Earth Notes: Combi Boiler Replacement Dilemma

Updated 2024-07-07.
Quick, green and legal, avoiding a distress heating system replacement when the cold hits! #DHW
Our old non-condensing gas combi is struggling. Under normal circumstances I would repair it again, or upgrade to a heat-pump. But as our house is due to be bought from under us and bulldozed in a few years, the normal things to do may not make sense in terms of money or upfront carbon emissions. Also, a heat battery was just added to the system just a few months ago, skewing the choices somewhat. I work through the dilemma/multilemma!

2022-09-09: Combi Struggling

(See some backstory.)

For a while now the pressure reported by the combi has been swinging between 0 and 3 bar. When it gets low I gently top it up, but I suspect a failed expansion vessel.

(Combi is Potterton Performa 24 GCNo 47 393 06.)

I'm asking for advice on possibilities (somehow fix, replace with condensing combi, replace with electric combi, somehow install ASHP) on Twitter and BuildHub:

Our old gas combi may be on its way out. Our house is due to be bulldozed in 5Y, so ASHP upgrade extra punitive. We are already a low user (<3MWh DHW and space heat per year). Winter heat demand typically 3kW, peak ~6kW. Already have a heat battery installed (Thermino).

Other gas appliances long since gone. Electricity is single-phase. What would the learned mob here advise: a condensing gas combi, an electric combi, something else?

Obvs, I want to minimise carbon, including embedded/upfront carbon. I'm prepared to be a bit spendy to do that.

No Panic

The Thermino can provide DHW even if the combi fails. We did not put the heating on until December last year. So thinking about this in September may avoid a hurried, expensive and unhappy distress replacement of the combi!


The main choices, each with complications and caveats, seem to be:

The number in brackets is the number of votes for each option in effect (it's complicated) on the Twitter and BuildHub threads as at . No spontaneous support for my (high-OpEx) idea of a ~8kW electric combi, lots of suggestions for variants of air-to-air (A2A).

One reason I am reluctant to invest the money and carbon in an ASHP is that having it all destroyed in a few years seems like the wrong thing. Several people on Twitter and BuildHub have suggested that I could take a heat-pump with me (or presumably it could be sold on). That was unexpected!

Note that the first option, repairing the existing boiler, may include adding an external expansion vessel. We are very short of space, and such a thing may well be long on ugliness, but it might turn out to be the least worst thing to do.

A good suggestion from Andy is to consider second-hand, eg boilers or even HPs I suppose! Thank you.

Questions / sub-options

2022-09-21: Assessment

Via The Heating Hub someone is due round tomorrow to assess. They are able to repair, or replace with a condensing combi, or with an ASHP, as appropriate.

2022-09-22: Probably Fixed

Today I had the visit from Tom of Tom Plumb London Limited, and he made the expansion vessel work again.

We also talked through the options if his fix does not hold, including new combi (and routing of condensate pipe around the kitchen), and AHSP.

We discussed for an ASHP were an external monobloc unit could go (in the back garden), routing the insulated pair of hot water pipes back to the current boiler cupboard. Also a buffer tank big enough to support the ASHP's reverse/defrost cycle. Also if we could get a big enough DHW tank in the existing boiler cupboard, or just let the Thermino do all the DHW.

Anyway, with luck the Potterton will Performa for another few years until the house comes down. The dilemma is done for now! A little over £100 all in.

I asked Tom to book in a full annual service of the combi early November. By that time we should have a decent idea if the expansion vessel will last or need replacing when he visits.

And to test things out we needed to run the heating for a short while. Felt dirty this early in autumn!

2022-09-23: Possibly Confused

At home just now I heard the boiler fire up briefly and is warm, and the circulation pump is running, even though:

  • No one is running hot water.
  • The boiler is in hot-water-only mode.
  • There is no call for heat from the thermostat or boiler controller.
  • The heating clock is (now) on never-on (0) mode.

I can only stop the pump by turning the boiler off entirely (0). I even tried the 'reset' position briefly to no effect.

I think that maybe a wire or two got jogged during yesterday's works, and the boiler now thinks that it is in 'both' (space and DHW) mode, and that it also has a demand for space heat.

