Economics calculator for replacing old back boiler with combi?

mr-b

Hi

My mother's 27YO back boiler has a 65% SAP seasonal efficiency but it's still "working fine". It has just an on/off timer with no room or hot water cylinder thermostats but the rads do have TRVs.

All the various energy advice sites vaguely hand wave about getting a new boiler to save money ("could save £580" p.a. according to https://energysavingtrust.org.uk/advice/boilers/), but none appear to have calculators that one can enter actual energy consumption figures, energy costs, standing charges, boiler efficiencies etc. in order to obtain realistic payback numbers. I recall hearing that the Energy Saving Trust hugely overrated the actual savings some time back. It seems all these energy sites have agendas to push.

Has anyone done a spreadsheet with all the various figures that one can plug into to view payback times?

For her, reliability is key, and it's "worth it" for her to keep with what is "working" rather than risk disruption from a multi-day installation plus the concern that new boilers are more complicated and so logically, less reliable. But what's "worth it" might change if the payback figures are truly compelling.

FWIW the annual energy consumption figures are (for an 8 rad, 60's detached, wall/loft/windows insulated, 1 bath/shower, single occupancy home) are:

Elec  1500  kWh

Gas   16,000  kWh

So low for elec, and high for gas compared to 'average' e.g. https://www.ukpower.co.uk/home_energy/average-household-gas-and-electricity-usage

There's also the fact that low carbon solutions apparently will require a hot water cylinder again, so is replacing with a combi a short-sighted idea if all the pipework will need to be redone again in future? Or are gas combis here to stay, even if gas prices are unlinked with elec, due to the sheer numbers of homes with them installed?

QrizB

mr-b said:

Has anyone done a spreadsheet with all the various figures that one can plug into to view payback times?

I think you can make a reasonable estimate without resorting to a spreadsheet. I'll hgave a go below; adjust the numbers as you feel necessary.

My mother's 27YO back boiler has a 65% SAP seasonal efficiency but it's still "working fine". I

FWIW the annual energy consumption figures are

Gas   16,000 kWh

OK, so if we say there's 1000 kWh/yr used for cooking, her actual heating energy requirement is (15000 x 0.65) 9750 kWh/yr.

To deliver 9750 kWh of heat with a 90% efficient condensing boiler would need (9750 / 0.9) 10800 kWh of gas.

That's an annual saving of 4200 kWh. At the current price of 7.4p/kWh that's worth £310 per year.

Although, gas prices are expected to double in October and then rise further in January and April, so over the coming winter it's probalby going to be worth more like £6-800.

There's also the fact that low carbon solutions apparently will require a hot water cylinder again, so is replacing with a combi a short-sighted idea if all the pipework will need to be redone again in future? Or are gas combis here to stay, even if gas prices are unlinked with elec, due to the sheer numbers of homes with them installed?

Whether you fit a combi and do away with the cylinder, or fit a regular boiler and keep it, is your (or her) decision. There's no right or wrong. Mains gas is expected to remain for at least another 20 years, which is likely to see out the life any new boiler.

FreeBear

mr-b said: My mother's 27YO back boiler has a 65% SAP seasonal efficiency but it's still "working fine". It has just an on/off timer with no room or hot water cylinder thermostats but the rads do have TRVs.

Something like a Baxi Bermuda ?

If so, very similar to what I have. A few relatively low cost things you can do to improve the performance -

• Fit a thermostat to the tank and wrap some more insulation around it [the tank].
• Get a programmable thermostat that allows for different temperatures to be set depending on the time of day/week - The Drayton Wiser system would probably work quite well.
• Get a zone valves fitted to the tank and the CH system so that you are not wasting energy heating one when you just want the other.

Another step to reducing the gas consumption is to turn the gas off completely during the summer months when heating & hot water is not required - This will save on the pilot light burning 24/7 which could save on 500KWh or more. It is possible to get a kit to convert to an intermittent pilot light, but these kits are not approved for retrofitting to boilers. A shame really....

mr-b

QrizB said:

Lots. Tx very much for the informative calculations and insights. I'll definitely consider those.

mr-b

FreeBear said:

mr-b said: My mother's 27YO back boiler has a 65% SAP seasonal efficiency but it's still "working fine". It has just an on/off timer with no room or hot water cylinder thermostats but the rads do have TRVs.

Something like a Baxi Bermuda ?

Great catch! Yes I will consider all those mods and add them into the options to consider.