Earth Notes: Combi Boiler Replacement Dilemma
Updated 2022-11-30 15:27 GMT.By Damon Hart-Davis.
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!
Choices
The main choices, each with complications and caveats, seem to be:
- Repair the existing (non-condensing) combi [2]
- Install an air-source (air to water: A2W) heat-pump [1]
- Install some sort of air-to-air (A2A) heat-pump(s) [4]
- Install a new (condensing) combi [2]
- Install a new electric boiler [0]
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
- Repair the existing (non-condensing) combi:
- Replace the internal expansion vessel (if that is the issue) or find space to fit an external one?
- Install an air-source (air to water: A2W) heat-pump:
- Will if fit where the gas boiler currently does, wall hung, for space heat and DHW?
- Can I hang any fan unit on the wall behind the boiler, and will that require planning and other permissions? Will the wall bear the weight? Will noise transmit into the house, in particular the bedrooms?
- What places are suitable to put the external unit? Am I running water or refrigerant from the external into the house?
- What power/rating should I allow for given our old single-phase infeed? 3kW--6kW should cover space heat, but what hot-water parameters (tank size, device rating, other) for our ~4kWh/d DHW demand?
- Will it accept hot water in direct from the heat battery?
- Will I need a buffer store of some sort?
- Will it continue to work with my Radbot TRV TTZC controls?
- Can I integrate the controls (preferably locally, not over the Internet) with the rest of my system, eg from my RPis?
- What is available / within regs second-hand?
- What will it cost (then save) to decommission the existing gas boiler and supply?
- Install some sort of air-to-air (A2A) heat-pump(s):
- What (S)COP can I expect for heating?
- Can it do cooling?
- What (S)COP/SEER can I expect for cooling?
- Can it dehumidify in winter/summer while heating/cooling/neither?
- What do I do for hot water? Convert the heat battery to be the entire DHW source?
- Am I limited to one external fan under GDPOs? Eg could I have multiple mini-splits A2A, maybe with a separate small A2W for DHW?
- Am I limited in the number of separate (possibly independent) units if they don't have external fans?
- For a multisplit, can rooms run at different temperatures, and set-back if the room is vacant?
- For a multisplit, how big is the pipework, what's in it (water), and how far can it run?
- Can I integrate the controls (preferably locally, not over the Internet) with the rest of my system, eg from my RPis?
- What is available / within regs second-hand?
- What will it cost (then save) to decommission the existing gas boiler and supply?
- Install a new (condensing) combi:
- What instantaneous power should I allow for? 3kW--6kW should cover space heat, but should I allow 20kW, 24kW for DHW when the heat battery is empty?
- What should I be looking out for in efficiency rating etc?
- Will it accept hot water in direct from the heat battery?
- Can I delay DHW response to turning on a tap to avoid any gas use if the heat battery is hot enough?
- What should my flow temperature be?
- Where should the condensate drain be routed given the footpath behind the boiler location, with no obvious drain?
- Will it continue to work with my Radbot TRV TTZC controls?
- What is available / within regs second-hand?
- Install a new electric boiler:
- What instantaneous power should I allow for given our our infeed? 3kW--6kW should cover space heat, but should I allow 8kW, 10kW, 12kW for DHW when the heat battery is empty?
- Should there be any storage from overnight low-carbon electricity, eg another heat battery, or even steal heat via the second pipe in the existing Thermino?
- Will it continue to work with my Radbot TRV TTZC controls?
- Can I easily continue to block out peak grid times eg 4pm to 7pm? for space heat at least?
- Will I need to replace/expand the consumer unit again?
- What is available / within regs second-hand?
- What will it cost (then save) to decommission the existing gas boiler and supply?
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!