For now I have turned the boiler off and I am putting my heat-battery system into the alternate mode to supply all hot water. Any bugs may sabotage my hot bath tomorrow, so I'm checking closely!

2022-10-04: Fixed

The combi was fixed this afternoon. A thorough service is pending though.

2022-11-17: Serviced

A thorough boiler service today, including (second-hand, refurbished) heat exchanger (HX), new diverter valve, and a PRV (pressure-relief valve) fix.

On the old HX, black gunge could be seen on one side, and scale on the other.

The old HX will go off to be cleaned and refurbished and reused.

~1.4kWh of gas was burnt during servicing and testing including DHW and radiators/heating. If this Performa does pack in before 16WW is bulldozed, a second-hand condensing boiler is likely to be available for ~£200.

I really need to have a copy of the heat-battery plumbing schematic, including isolation valves to help service and test the combi, filed next to the combi.

£288 all in. Less than I had feared it might be!

2023-12-13: Failed Again

The boiler dripped and then failed again, with the circulation pump running continuously, and no response to a call for hot water even with the Thermino only tepid.

So I have put the Thermino system back on emergency 'primary DHW' mode, so we should have hot water, but no heating. And I have emailed Tom Plumb!

2023-12-18: Octopus Heat-pump Offer

I requested an on-line Octopus heat-pump quote (valid for 30 days) for our address; just short of £2,700 including VAT after the BUS grant, but we would need to get an EPC (£60) and maybe planning permission (~£270). Having spoken to the helpline Octopus cannot do heat only, but can integrate with a solar DHW system. I have put down a £500 deposit to probably get a survey/install in spring, but we can cancel up to right before install and get all our money back. This would work out cheaper than the the extra electricity to run a simple electric boiler for ~5 years, which is how long we may have before demolition. No gas used for a week!

(Other installers tell me that they cannot get near this price: Octopus is likely to be hugely subsidising it on top of the Boiler Update Scheme grant.)

Two days later I took a call in the afternoon from Octopus checking eligibility: one question was about how much microbore pipework we have, another about where we would site the external and internal units and the DHW tank.

My head was full of cold/flu/covid, so my memory may be bad, but I think Octopus wanted 25mm copper main pipes (I think our non-microbore radiator pipes are 15mm), and gave the system internal unit dimensions as 1m by 1m by 2m, which would be tricky with the Thermino! That might include an integrated DHW tank though.

2024-02-05: provisional survey date booked

I was called and now have a provisional date for the Octopus heat-pump survey, in April (with a window of a week).

2024-02-10: new EPC

20240210 EPC A chart

In order to allow a BUS grant, an in-date EPC is required, and it must not be suggesting any easy insulation works. Ours expired in 2019, so I booked in a new one, with the assessor here earlier in the week. Not everything done to improve this house can be captured by rdSAP.

The renewed 16WW EPC certificate came in from Ecoalex and it is an A (was previously B), which is ~0.3% of existing England and Wales homes.

2023-12-21: Fixed, Momentarily, then Fully?

The boiler dripping was a fault on the flow part of the diverter valve, ably fixed on the spot after diagnosis by Martin Elsby, thank you. £120 well spent.

However, it seems that the main PCB is probably getting flaky. Everything restarted while Martin had Baxi on the phone to try to diagnose the fault given some strange behaviour (no lockout when expected). So I expect to replace that in the new year at significant expense...

I may leave the Thermino system on emergency 'primary DHW' mode, given how low carbon the grid is at the moment, and given the potentially still-flaky state of the boiler.

2023-01-02: board-swap estimate after visit: £330.

Martin made an interesting point about electric boilers: it is difficult to find one person with the right qualifications and interest for electricity and wet heating, eg for maintenance!

Board swap started a little before 11am and was done in under half an hour, and we have heating again.

2024-01-03: Martin observed that on the old board a resistor was discoloured and seemed to have a dry joint: maybe that was the failure locus.

All done for £450 from diagnosis to today's fix. More than our annual gas bill!

I am taking the Thermino off 'primary' DHW duty.

2024-01-11: Low Temperature Experiment

It is ~6°C outside and is due to be similar over the next few days. I have turned the boiler flow temperature down to minimum (~35°C) to see if the house is warm enough, for a weather compensation data point. We barely need heat at ~12°C externally, so that is another data point.

