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Tim Cooke
Tim Cooke,
Category: HVAC
Satisfied Customers: 800
Experience:  Owner at Self-employed
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Worcester Bosch Greenstar 28i Junior - checking output over

Customer Question

Worcester Bosch Greenstar 28i Junior - checking output over one hour on central heating only - gives 12.19kW-H.
JA: How old is your Bosch central heating?
Customer: previous owner-probably 2009
JA: How long has this been going on with your Bosch central heating? What have you tried so far?
Customer: don't know but gas bills have seemed high for our usage (in house 3 years)
JA: Anything else we should know to help you best?
Customer: gas central heating, electric cooking, water heated shower, 3 adults , 4-bed detached, no other heavy usage
Submitted: 6 months ago.
Category: HVAC
Expert:  Tim Cooke replied 6 months ago.

Hello, over what period have you monitored your gas usage?
Do you have a smart-meter?
Have you ensured that you're comparing actual readings, and not estimates?

Customer: replied 6 months ago.
Annual gas summary is 20,251 kW-H (shown as much more than UK average) of lesst5han 12,500 kW-h)so took a reading of one hours usage of the central heating to get 12.19 kW-h from 24 kW output boiler
Expert:  Tim Cooke replied 6 months ago.

It's excellent that you have the precise numbers!

Please tell me more about your house - age, construction type, loft insulation, cavity insulation, external cladding, draughty doors or windows? Also. any extensions or conversions?

Customer: replied 6 months ago.
failed to send-try again
4 bed detached, circa 1930s, double-glazed all round, attic is part floored-remainder fibre glass insulation (cold-attic construction), supposed cavity wall insulation, rear kitchen extension with 4th bedroom over, upvc conservatory with internal upvc internal wall & door, solid timber front door with perimeter draught-proofing, rear door upvc, upstairs fire places sealed up, two downstairs fire places with 'modern' gas fires (not used in my usage assessment), 3-adults living.What I want to know is: do I have a central heating problem i.e. is it running at near half efficiency?
Expert:  Tim Cooke replied 6 months ago.

There are two aspects here:

1. Energy consumption;
2. Boiler efficiency.

--- Energy consumption ---

If you had three 10 minute showers per day, at 11kW, you'd used 5.5kWh. That would leave an average of 50kWh per day, over the day, going into heating the house. This would be a lot less in the summer and a lot more in the dead of winter. 50kWh equates to heating the house for 10 hours, with total heat losses of 5kW. If the losses are 10kW then you'd be having the CH on for an average of 5 hours per day. I'm sure you're getting the maths here. On the basis of these numbers, your usage doesn't seem outlandish. That doesn't mean that you can't effect some savings. The bot***** *****ne is that the energy you obtain from burning the gas has to go somewhere. You can visit various sites and use an online heat loss calculator to see if 5kW is a reasonable estimate for your house.

--- Boiler efficiency ---

If the boiler has been configured correctly, there's nothing much you can do to affect its efficiency.

When you say that it's running at near half efficiency, are you comparing "55.5kWh" with "24kW"? If so, that's a mistake, as the latter is a measure of power, not of energy. We don't actually know how much power the boiler is producing when it's burning, only that it's a maximum of 24kW. Most of the time I would expect it to be modulating down.

I'll pause there, in case you have any comments so far.

Customer: replied 6 months ago.
This is a pensioner family and much more frugal with gas usage than you suggest. The central heating has been 'shut off' since spring and not switched on until the end weeks of September say 6 months usage only ( of 2 x one hour a day then until say six hours per day this month) typical. In the Summer the boiler has only been serving a weekly washing machine, say 1 shower per day, hand washing up of breakfast and dinner plates, etc. Is it normal for a 24kW boiler to output 12kW when going full blast, regardless of for how many days?
Expert:  Tim Cooke replied 6 months ago.

You still seemed concerned about efficiency, but that relates only to (a) the combustion efficiency and (b) recovery of heat from exhaust gases. It isn’t something that can be calculated from the measurements you’ve taken.

Above all, you don’t have an estimate of the external heat losses from the property; once you have that, you'll have a better idea of whether or not the gas usage is reasonable.

Your measurement of 19kWh could become useful, but only if you also know the starting and ending temperatures in the dwelling for that hour of measurement. If it didn’t increase in temperature then 19kWh is crazily high, but as a figure on its own it’s not very meaningful. I’ll check to see if this model can be rate limited but, in the meantime, why do you think it was, or should have been, “going full blast” during that hour?

Regarding the washing machine, it would be unusual if it were such an old model that it takes stored hot water instead of heating it. Are you certain about that?

Going back to an earlier post, you mentioned that gas charges didn’t seem right for the amount of gas used. I’m still unclear about why you think that - isn’t that a billing issue?

Expert:  Tim Cooke replied 5 months ago.

Hello, how are you getting on with your boiler?