Which Heat Pump recommendation would you pick

mta999

Before you go spending 10 grand or whatever it is you might want to get Octopus back and go over what you plan and find out why they think it won't work - you never know they may be right!

NedS

Whilst a buffer tank can improve efficiency by reducing short cycling, it can also have a negative affect. because it provides hydraulic separation between the heat pump side and the heating system, the HP has to operate at a higher flow/return temperature compared to the CH side. The difference may only be a couple of degrees, but it is likely be enough to go from a top performing system to a meh installation (or worse). Worth discussing the options with your installer. Also read up on the downsides to buffer tanks before trusting anything ChatGPT has to say.

Another reason often given for installing a buffer tank is to ensure the system has sufficient volume to pump round if/when a heating zone is shut off - Wrong solution to the problem in my opinion. A small volumizer tank is a better solution if zoning is baked in to the system design.

Couldn't agree more. A buffer tank is normally used to mask (solve) other issues. It's an easy option for an installer at the cost, in terms of efficiency, of the home owner. The system will work, but running costs will be higher as a result of reduced efficiency from running at higher temps due to heat losses across the buffer.

Any installer who recommends a buffer tank (not a volumiser) on all installs regardless is a huge red flag. In this case there does at least appear to be a genuine reason for considering a buffer tank, and it may likely solve the issue at hand. Given the offending pipework cannot easily be replaced, solving it with an additional pump or radiator is likely a far better solution for the home owner (buffer tanks introduce their own set of problems if flow rates are not evenly matched across each side of the buffer due to hydraulic distortion effects).

swanny65

Whilst a buffer tank can improve efficiency by reducing short cycling, it can also have a negative affect. because it provides hydraulic separation between the heat pump side and the heating system, the HP has to operate at a higher flow/return temperature compared to the CH side. The difference may only be a couple of degrees, but it is likely be enough to go from a top performing system to a meh installation (or worse). Worth discussing the options with your installer. Also read up on the downsides to buffer tanks before trusting anything ChatGPT has to say.

Thanks again. Clear what I need do next and will speak with the installer next week.

swanny65

https://forums.moneysavingexpert.com/discussion/comment/82103775#Comment_82103775

Sadly @mta999 not worth it for two reasons.

1. The Octopus assessor admitted the floor probably didnt need ripping up. He called a supervisor to check if they could disregard the companies insistance on a 28mm supply from the heat pump to the hot water cylinder, but he was told it had to be 28mm on all their installations. I suppose this prevents any comeback on them if 22mm pipe, no matter how short a length that may be, is used.
2. The other reason, which I didn't go into much detail in the initial post, is they wont fix the UFH heating issue in the extension and its "cold" room.

a) Why are Octopus happy to replace and/or fit radiators, if I had them, but not touch UFH.

b) Secondly why would I get a separate installer to do this work, which would also be VAT charageable if Octopus they, before or after this work is done, fit a heat pump, and not have that installer fit the heat pump ? In my case this is not a small job as the UFH pipes need replacing all the way back to the 22mm upstairs supply - this is along a roof void, across a ceiling and down through a further room to where a new manifold, and return pump, would need installing.

Quite simple Octopus are cherry picking the easy stuff and leaving the difficult stuff with me to solve.

silverwhistle

I found this from NedS very useful:

@swanny65 do you have smart meter data for your gas usage on the coldest day in
January of this year, as this should give us an excellent indication of
your actual heat loss? Just look at your highest daily gas usage figure
in kWh (adjusted for any cooking usage) and divide it by 24 to get your
heat loss figure.

Unfortunately my gas usage figures for my 3 bed terrace are low 30's or under, apart from one inexplicable outlier at 70. January total was 386, and 4 days with no usage at all. By March I only used gas on 3 days of the month..

Does anyone have any idea how to account for having a wood burner?!

FreeBear

Does anyone have any idea how to account for having a wood burner?!

Wood has an energy density of around 4kWh per kg. Your typical wood burner is around 70-80% efficient. So a rough assumption would be 3kWh for every kg of wood burnt.

I'm also in the same boat in as much as having a wood stove to supplement the gas central heating. For me, I got through 660kWh of gas in January . Highest daily usage was 33kWh, and the lowest around 10kWh, yet these were not even the coldest days.

Comparing my annual usage (3200kWh) with neighbouring properties of the same age and approximate size, some are using around 8000kWh p.a. and others are hitting 10000kWh. Or if I look at historical usage before the stove was installed (along with various thermal upgrades) I would have been using 5-8000kWh of gas - But that was with an old Baxi back boiler, whereas I now have a much more efficient combi. Also benefit greatly from solar gain when the sun is out which also skews the figures.

If I ignore the stove, the heat meter suggests heat loss is coming in at around 2.5kW, and I need to pump in 4-5kW of heat to raise the temperature by 1°C per hour. Not too bad for a 1920s 3 bed semi.

Really need a few days of sustained subzero temperatures and resist the temptation to light the fire.

swanny65

@FreeBear @silverwhistle we also have a small multi fuel stove. We tend to use it occasionally during winter. When we open the doors to the cold room, in the extension I mentioned, the additional heat helps make that room a lot warmer. TBH its more for show when we have visitors than a useful/needed additional heat source.

silverwhistle

Apologies, I've only just noticed the last couple of helpful posts.

I'm hoping that going for a low capacity A2A would be fine for my location and house, and any cold snaps, or need to get the house warmer after an absence, easily dealt with by the addition of the stove's input. I've plenty of free scrounged and self-processed wood. In my case it would have slightly more utility than a nice to have for visitors, and as I go sea rowing all year round the radiant heat is wonderful for the muscles afterwards!

mk321123

swanny65, a gap like ~10.7 kW from the installer vs ~5.6 kW from your smart meter is normal, it doesn't mean your meter is wrong.

The installer figure is (or should be) steady-state loss at a design outdoor temp (often around -1 to -3 °C in much of England, lower in colder spots), with design room temps and standard ventilation assumptions. Your 5.6 kW is daily gas / 24, and that differs in three ways:

• the days were probably milder than design, so it understates design loss
• it includes DHW and cooking, so it overstates space heating
• it's gas in, not heat out, a boiler delivers roughly 85 to 90% of it

To compare like with like, take space-heating kWh for a cold day (gas minus a DHW estimate, times boiler efficiency), divide by 24, then scale by (indoor temp minus design outdoor) / (indoor temp minus that day's average outdoor). That gives you a meter-based design loss to put next to the MCS sheet. Standard-assumption calcs often come out above measured figures, but check it for your house rather than assuming.

Sizing then comes down to output at your design outdoor temp and your likely flow temp, not nameplate kW. It should cover the design loss with a bit of margin while still modulating low enough for mild weather. Oversizing mostly hurts through cycling, undersizing shows up on the few deep-cold days and during defrost.

Separately, a cold UFH extension is usually a hydraulics problem, not something a bigger heat pump fixes. That fits the Nibe installer's hydraulic-fix approach. An extra rad is a workaround if the loop can't be fixed, not proof the whole-house loss is higher.

I'd ask each installer for the room-by-room loss table and the flow temp they assumed for the UFH, then compare the cold room's watts to what that loop can emit at 35 to 40 °C. That one sheet usually settles both which heat pump and whether to fit a rad or fix the pipework.

For what it's worth I drew my own place room by room to get the same kind of table, if seeing a worked example helps: https://heatlosspro.cloud/s/YAzppFumRwF3TzRGRBaV0g