The boiler seems to be running in bursts of 1 minute or shorter, maybe ~20% duty cycle, with two radiators ~fully open (out of six) and most of the rest slightly open.

As of ~21:30Z and nominal end of the heating period, I think it has worked well.

2024-01-13: up a bit

TRVs 1g and 3l were not turning off, though the rooms were not cold, so the ~35°C flow temperature may have been marginal for those rooms at the ~5°C outside at .

20240113 16WWvpc
20240113 16WWmultisensortempL

Temperatures are due to drop significantly over the next few days (to below freezing overnight), so I have put the flow temperature back up to a more typical ~45°C. At this level 1g and 3l are usually turning off before the end of each heating period on .

2024-01-16: winter design temperature

The weather forecast claims -3°C outside, my porch sensor says below -1°C. Thus near enough London winter design temperature of -2°C.

Although slightly tricky to tell given all the comings and goings before the heating goes off at ~9am (on at ~07:30), the 45°C flow temperature seems to be sufficient: rooms are coming up to temperature and TRVs are turning down.

2024-01-18: winter design temperature or lower

The weather forecast claims -4°C outside, my porch sensor says near -2°C. Thus near enough London winter design temperature of -2°C again. (The hallway Radbot on frost protection ~6°C came on this morning...)

The living room is struggling a little to get up to temperature (with the RadFan off), so I have opened up the lockshield a little.

2024-01-19: winter design temperature or lower, again

Close enough to London winter outside design temperature (-2°C) again. Towards the end of the 07:30 to 08:59 nominal morning heating on-time, only the living room (4 on Radbot, ie ~21°C target; RadFan off) is still calling for heat.

(Mains water temperature was ~9°C.)

2024-02-19: Gas Disconnection

I asked Ecotricity the procedure for disconnecting the gas once we no longer needed it, and today I received an email:

In order to have your Gas meter removed and your account closed, you would first need to contact your distributor and ask them to cap the supply. Once you have a time frame for this, if you could then let us know, we can book an appointment for an engineer to attend and remove the meter for you. This will cost £92.19 ex VAT.

Though it occurs to me that I do not know if the distributor will charge me separately to cap the supply.

And indeed SGN estimates £1222+VAT (minimum)!

I have sent the following to OFGEM, CCed to my MP:

To whom it may concern,

Given that OFGEM now has a clear legal duty to support Net Zero, why is SGN allowed to charge a punitive amount (estimated at over £1000+VAT) for a voluntary gas disconnection (after moving to a heat pump)? My retailer asks for under £100 for its part of the process.

Disconnection should be free or a token admin charge to encourage the move from fossil fuels.

Can you justify this continuing charge?

2024-04-22: gas deflation

It was confirmed today by Ecotricity that disconnecting the meter and stopping the standing charge would cost £92.19 + VAT. The SGN work does not have to be done to make the gas bills stop.


If you are no longer using gas in the property, you can have the meter removed and this will close your gas agreement and stop any standing charges.

The engineer will cap the supply point and make it safe. As the engineer is only responsible for the meter, any additional pipework you want removing coming into the property, would need removing by your gas network.


2024-04-15: Warming Up for a Heat-pump Survey

The Octopus survey is due soon. A heat-loss calculation will need to be done, during the survey or in any case before an installation. To that end, I am gathering data to help with that process and calculation.

  • 1960's timber-framed end-of-terrace ~76m^2 over two floors, EPC A 100.
  • ~300mm loft insulation.
  • Smallest bedroom (and other bedrooms) 30mm aerogel external wall IWI, U=0.3W/m^2K.
  • Front and back (Ug=1.0W/Km^2 with SolaGlas Climacontrol Vertical) doors both new-ish good double-glazed.
  • All windows triple glazed Uw=0.8W/m^2K.
  • Living room 40mm aerogel external wall IWI U=0.21W/m^2K, and insulation above the ceiling to 0.33W/m^2K.
  • Kitchen 40mm aerogel external wall IWI, U=0.24W/m^2K.
  • Downstairs solid floors, U=0.63W/m^2K.

