Its a big house Alan so getting an accurate heat loss will be important. If I were you I would try and go lower than 54C with your boiler during a cold snap and see how you cope. For efficiency (and cost) I would see if you can get to around 45C, obviously the lower the better.

Your battery and PV will help a lot and if you get an EV, Intelligent Go is a great tariff. I averaged 10p per unit this month.

Hi Alan,

I'm in a similar position to you, assessing gas consumption (in a large and old but well-insulated house) to aid in correctly sizing a heat pump. One thing I am finding useful is to use half-hourly gas usage data on the coldest days to see what my maximum heating demand is, rather than relying on an average across a whole day (or longer). If you have a smart meter to give you access to such data, I think it's a worthwhile exercise.

The challenge is then to reduce peak demand to a minimum by tweaking the heating settings - I already have weather compensation for my gas boiler, so the system is already essentially as it will be when I have a heat pump except for the different heat source. This gives more variables to play with, especially heat curves and set-back temperatures.

Also take care over the COP/output you are assuming that the heat pump can manage on the coldest days (particularly at high flow temperatures such as 50C or 55C). For instance, I'm looking at a nominally 12kW Vaillant arotherm+. According to the product specifications it can actually deliver 14.9kW for an outside temperature of +2C and a 45C flow temperature.  But it can only manage 11.2kW output at -2C for a 55C flow temperature, so just when you need it most, it delivers the least.

Posted by: @jamesw

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According to the product specifications it can actually deliver 14.9kW for an outside temperature of +2C and a 45C flow temperature.  But it can only manage 11.2kW output at -2C for a 55C flow temperature, so just when you need it most, it delivers the least.

In general terms most heat pumps have this kind of characteristic unless output is capped in the firmware to flatten the curve artificially.

Do you need to design for 55?  It's OK but lower would be better.  May I ask what is stopping you designing for a lower FT?

If using gas data from only a few days I would suggest to look at the max 3hrly averages (or even 6hrly) not the max half hourly data. Heat pumps are designed to be operated low and slow not peaky like boilers are often used.  

Evening all. 

Thanks for the replies. Great info. 

Looks like my gas average hourly usage is 12kw running the boiler 24/7

Looking back when the system was running the old on and off way the peak usage was 17kwh.crazy!

I think a viessmann 16kw ASHP can output 13.5kwh at - 3 (I think) 

I'll get a local heat geek elite to do a heat loss survey once we're ready to move forward with a ASHP. Would be great if I can get a flow temp of 45c. I don't see a reason as to why I can't. I can replace the two lounge rads with two 2400x500 type 33 if need be. I'll get the experts in next year to see what's best. 

I don't think my old boiler is very good at regulating temp as the rad temps are jumping between 40c and 48c. House comfort has been surprisingly good though. 

The house is a big old girl. 100 years young this year. House was empty for a few years before we bought it. That first winter here I'll never forget! 

I've plastered over 90% of the walls and ceilings. Artex everywhere. Yes. I'm mad. The list goes on and on... Nearly finished after 11 years. Last big job is to get a new kitchen next year but a new ASHP would be the next sensible step. 

Thanks 

Alan