Pre-HP questions

Hello all,

I am starting to plan for getting a heat pump installed at my home, although due to the gas combi boiler so far working fine (touches wood) at the moment this is unlikely to be before 2026. I’m intending to make use of Nationwide’s 0% green additional borrowing scheme and the BUS (if both are still available) so although prices are likely to change in the meantime, I have tried to budget something out in case help stops being available. Nearer the time I will get some proper quotes, but I don’t want to waste the time of an installer so far ahead, so does this seem realistic to anyone who has already been through the process?

HP budget
Currently have a P+ rad in each bedroom and towel radiators in the bathroom and ensuite in one zone upstairs, then ufh downstairs split into one zone for the open plan living/kitchen/dining room and another zone for the hall, utility and downstairs loo. 118m^2 detached house built in 2017, so estimating 2.5-5kW loss (from Heat Geek estimate values). At the end of last year 4.5kW of solar was added to the roof, so hoping to divert excess to hot water, but not much room in the utility for a tank.

I also have a few questions that people who have already installed heat pumps might be able to answer for me…

  1. Generally a single ASHP should not require planning permission. Is this the case if you already have an air-to-air split aircon unit? I have been considering this but would probably avoid it if that would mean an ASHP would be the second unit and thus require planning permission.

  2. I already have a 7.4kW Zappi and a Sonnen house battery with a 3.3kW inverter. Am I likely to run into problems with the DNO if I want to add an ASHP? I believe that I currently have an 100A main fuse. I understand that I would be liable to pay if my connection needed to be updated to 3ph, and that this likely wouldn’t be cheap… (Especially if I did go ahead with aircon as above.)

  3. Is there a way to tell if my ufh downstairs is heat pump ready? I’ve found a lot about radiator sizing, but not much about pre-existing ufh. I’ve had a look under the stairs where the equipment is, and I think it’s currently running at 35degC.

Thanks in advance to anyone who can offer advice. :slight_smile:

Hi Libby,
Have you considered using a heat pump to heat your DHW? This could be more efficient than
using the electricity direct.
Do you have space for a small hot tank. The space required would be bigger than the Sunamp but would be much cheaper.
As your house is modern I wonder if you need K3 rads, even though big is best…it may be more than needed?
It is likely that your UFH can heat at low flow temps.
If you do not ‘request’ a 3 phase connection but ‘need’ one for renewable purposes your connections costs should be low or perhaps zero. This is a new directive from the power companies.

Best of luck

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The Zappi I believe and definitely the Eddi (if you are using cheap period overnight DHW heating) have load management built in so they will throttle back based on house load to avoid popping the 100A fuse.

My OpenEVSE charger, Eddi and ME3000 battery inverter will all back off so my ASHP can pull whatever it needs and at full tilt, together with possibly two dishwashers and two washing machines running during the Octopus Go cheap period, I’m still well within 100A.

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Hello @xlibbyx, Not sure about the planning

That sounds promising if it is indeed running at 35C, do you know the pipe spacing, pipe diameter and floor covering? Your installer should be able to assess as part of the heat loss calculation and system design, feel free to post details here if you have any figures on the calculation?

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I don’t have an calipers at home, but I think the diameter of the pipe going into the system is about 25mm.

This is the setup under the stairs for the ufh:

And this is what makes me think it’s running at 35degC:

I /think/ the rads upstairs were being supplied at 45degC last winter. I definitely turned it down when there was publicity about boilers not actually running at condensing temps, but I think the plumber might have turned it back up again when the boiler was serviced the other week. Anyway, I will see if it works out at 45degC this winter.

Where the boiler is currently there definitely isn’t space for a hot water tank, as it’s in a wall mounted kitchen unit above the washing machine. I think even to fit the Sunamp in would probably require removing the full height cupboard at the other end of the utility space. It’s 60cmx50cm and ideally I’d like to keep some storage space too.

Hopefully the rads won’t need replacing, but there isn’t a rad at all in the hallway and I’m not sure how to figure out if the towel rads are sufficient in the bathrooms, so budgeted for replacing the bedrooms in case.

Thank you very much for the info on 3ph connections. Hopefully it wouldn’t be needed, but good to know that it’s likely to be more affordable when trying to get away from gas!

That is a blending valve. It adds hot water to the UFH return water to meet the temperature set on the UFH flow.

35°C is quite hot, I run mine at about 28°C, though I do have a well insulated house. It generally depends on floor coverings; Tiles best, carpet worst so if you have carpet, 35°C is probably about right.

Ideally, you match the temperature of the UFH to the rate of loss of heat so it is largely running at the right temperature and you do not get a lot of overshooting.

Only way to know is to measure!

It’s laminate flooring, so not as good as tiles but not as bad as carpet as far as heat transmission goes. The house is quite new, so should be pretty well insulated. Certainly on the occasion that we get a nice sunny day the ufh gets warmed up a lot via the rays in through the patio doors, and although it doesn’t always mean that much, the EPC rating is B.

