Earth Notes: Combi Boiler Replacement Dilemma

Updated 2022-11-30 15:27 GMT.
By Damon Hart-Davis.
Quick, green and legal, avoiding a distress heating system replacement when the cold hits! #DHW
under combi 20191215
Our old non-condensing gas combi is struggling. Under normal circumstances I'd 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!

~1946 words.