Posted by: @ian-s 

Could always use the money saved on water cylinder to get additional battery storage to cover most of the heat pump usage especially during the winter months

That sounds like a much more cost effective use of funds than a replacement hot water tank. 

I have 6.56kW Solar PV, and 9.6kWh battery. Recently I have being doing some detailed modeling of how much of the electricity requirements of a heat pump I could meet with cheap rate overnight electricity and solar. At a SCOP of 3.8, I need about 2900kWh over winter to replace 11000kWh from gas. I've broken this down into daily gas usage and solar production for the period Oct 2024 to March 2025. The end result is that I could feed a heat pump with 600kWh directly at the overnight rate and 1200kWh from solar leaving 1100kWh at standard electricity rates. This assumes that my battery is used to provide all other day-to-day background electricity use. 

If I added a further 9.6kWh battery, then the 1100kWh at standard electricity prices would reduce to just 200kWh saving roughly (0.26 - 0.09) x 900 = £150 per year.

That may work as a rough estimate or at least ballpark figure for the savings that you could make.

If you go down that route, I'd suggest waiting one winter to see how your electricity use works out with a heat pump and your existing battery. That way you will be able to collect figures for how much electricity you used at standard rates and hence get a much more accurate picture of what any savings would be if you added another battery.

Posted by: @ian-s

The bigger item think is do we upgrade our 15 year old 180l slim water cylinder for the Vaillant uniSTOR PURE 175l Slim, 20% increase in efficiency but an extra £1650.00.

I would suggest working out a reasonable estimate of the savings that you might make based on your own hot water consumption, since that could differ significantly from what someone else may be using. This can be done simply from your gas usage in a summer month when there is no heating, but gas is used for hot water and potentially cooking, which is usually negligible so you can just ignore that.

Let's say you used 200 units of gas in July and you were there all month, no holidays. That would equate to about 100 kWh input electricity for a heat pump, since gas boilers are about 0.85 efficient on heating hot water and a heat pump is about 1.7 efficient. So over the year, that's roughly 1200 kWh. Now at standard electricity prices, say 25p as a quick estimate, that's £300 for hot water heating per year using a heat pump. A 20% saving is then £60 per year. In that case, an extra £1650 for a new hot water tank has a payback period of 27 1/2 years. However, that is assuming very pessimistic figures for electricity costs. Most likely you will move onto a time-of-use tariff, heat your hot water at night, and pay 10p per unit or less. Then the payback period is over 68 years...

If you have really high hot water usage, then the economics might work out for you, but for most people they simply don't.

Even if the numbers don't add up, you may still want to replace the hot water tank for other reasons, and that's fair enough. Either way, it's always good to understand the financial position.

Any advice from people who did and didn’t change their hot water cylinder.

I upgraded mine but only because it was vented. 

Had it been unvented I would not have bothered, the payback time is too long for the very small amount of DHW we use, particularly as we heat at night on electricity costing 8p/kWh, so less than 4p/kWh delivered.

Any advice from people who did and didn’t change their hot water cylinder.

Cheers

I wouldn't be too concerned about the design flow temperature. As that's only the flow temperature when it's very cold outside, for example -3C. The rest of the time, weather compensation lowers the flow temperature and hence radiator power output with the aim of matching the heat loss that occurs with a higher outdoor temperature to the amount of heat going into the house. 

In short, you'll see the design flow temperature about 1% of the time, the other 99% it'll be lower and the heat pump more efficient.

Knocking 5C off the design flow temperature will likely cost you a lot of radiator upgrades (possibly all of them) to save less than £50 a year. So needs a careful look at whether it's cost effective and what the payback period is. (I'd be surprised if it makes economic sense).

In a sense a performance guarantee rather than a guarantee of particular parameters is what a customer needs. 

If I were otherwise happy with the proposal I would personally accept this as sufficient.  We don't ask what the temperature of the engine in a (fossil) car gets to, we ask how many mpg, what's the acceleration and what's the top speed.  The focus on performance is a sign of a maturing industry and thus to be welcomed.

Thanks for all of your comments, I have been asking HG some additional questions especially around the heat pump size and the flow temperature they have advised. There response is below, it would appear they are confident 5kW would meet our house needs also get the impression they don’t like being questioned about their advice. They also do not appear to design systems that will achieve a 45° flow temperature so if we went with them it would be 49° to 50°. We tend to run our house around 19° upping to 20° on colder days only going higher very occasionally when elderly parents stay.

Heat Geek Response:

RE: the missing 3D room renderings - it's a glitch that our software engineers are working on to fix

RE: flow temperature - unfortunately we don't have a design configuration where we could get the flow temperature down to 45 C, even with 5+ radiator upgrades

In regards to offering a 5kW plus as an option, we stand by our Heat Geek comfort and efficiency guarantee and if the heat pump is found to be the reason the system is not performing up to the agreed upon standard we will replace it and cover the labour costs at no cost to you.