I guess I’ll need to wait until closer to when I’m ready to go ahead and therefore in a position to get a proper survey done.

Nice work doing some pre planning yourself. All sounds good, although if your getting a heatpump I would do without the Eddie. Using the ASHP to heat hot water is much more efficient than using the Eddie, in the summer you’ll get a COP of around 4 heating DHW while the Eddie will only get a COP of 1. Especially since you’ve got a home battey, you can charge the battey up with excess solar the use this to run the ASHP for DHW.

If possible try and use lager K2 radiators, they are much cheaper and often less intrusive than K3s. You can use https://heat to calculate the heatloss and radiator sizing required at different flow temperatures.

Of your already running UFH heating at 35C it will be perfect for a heatpump. I would remove any bending valves and it should be possible to remove any additional pumps, but this will require some calculation.

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Makes sense that the heat pump would be more efficient for DWH now that you’ve pointed it out. I’ll have to think about whether it’s better to add another battery module instead of the Eddi, as at the moment when it’s sunny I’m filling my current 7.5kWh of battery quite early and then exporting a fair bit at 4p/kWh while at work in the week. (Or I was before my inverter failed again, but that’s a whole other saga!) Maybe I can get around paying VAT on the extra battery capacity if it’s done with the heat pump install too…

Yes, I’ve got the Eddi and whilst it was worth it first summer (before ASHP), i’m not so sure now we have the heatpump as Glyn says… 3:1 and 4:1 COP during the summer.

Depending on your energy tariff, you might be better selling the excess back to the grid? Octopus Outgoing is 15p and their Flux is upto 30p per unit.

You mention 4p, so you’re probably on Go. Cheap overnight though, so swings and roundabouts.

I managed to average 14p per unit across the whole of Nov to end April on Go using the batteries, solar and EV.

Yep, K3 are ‘very’ expensive… good yes, but very pricey.

K2 are much better value if you can manage them.

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Lots of useful info there! Thanks for that.

I was on Go but when the fixed period ended at the start of August, I switched to Intelligent. Unfortunately due to the inverter issues I’ve only had access to solar power for about 60% of the time since the panels were installed last November, so it’s hard to tell if I’d be better off with another tariff, especially as since mid-April I’ve only had a functioning inverter during July. :cry:

It does appear that increasing my battery capacity would be a better way to spend than adding in the Eddi. Having a bit of a look at the info on VAT though, I’m not sure that I could get the battery increase signed off as being part of the ASHP install if the original installer came back to do that, and I’m loathe to do anything that would effect the warranty of the hybrid unit given all the issues I’ve already had, although hopefully that will all be in the distant past by 2026!

Oh, and for anyone who comes across this with similar questions about planning permission, this was useful: Air Source Heat Pump - Planning permission required! — MoneySavingExpert Forum

Tl;dr if you have a split air-con system then you’ve used your one permitted development ASHP, and certainly in Ipswich it seems getting planning permission for an ASHP could be a bit of a nightmare!

And also some other opinions here, which considers the difference between aircon units and ASHPs: Planning Permission for Air Conditioning Units - Page 1 - Homes, Gardens and DIY - PistonHeads UK

Plus: RevK®'s ramblings: Air conditioning at home (planning permission!)

In light of all your useful feedback (thank you!) about the hot water situation, and having looked a bit more into what’s available and the sizing of cylinders, I’ve changed my plans from a Sunamp with Eddi to a slimline cylinder and then ‘spent’ the difference on additional battery capacity.

It’s probably on the small side for the size of the house, but it’s usually just me and so I’d prefer efficiency anyway. Plus the reheat on the Valliant slimline seems to be to be quite quick and if you can use electricity out of the batteries to do a reheat then hopefully 150L is fine for when I have guests and also future proof enough. I’ll have to make do with gaining the boiler cupboard space to make up for losing the tall cabinet!



Note the rules are different in the different countries of the UK. Presumably this is English Planning :slight_smile:

Good point! I’m in Oxford so only been looking at English regs, yep.

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ASHPs are permitted development therefore usually don’t require planning. In England, they are permitted development if located 1m or more away from a boundary and the outdoor unit is 0.6m3 or less. A 5kW Arotherm is only 0.38m3 so as long as you’re 1m or more away from a boundary you should be fine.

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I was thinking it would go where the square is, as the boiler and breaker boards are on the external wall down the alley.

Ideally it would go where the question mark is, and then the hot water tank would go in the loft, but I think between more complicated pipe routing and access issues that’s probably a non-starter.

Is that not m³ (i.e. the volume of the unit)?

Yes. The 0.6m³ refers to the volume of the compressor in the outdoor unit.

I have seen a lot of these permitted development requirements listed on various websites - most differ from each other.

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