1920s/30s semi small retrofit - what should i be thinking about?

Air Source Heat Pumps (ASHPs)

Last Post by lbj 3 years ago

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lbj

(@lbj)

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Topic starter 11/09/2022 5:49 pm

Our 2.5 bed semi has a traditional gas boiler and more recent HW tank, fairly old radiators, reasonable loft insulation and fairly new double glazing. we have a full roof of solar and an iBoost immersion heater from that. I can't see that we can do much with wall insulation, and we've made some small improvements to air tightness (sheep thing in chimneys, sealant around various pipe inlets) but we do like fresh air so windows are often open a crack even in winter. Since the boiler won't last forever *crosses fingers* and we would like to cut back on gas for climate reasons, I've been looking at ASHP. My original attempt at this a year ago was delayed by family bereavement, and i'm now clearly hitting the rush season where 2/3 of the ASHP companies i contact don't respond. I have had a survey and quote from a local installer who was recommended. They are suggesting a Vaillant AroTherm 12kW and a unistore 150 litre HW tank, and new radiators in a handful of rooms. The heat survey came out at 9.6kW for 98m2, so 12kW feels kind of high, but I wonder if that's also a 'what stock is available' thing. I had hoped to have a couple of options but so far it seems like this is the only quote i'm getting.

What should I be thinking about / asking about? The surveyor made a couple comments about old pipework and whether it would stand a newly pressurised system, but then was super relaxed when he realised the ground floor is boards not solid (because any leaks would be accessible? which i guess is ok but also worrying). I want to check it will all be Vaillant controls/ancillaries (thanks to reading past threads here) and I guess i'll also ask about servicing costs.

One small worry is whether we have enough space in the consumer unit (we already have a car and bike charger). I'm not sure the surveyor really checked... and I'd like to understand better what happens in the summer - currently the gas CH goes off and the iboost covers the HW, but i understand i'd need to keep the ASHP running at a low level all the time?

I've also not been able to figure out whether the AroTherm needs to sit on top of a french drain or similar - the pictures often seems to show it on feet over a gravel bed. currently the logical site for it is on top of concrete driveway adjacent to the house wall, and i'm not super keen on excavating that as it feels like a precursor to structural problems. Any pointers?

Thanks for all the helpful info elsewhere on the forum! looking forward to figuring out whether this is a viable project for us, or not...

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Kev M

(@kev-m)

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13/09/2022 7:46 am

Hi and welcome to the forum. A few comments.

- Although 12kW is higher than your heat loss survey it's probably fine. The next Arotherm down may be too small and ASHPs can normally modulate down to about 40% of their maximum output. Also, many manufacturers overstate the badged output in real life conditions (I don't know if Vaillant do this). Remember also that when the ASHP is heating the water tank, it's not doing the heating. That 9.6kW should be a continuous output to heat your house at about -3C outside (the number depends where you live) and if you use a lot of HW it'll have a bit of catching up to do when it's very cold. Having said that, 9.6kW sounds a lot for 98m2, especially a semi. Is it solid walls? I would bump up the loft insulation to 300mm as it's cheap and easy.

- The pipework comment is a bit worrying and the flow rates will be a lot higher than a boiler. Is there any way to pressure test your heating system before the installation? (I don't know)

- 150 litres is quite a small HW tank. If you are big HW users the water heating will be on a lot. But a bigger tank takes up more space.

- Do get it set up with Vaillant controls rather than external thermostats - I understand they are good

- You'll need a qualified electrician to do the electrics - they will advise on the consumer unit. You also need to apply to/tell the electricity network operator you're installing an ASHP (at least we did). People do power down their ASHP in the summer. We don't as we need it for HW.

- ASHPs have a condensate drain pipe. It's just a little water that dribbles out. I think the recommendation for a drain is to avoid it being a hazard if it lands on a hard surface and freezes. My ASHP is on a concrete slab set on gravel!

It would be good to get another quote.

Good luck and feel free to ask more questions.

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lbj

(@lbj)

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Joined: 3 years ago

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Topic starter 16/09/2022 8:32 am

@kev-m Thanks so much! Really helpful.

I've confirmed it will all be Vaillant controls and the consumer unit will have a spare slot (given that the old CH circuit will be coming out).

Trying to get a second quote now - if anyone would respond to calls/emails! I guess the energy cost push has increased demand, even if the cost of living worries might be reducing it... Interesting times.

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