2024-04-19: Surveyed

The Octopus survey has landed; their tool's suggested heat loss (50°C flow, and presumably at -2°C design temperature) is marginally under 4kW for 16WW, though their estimated heat loss for the living room of a little under 1kW feels a bit high to me. But I don't mind upgrading the living room radiator, since it clearly is a bit marginal; a K2 vertical 1800mm might do the job. Their calculations also suggest that the hallway radiator is ~20% undersized given that this is nominally providing heat for the upstairs landing also, but it is barely ever even on, so I am contesting that! Their calculations also suggest that the kitchen radiator is ~20% undersized, but a 600mm high rad in the same spot and the same length would pass muster. Again, in practice, that room is never cold, likely in part because of incidental gains from the gas boiler, Thermino, cooking, and dishwasher. At one point the solar PV and Enphase battery had to be turned off for a few minutes so that a load test could be done.

I asked for maximum data access, and possibly control, to better make the system grid-responsive and low carbon. I asked for heat flow metering of space heat and DHW (so I can do H1, H2, H3, H4 boundaries)!. I asked for a way to do the data without it leaving the house (like the Enphase, unlike the Eddi).

The survey took two people (including a trainee) approximately 4 hours.

It seems that everything we need can fit, physically. I will need to do some work to see how the end result can be made to look good, and allow for easy maintenance. One initial idea is to effectively rotate a cupboard and a half to match the existing ones on the same wall, on the other side of the fridge/freezer.

2024-06-17: DHW CoP Thoughts

Looking at the spec for the suggested DHW tank, Joule Cyclone Slim High Gain Solar 200l - TCIMVG-0200SFC, it seems to have an actual capacity of 188l (similar to the Thermino) but a heat loss of ~2kWh/d, so about twice that of the Thermino.

Given an expected DHW CoP of ~2 for the heat pump, and our typical ~4kWh/d DHW demand, it seems that resistance heating (eg in the Thermino) would have similar energy footprint to using the heat-pump, because through the raw CoP is ~2 we lose half the heat pumped thus back to ~1.

In summer that may be true, ie let the Eddi divert excess PV. (Maybe first to the cylinder then the Thermino to provide a ~2d store.)

In summer that may be true, ie let the Eddi divert excess PV.

But in the winter, lost heat from the DHW would be warming the kitchen, and in particular if we can avoid heating the full tank if not needed and maybe heat only to ~50°C, so as to reduce losses, then the heat pump set up should be providing some gain over straight resistance heating, reducing grid demand. It still feels like a lot of space for a relatively marginal gain, given that the Thermino is already there!

2024-07-07: more thoughts

Assume a simple consistent 4kWh/d of DHW demand, a Thermino resistive CoP of ~1 and a heat-pump DHW CoP of ~2. Assume losses from the Thermino ~1kWh/d, from the proposed water tank ~2kWh/d.

Initially ignore where the electricity comes from (PV diversion or grid), as any we do not use because of better efficiency can spill to grid to make the grid greener.

Assume for now that tanks are generally kept full-ish, though less so during grid intensity and demand peaks. Either the tank or the Thermino can cover a whole day's demand, so when in series the (upstream) Thermino usually only need cover its own losses (assuming that the Thermino is only heated when once the water tank is full and a big lull in available green energy is predicted).

I am thinking about three ways of setting up DHW:

  • Hot water tank only: use + loss = 6kWh/d, so 3kWh/d electricity demand.
  • Thermino only: use + loss = 5kWh/d, so 5kWh/d electricity demand.
  • Thermino as pre-heat for hot water tank: 7kWh/d, so 4kWh/d electricity demand. (This provides up to two or three full days' supply, so allowing better times to top-up to minimise footprint.)

If we had the heat-pump only provide space heating, and so saved the space for and upfront carbon of the water tank, we would use ~2kWh/d more electricity (thus an extra ~700kWh/y). That extra will be increasingly low carbon as the grid is. In practice most would come from our PV so maybe we would import and pay for (and not export, and thus not be paid for) ~300kWh/y costing maybe £100/y, for the remaining ~5 year life of 16WW. So if omitting the tank saves over ~£500 we should probably do it.

Tuning the heat pump to maximise space heating efficiency for its ~2MWh/y demand should be the primary goal.


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