Boiler output, old boiler verses new

[PC Plod]

I am just starting to think about replacing my boiler so no doubt this will be the first of a few question posted here.

I live in a 1990 built 4 bedroom detached house. My existing boiler which is now getting on for 18 years old is rated at only 50,000 btu/ 14.65kW. I say only because when I look at websites for boilers I apparently need a larger output. My existing boiler serves a Flowmax thermal heat store, so the tank is full of central heating water while the hot water runs through a copper tube inside the tank which serves all taps at high pressure. The boiler is only set at 50% with the room and tank thermostats controlling the temperatures

It works great so I intend to keep it. So the first question is do I really need a higher output heat only condensing boiler.

[Andrew Gabriel]

The higher outputs you'll see on many modern boilers are because they are combis, so they have to have enough power to instantly heat a stream of shower/bath water. For a stored system like you have, this isn't the case. However, a stored system can take a more powerful boiler without any negative effects, and it would give you a faster heat up from cold.

Start by reading Ed's Choosing a Boiler FAQ...

[The Natural Philosopher]

If what you have works, of course you don't need a higher output.

The max output of the boiler should equal or exceed the max heat loss from the house in the coldest weather you are likely to encounter.

After that, its pure cost benefit analysis..if the capital outlay on the new system will be offset in terms of fuel cost savings over the period its likely to last, or you are likely to stay living there.

(IME as long as you sell a house with functional systems, whether or not its the latest and greatest is not usually a thing that affects price greatly anyway)

Example: if your fuel bill is £500 a year, and your existing boiler is

60% efficient, there is £200 a year being sent 'up the chimney'

And 80% efficient boiler would cut that in half. So £100 a year.

If the boiler itself costs - say - £1300 to buy and install, that's a payback period of around 13 years. Or a cost of capital at 10% borrowing of £130 a year..not worth it is it?

Of course that isn't a real world example. YMMV.

[PC Plod]

Thanks guys for those great replies.

My old boiler which is 18 years old has cut out twice in the last 2 weeks. The overheat thermostat trips and the pilot light goes out. If I could get someone to fix it for under £100 and I got another 5 years out of it I would get it repaired as it has never gone wrong. But I am thinking that I might be just throwing money away due to its age.

I pay £30 a month for gas so with a 30% efficiency saving I would save around £108 a year. This would no doubt be written off with a required annual service and parts due to the complexity of modern designs. I have to confess I have only bothered getting the old boiler serviced once so what I have saved in maintenance visits will cover the cost of the new boiler.

[Touchy]

This isn't strictly true. The boiler has also to have sufficient excess capacity to both achieve the above situation and to also heat up the system in a reasonable time - say 20% overcapacity.

[The Natural Philosopher]

Mmm., Your choice then.

£108 a year probably just covers a decent service.

Unless you want the boiler renewed for other reasons., I'd just get it serviced...

[Andrew Gabriel]

I doubt that it's anything major wrong. Might be a new pump, and system might benefit from a drain down and flush.

If it's not a room sealed system, you really should get it checked to ensure the flue is operating correctly and it isn't generating any significant Carbon Monoxide. Otherwise it could kill you. If you can see the burner flames, do they look a clean fierce blue directed up the heat exchenger (good) or is there any yellow/white flame, or any flame at all wafting out from under the heat exchanger (bad)?

I wouldn't write the old boiler off soley on the basis of the reported symptoms just yet.

[The Natural Philosopher]

'or exceed'

Depends on whether you run 24x7 in very cold weather. I do. ;-)

[PC Plod]

"The Natural Philosopher" wrote

Thanks everyone for your input. I think I might now well go down the path of getting it serviced. So what is the life expectancy of a 1990 Potterton Flamingo boiler? I guess it must be or borrowed time.

[Ed Sirett]

I'd be interested to know what web sites are suggesting a larger output unit. This existing unit will be sized for the space heating requirements which for a modern house will be relative modest.

So it comes down to what 15kW condensing plain heating boiler you need. Some thermal stores have a coil for indirect primary heating and so a sealed boiler can be used. Otherwise you are needing a plain heating boiler.

[Ed Sirett]

There is little to go wrong. If the main heat exchanger is intact and you keep it cleaned out from time to time. Then the only item of hassle will be the pump, the themocouple, very very occasionally the gas valve, ocassionally the overheat stat.

It will send a sizeable chunk of the gas bill up the chimney.

[Andy Champ]

I don't know if Ed reads this group any more, but the boiler calculator at

has gone - with the whole web site.

Andy

[PC Plod]

"Ed Sirett" wrote

They recommend the ecoTEC plus 428 which is a 28.2Kw condensing boiler for a 3-4 bedroom house. I have seen another website but can't find it now

[PC Plod]

You have an "e" missing

Try this

after doing the calculations I only need a 10KW boiler

[Ed Sirett]

Thanks for pointing out the broken link. Time for an edit, I think.

Which rather illustrates the point I make in the FAQ.

... This calculator tend to suggest boiler a little less than I would fell comfortable installing. ...

[Ed Sirett]

They are playing safe. A large 4 bedroom house with uninsulated everything, draughty ill-fitting sash windows, old unblocked fire places,....might just need 28kW