Cheers

Ian

I would encourage you to have a conversation with HG at this stage rather than stop at this point. You might still call a halt later, but you would have a more complete picture. Getting extra information before coming to a final decision is nearly always a good thing.

I completely agree with @jamespa re the legislation, which I have also read carefully, as I have two A2A units that can serve a similar purpose to your electric radiators. The key point is that the heat pump has to meet the overall heat demand of the property. With that in mind, given your quotes, I'd be looking at the 7kW option. For the Aerotherm plus, the 7kW unit has a better SCOP above 40C flow temperatures than the 5kW, which makes a good case for it given the negligible extra cost. (Not sure if this applies to the pro, you'd need to dig a bit deeper).

There are often some nuances, for example the point I made about ACH values. It can take time for the software that different installers use to be updated inline with changed guidance. If like us you treat the whole house as open plan, doors open to all rooms, then modeling 21C everywhere is sensible. It is also a conservative assumption, as is using the larger ACH values even if it is unnecessary with a well sealed house. If it doesn't change the selection of heat pump, then such overestimates don't really matter, and will simply mean that the achieved efficiency is slightly higher than expected, which no one will have a problem with.

I suggest that you raise your concerns with HG. If you felt it necessary, you could ask them to put in writing why they do not require radiators in the hall and utility.

It's important to understand the basic legal position. The BUS grant is not paid to you as the home owner, it is paid to the MCS certified installer and comes back from them if there is a compliance issue.

The Ofgem guidance is useful in this respect, see https://www.ofgem.gov.uk/sites/default/files/2026-07/boiler_upgrade_scheme_guidance_for_installers_v5.1_20260630135616-20260701135531.pdf

5.11 Where we identify non-compliance with the BUS regulations, Ofgem may take
compliance actions against the installer, including withholding or recouping payments, or
revoking vouchers, and we will inform both you and the installer of this.

It looks like our heat pump journey is coming to an end. After speaking with Dwellow it looks like we have an issue with our new hall and utility, these were created by filling in the space between the house and the garage. When we did this we fitted 2 electric rads as getting central heating pipe work to the rooms was pretty much impossible due to no pipe work near that side of the house. Dwellow said we have to have rads that run from the heat pump heating circuit to qualify for the BUS grant and could surface mount pipe work to get to the rooms and in the rooms, that is a total no from us.

Others would have a different interpretation of the BUS rules to this.  There are examples on this forum of alternative interpretations, also read on.

Bit confused if we went with Heat Geek and there was a subsequent inspection and it failed we could have had to pay the BUS grant back which really makes transitioning at this time a financial no no.

Still appears there is differing advice out there, are companies ignoring some of the rules and regulations or interpretating them differently?

Definitely the latter.  The actual relevant requirements, directly quoted from the legislation, are 

(b)it is capable of meeting the full space heating and hot water heating demands of that property, and

(c)it replaces the heat generating components of the original heating system installed in that property (where applicable), other than any—

(i)supplementary electric heater, including any immersion heater,

(ii)circulation pump, or

(iii)solar thermal collector.

In your case the heating requirement for the hall and utility could, I would imagine, reasonably be met by transfer of heat from adjacent rooms.  Neither is a habitable room so if its a bit chillier who cares?  Thus as long as the rads in the adjacent rooms are oversized a bit and the total capacity is OK the BUS requirements are met by any reasonable interpretation.  Thats how Heat Geek are looking at it.

My bathroom and downstairs toilet are heated by the same means (downstairs toilet no heating but hall is adjacent and rad oversized, bathroom wholly inadequate landing but hall is adjacent and rad oversized).  This is quite common. 

There have been similar arguments in relation to low loss houses where its pretty obvious that just heating the downstairs, or mostly only the downstairs, is going to work.  The legislation does not specify a radiator in every room, it specifies that it must be capable of meeting the full space heating requirements without prescribing how.  It is also, at least arguably, a requirement placed on the Heat pump not the system (the section is specifically headed 'requirements for heat pumps' and clearly differentiates the heat pump from delivery of the heat).

Finally is it really likely that the intention was to rule out properties with one or two non habitable impossible to get to rooms if they could be heated to a reasonable extent by spill over from adjacent rooms?

I was half way to suggesting 'just go with Heat Geek' based on the quotes you posted.  I think, with a bit of interrogation of Heat Geek (who have a very good reputation), the Dwellow response tilts further towards that. 

Of course the above is not a guarantee, but I would suggest that the risk you have to refund the BUS grant provided that at least some heat transfers from adjacent rooms and the capacity overall is sufficient is negligible (and its probably negligible even if those conditions aren't met).