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Heat loss with gas usage.

Air Source Heat Pumps (ASHPs)

Last Post by TheHolyHeatPump 1 year ago

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TheHolyHeatPump

(@theholyheatpump)

Active Member Member

Joined: 1 year ago

Posts: 3

Topic starter 27/11/2024 4:13 pm

Hi all.

Great info on here. Thanks.

We all know how it goes once you start to researching new tech. It never stops!

Some house info.

306m²

9" solid walls.

Average yearly gas usage has been 32000kwh. Around £2k on gas.

I'm in the process of insulating the crawl space under the floor. Using ybs superquilt. There will be 116m² installed. (what fun fitting it)

6 bed, 4 bathrooms. 2 bathrooms are fed via the 120 litre DHW. The other two are electric showers.

Boiler is 16 years old. Worcester 40cdi. Primary pipe work is 28mm. It's then tead off to 22mm to radiators.

18 rads.

4 are bathroom. Towel radiators.

1 is an old solid type.

2 are type 21

11 are type 22.

New roof in 2013. Living spaces in the roof has multifoil insulation. Loft areas has over 300mm insulation. Dormer roofs insulated.

All windows replaced in 2013. Pilkington k glass argon filled.

I have a solaredge system

9.24KWp

15 panels south. 7 west.

6kw solaredge Homehub inverter

10kwh solaredge battery

DNO of 6kw.

100amp main fuse.

Phew. So. After reading about heat pumps we need to know the heat loss for - 3 to 21c. I decided to run the boiler at 54c for 24/7 as there was going to be a cold spell for a few days. We've had -1 over night for a day. I thought it would be a good idea to see what the old house got up to.

House stat set to 19.5c

Rads max temp was 48c and to my surprise the house held steady at 19.5c. I've noticed that the thermostat isn't very accurate. Probably a 1c out.

I don't think the gas boiler is very good at holding the temp at 19.5c as the temp drops to 19c then gets to 20.5c. I have little thermometers dotted around the house and the average house temp is 20.3c.

I've had a few ASHP quotes. Both suggested (guessing) that my heat loss would be around 18kwh to 21kwh.

I've read up and looked at the info about using your gas usage to calculate heat loss. So. 32000÷2900= 11kwh Heat pump.

20th November, for 24hr I used 223 kwh of gas.

If I say boiler is 8o% efficient, so 203kwh for heat.

203÷3.5(cop)= 58kwh needed for heat.

My average electric prices is 19.23 p/kwh. I'm on Agile.

Gas cost £14.57

A heat pump would be around £11.

I think it's no brainier to get a heat pump in the next year as my solar will help towards the running costs.

I'll get a heat loss survey done in the new year. Fingers crossed a 14kw ASHP should cover my needs.

I've posted a picture of my solar production for that day. I'm also surprised how much the sun makes a difference to the heat loss at around 12pm.

Thanks all.

Alan

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bontwoody

(@bontwoody)

Noble Member Contributor

Joined: 4 years ago

Posts: 960

28/11/2024 10:29 am

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.

House-2 bed partial stone bungalow, 5kW Samsung Gen 6 ASHP (Self install)
6.9 kWp of PV
5kWh DC coupled battery
Blog: https://thegreeningofrosecottage.weebly.com/
Heatpump Stats: http://heatpumpmonitor.org/system/view?id=60

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jamesw

(@jamesw)

Eminent Member Member

Joined: 2 years ago

Posts: 10

28/11/2024 12:26 pm

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.

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JamesPa

(@jamespa)

Illustrious Member Moderator

Joined: 3 years ago

Posts: 3774

28/11/2024 2:17 pm

Posted by: @jamesw

↑

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.

This post was modified 1 year ago by JamesPa

4kW peak of solar PV since 2011; EV and a 1930s house which has been partially renovated to improve its efficiency. 7kW Vaillant heat pump.

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TheHolyHeatPump

(@theholyheatpump)

Active Member Member

Joined: 1 year ago

Posts: 3

Topic starter 29/11/2024 12:19 am

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