New boiler efficiency

[winston1]

I have a Baxi Bermuda 552 back boiler which works OK. However there is a big ugly gas fire in front of it which I understand cannot be changed or got rid of. Baxi do a complete replacement back boiler and fire which I have not looked into yet. I'm told a replacement wall hung boiler at approx £2200 will save around £330 gas per year and pay for itself in 7 years. Sounds good, time to do some calculations.

Checking my gas bills (I only use gas for the boiler) 23/11/2012 to 28/11/2013 the total was £832.07 for 371 days just over a year. There was a standing charge which does not change whatever the boiler of 23.835p per day = £88.43 for 371 days. Therefore cost of gas used is £743.64. Now according to the Baxi leaflet I still have the boiler input is 21.2kW and output is 16.1kW which makes it 76% efficient. 76% of £743.64 = £565.16 meaning £178.48 is lost up the chimney, somewhat less than the £330 quoted.

Not only that a new boiler is at best 93% efficient so cost of gas will be £565.16/0.93 = £607.70. Subtract this from £743.64 the previous gas used gives a saving of £135.94. Hm payback time 16 years.

The only thing I may have missed is that the Baxi has a permanent pilot light which costs maybe £15 a year. However it will still heat the heat exchanger so not all lost. Also a new boiler has an electric fan, I wonder if the electricity cost is included in efficiency figures.

Then there is the fact that new boilers are high maintenance. Gosh can I live with that ugly fire? Paint the wood surround white perhaps.

 

[footprints]

Something that is often over looked is while input /output gives you the appliance 76%, the back boiler is open flued so requires a large amount of air through the draught diverter to dilute the products and aid the flue operation.

In effect a large portion of the air you have heated using your 76% is drawn up the chimney, and replaced from the air brick (which I sincerely hope you have not blocked up ) with cold air from outside. This can't be changed because it is vital for the safe operation of an open flue.

Your new boiler should be a balanced flue or room sealed appliance, the air for combustion and product dilution is drawn directly from outside and not from heated air in the house. Also the old cast iron heat exchanger is slow to heat up and holds a lot of heat after the heating is no longer required.

 

[Jackrae]

Congratulations, you've just discovered what they don't tell you when extolling the benefits of a new , high-efficiency boiler. If your old one still works, it may well pay for itself when costed against loss of capital, return on "investment" and possibility (or probability) that controller complexity will render the new one unserviceable before the pros exceed the cons.

 

[Steelmasons]

Painting the fire surround is a no no so I guess live with it or remove it.

 

[gas112]

firstly all that heat is not lost up the chimney a percentage of it actually heats the fabric of the building so you get this benefit . A pilot is now a lot more than 15 quid a year.
And a condensing boiler is only 90 odd percent efficient for a small percentage of the time the boiler is on , it drops to low to mid 80`s for the majority , the 90 odd is in lab conditions with everything perfectly designed and set up.


If you are changing purely for a monetary reason then its a big mistake , what you will get is faster heat up times and more controllable comfort levels

 

[Slugbabydotcom]

I have an old circa 1983 Thorn Olympic wall mounted boiler. I last did a calculation a couple of years ago and concluded that I could save the vast sum of £138 a year by switching to a modern combi.
During the summer I often turn the pilot light off! I don't need any hot water at all as I have an electric shower and dishwasher.
Last year however I left it on a little bit longer than usual and discovered that my gravity circ fed hot water cylinder was preheated to about 30ºC. I concluded that the heat from the pilot in this case was probably the most efficiently consumed gas of all that is burnt!
Another frequently unrecognised feature of a pilot light is that it also provides a degree of frost protection.
I for one will miss these old dinosaurs as they become extinct.

 

[dcawkwell]

My mums back boiler is 40 years old this year and has required a £5 thermocouple in all the time. It will probably last another 10 or 20 years as it looks spotless.

Keep the back boiler.

 

[winston1]

And a condensing boiler is only 90 odd percent efficient for a small percentage of the time the boiler is on , it drops to low to mid 80`s for the majority.

Interesting, why is that?

 

[Dan Robinson]

Depends on the flow and return temperatures and the flue gas temps.

As soon as the return goes over 56 the boiler is no longer condensing (although still more efficient that the older boiler.

This is where weather compensation and dual running temps come in to make the most out of the boiler, but the extra installation costs don't always make it worth while.