Boiler efficiency (or not!)

[giw12]

Morning All

I've been swithering for some time about replacing the boiler in a house I recently moved to. It's a Potterton Osprey 2 (43kW) and from what I can gather seems to be aimed at very large homes and smaller factories/offices. I've got a fairly spacious 4 bed detached house.

My gas bills have been eye-watering (£1400 per year) and a plumber who was round recently advised that he'd never seen such a high output boiler in a house (he said it's 150000BTUs).

Interestingly, my father in law has a similar size house. It's warmer than my place, he keeps the water on pretty much all the time and he runs a gas-fired AGA 6 months of the year. He has an Ideal Mexico 2 (37kW) which he runs at about 3/4 of max power (my plumber recommends running boilers at full power so I do).

Aside from the differences in boiler I'd have imagined that he uses significantly more gas than I do. In actual gas consumption terms though I use more than half as much again as he uses. I'm amazed to be honest.

Plus, his boiler is actually less efficient according to SEDBUK (78% v 72%).

Can such a big difference be down to the 'over powerful' Osprey?
Is it time to change to a higher efficiency model?

Thoughts very welcome!

Thanks.
Gary

 

[twgas]

a house I recently moved to

My gas bills have been eye-watering (£1400 per year)

 

[giw12]

a house I recently moved to

My gas bills have been eye-watering (£1400 per year)

Ok..recent as in 2 years!

Good spot though!

 

[twgas]

43kw does sound a lot for a 4 bed, how many rads do you have? try getting a company that does free estimates to give you a price and see what size boiler they recommend

 

[gas4you]

I would be surprised if you needed more than 30 Kw. It will also depend on the type of controls you have on the system.

 

[D_Hailsham]

The EST online calculator will help you find out what size boiler you need.

 

[giw12]

Thanks for the link...when I'm home tomorrow I'll measure up and see what comes out.

In terms of radiators, I've got 15.

For the same space to heat and water to get hot, would a 43kW boiler consume significantly more gas than a 30kW one (assuming they are the same efficiency? Wouldn't it just do the job much quicker?

 

[TicklyT]

My first step would be to compare gas consumption in KWh, cubic feet or cubic metres, whichever is convenient. Just make sure you are comparing like with like. Base it on actual meter readings, not estimated readings.

It's certainly not unknown for a gas company to send out bills based on erroneous estimated readings, or to base monthly payments on over-inflated annual consumption estimates, not to mention the multitude of different tariffs they charge per unit of gas.

Otherwise you may be comparing apples and bananas

 

[giw12]

Thanks for the tip!

I hope my figures are pretty accurate as they are both based on actual readings over an extended period (Nov 08 to Mar 09).

Would this give me a valid comparison?

 

[D_Hailsham]

In terms of radiators, I've got 15.

For the same space to heat and water to get hot, would a 43kW boiler consume significantly more gas than a 30kW one (assuming they are the same efficiency? Wouldn't it just do the job much quicker?

Unless you 15 radiators are 3kW each average, then a 43kW boiler is not much point. If you want to know approximately how many kW your rads are go to Stelrad Elite Catalogue.

A boiler is not like an electric fire, which gives out the heat stated on the label. A 43kW electric fire (assuming you can buy one!) gives out 43kW of heat and will heat a room faster than a 30kW.

But a boiler acts indirectly by heating water and the heat in the water is then emitted from the radiators. If you have 15kW if radiators, that's all the heat you will get. The boiler will heat the water up very quickly and then turn off until the water temperature has dropped sufficiently. If the boiler is three times the size of the rads, it will be idle 60-70% of the time.

Boilers are always sized for worst case, i.e outside temperature of -1C and a room temperature of, say, 21C. It that works out at a 10kW boiler then when the outside temperature is 10C, the boiler only has to produce 5kW to maintain a 21C room temperature. Older boilers will spend more time off; modern boilers will modulate the burner so it produces less heat.

 

[giw12]

Back home and I've done some research.

The Energy Trust boiler sizer tells me worst case scenario 25 kW (19kW if I count my loft insulation in terms of roof U-value).

My radiators based on the Stelrad catalogue (checked against the Myson Catalogue) come to approx. 23kW (80000 BTUs).

To my untrained eye it looks then like my 43 kW boiler is way too big for the job.

I don't really want to rush out and change it but I don't know how much additional gas I am burning because of the size of it. Would really appreciate if someone could give me an educated guess. Maybe another way of looking at it is....would an experienced central heating engineer change it if it were in an similar house to mine?

Last question (honest!)...by turning down the boiler am I effectively reducing it's kilowattage or isn't it that simple?

Many thanks both in advance and for the help so far.

Gary

 

[D_Hailsham]

Back home and I've done some research.

The Energy Trust boiler sizer tells me worst case scenario 25 kW (19kW if I count my loft insulation in terms of roof U-value).

My radiators based on the Stelrad catalogue (checked against the Myson Catalogue) come to approx. 23kW (80000 BTUs).

The first thing I would do is insulate the house better. My 2 story 4 bed house (130m² floor area) only needs about 10kW to heat it.

You may be able to get help with insulation costs, assuming you are not in a listed building or one with solid walls- you don't always need to be on benefits to qualify. Check out Energy Savings Trust Grant Search

To my untrained eye it looks then like my 43 kW boiler is way too big for the job.

That's what it looks like to me. The person who had installed was probably of the "bigger is better" school

I don't really want to rush out and change it but I don't know how much additional gas I am burning because of the size of it. Would really appreciate if someone could give me an educated guess. Maybe another way of looking at it is....would an experienced central heating engineer change it if it were in an similar house to mine?

If you have 23kW rads then the boiler will produce 23kW of heat. It will do this by spending about 55% of its time off. (A 43kW boiler running for 0.55 hours will use 23kWH of energy.) If it ain't broke there is no need to replace it.

by turning down the boiler am I effectively reducing it's kilowattage or isn't it that simple?

The boiler output is fixed at 43KW; you can't range rate it like some boilers. So when it is alight it is always going flat out. Reducing the boiler temperature will not reduce its output but it will mean that it does not burn so long each time it lights up. Try turning the temp down by 5°C and check the results.

 

[gas4you]

I don't know about your boiler personally, but it may be possible to range rate it.

Have a look in the MI's. It could be worth getting it serviced and asking the engineer to make sure it is set for minimum output

 

[simond]

An Osprey is a Potty light commercial.

You will be using more gas than necessary, but even if a new suitably sized replacement boiler halved your gas bill (v unlikely) you would save £700 a year.

So repayment in the next 3-4 years is the 'best case scenario'. More likely to be 7 years IMHO.