence than the crude calculation based on annual consumption.

We are with Octopus Energy for gas as well so I can pull the 30 mins data. I am not sure how far back I can go, but I am attaching 2yrs. Curious about what it says.. The last year is with the current insulation..

Hopefully you can read this.  The various tabs in the xlsx show average power over 3, 6, 12, 24 hrs.  If you can get average temp or degree days for your location you can plot vs one of these which will give you more info.  The pictures are the plots from your data which appear in the spreadsheet.

The file 'Measured consumption - Copy' is my data for comparison.

Posted by: @jamespa

↑

Hopefully you can read this.  The various tabs in the xlsx show average power over 3, 6, 12, 24 hrs.  If you can get average temp or degree days for your location you can plot vs one of these which will give you more info.  The pictures are the plots from your data which appear in the spreadsheet.

Thanks, I think most of the 10kW peaks at the 3hr graph are since we have the boiler on setback overnight, which is followed by a surge in the early hours. We would not have such a low setback with an ASHP. But there are a few cold days where power used is above 7kW for more several 3 hour periods. 

From the below table of Vaillant heat pumps, if the external temperature is -3C, a 5kW boiler would deliver 6.8kw at a 35C flow. Tempting to get a simple 5kW, but perhaps we need a 7kW pump?

The first pre survey quote we can actually consider calculated a 7.1kW @ -2C and used a 45C Flow Temp at -2C 

Posted by: @batpred

↑

From the below table of Vaillant heat pumps, if the external temperature is -3C, a 5kW boiler would deliver 6.8kw at a 35C flow. Tempting to get a simple 5kW, but perhaps we need a 7kW pump?

You need to make an allowance for defrost, perhaps 10-20%.

That said I would also be tempted to go with the 5kW based on this data.  Before coming to a conclusion I would definitely plot 24 hr consumption vs degree days/average OAT and work out for myself what I would do on coldest days if it turns out to be undersized, and whether I am truly happy with that.  Otherwise I would go with 7kW which you anyway may not get a choice over because I think you might struggle to convince an installer.

The other things to consider before considering 5 and given its so marginal are

• How old is your gas boiler and what temp do you run it at (affects efficiency), whats the current heating pattern, is the house adequately warm?
• How can the measurements be reconciled with the survey, is it wrong fabric assumptions, ventilation loss or its impossible to reconcile?

@jamespa 

Thanks, that is a good point, the installers would be concerned about it. Plus there´s little for them to gain by taking the risk with 5kW. 

My main concern is whether a 7kW would cycle too much in warmer weather. But reading energy stats etc, it seems that the Vaillant algorithm manages to avoid it.

And of course, most importantly, it will be outlast the gulf-stream collapse.. 😀 

Of course that after 7 years, we should expect a growing argument about lifetime, etc... 

Posted by: @batpred

↑

Thanks, that is a good point, the installers would be concerned about it. Plus there´s little for them to gain by taking the risk with 5kW. 

My main concern is whether a 7kW would cycle too much in warmer weather. But reading energy stats etc, it seems that the Vaillant algorithm manages to avoid it.

True (first sentence)

Thats the reason to consider the 5kW (second sentence).

See my previous email for suggestions/questions if you wished to take the analysis further.  At this stage its up to you (or your installer!)

Posted by: @jamespa

↑

Posted by: @batpred

↑

From the below table of Vaillant heat pumps, if the external temperature is -3C, a 5kW boiler would deliver 6.8kw at a 35C flow. Tempting to get a simple 5kW, but perhaps we need a 7kW pump?

You need to make an allowance for defrost, perhaps 10-20%.

That said I would also be tempted to go with the 5kW based on this data.  Before coming to a conclusion I would definitely plot 24 hr consumption vs degree days/average OAT and work out for myself what I would do on coldest days if it turns out to be undersized, and whether I am truly happy with that.  Otherwise I would go with 7kW which you anyway may not get a choice over because I think you might struggle to convince an installer.

So our heat loss survey came up at 5.1kW on the coldest -2C day. The installer surveyor is still recommending the 7kW Arotherm. I had a look at https://energy-stats.uk/heat-pump-cycling/, the lower modulating would is seen as 2.2 for the 5kW vs 3.2 for the 7kW. Anyway, the 5kW could help bring the outlay below the budget ceiling...

The Grant could be cheaper as it is imported from countries where standards are not as strict as in our region?

@batpred As my installation survey was carried out under the HG assured warranty scheme, my installer didn’t get much say as to the size of the heat pump. What I can say is that with no radiator changes, the HG survey was looking at a 5kW Valliant with a SCOP of 3.7. I elected to go for some radiator changes based on running my boiler at 50C for past 2 years. These radiator changes increased the minimum SCOP to 4 based on a 7kW Valliant. My predicted heat loss is 5.9kW at -3C.

A month or so on my system has stabilised on a 0.2 heat curve with an internal temp of 19.7C. All radiators fully open in Weather Compensation mode manual with room temp mod ACTIVE. Yes, I do get HP cycling: 338 cycles with just over 200 hours of operation. Having the heat pump cutout temperature set at 14C seems to be working well. So far in May, the heat pump has consumed 50kWh of electricity to cover heating; hot water and cylinder disinfection. I don’t have heat pump monitoring but looking at a similar sized heat pump (SCOP4.5) in my location which does have live monitoring, my recorded performance is closely matching it - particularly in respect of flow temps; daily usage and costs.