The simple fact of the matter is that whatever insulation is used, the manufacturer will provide detailed installation instructions which in the case of Primary Pro includes lots of ‘Bond and Seal’. Fitted properly, Primary Pro gets very good reviews but I accept that from an installer’s point-of-view it is a time-consuming and messy installation process.

I think some people value the lower disruption service and they're giving you the option to have it or not. I don't think either way would be a bad decision. 

It seems you did very well.

With our HG heat loss, we did not have anything proposed close to a SCOP of 4. Our heat loss is also outside the bands calculated based on 30 mins gas usage. 

Thanks, that's very useful to know. I believe us using -2C is due to being in the SE. 

We are still waiting for HG to come back to us on the specifics, hence we are self evaluating the pump modulation bands. 

Vaillant is our preference, but the pricing is hitting the tight budgetary constraint! And we will not postpone the rad upgrades.. 

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.

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.

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?

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!)

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... 

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?