boiler size

Modern condensing boilers modulate their output to try and keep in condensing mode - unlike older types which simply switched on and off at full blast.

It would depend on whether your rads could make use of the extra capacity.

Do they condense at low load or when going flat out?

I see. So if I had 19kW of radiators, it would not matter whether I had a 21 or 26kW boiler. I have added up the rads, though I can't remember off hand what that came to, but I suppose I need to allow a bit extra in case I ever fit any more. How much should I add for the HW (I have a cylinder)?

Thanks.

The boiler will condense when the return water temperature is cool enough. In order that the boiler will condense you need to have enough radiators.

Do you just want to replace the boiler? How many bathrooms? Do you have a tank in the loft? Cylinder? If you have good water mains pressure and flow a quality high flow modulating combi may be just what you need giving power shower performance.

Back to answering a question that wasn't asked, I see.

Please eff off as you are a total plantpot.

QED

I downloaded the program to calculate heat requirements. I don't know how accurate the results are because I didn't know what some of the walls were made of so I made guesses. I wasn't sure what to put in the heat above and below boxes either. Anyhow, the total heat required came to just under 15kW for the rads. I don't know whether the existing rads meet that demand because I don't know their ratings (they are old single rads with no fins). The 15-21kW boilers seem to be the smallest I can find, so I guess I will buy one of these.

Please eff off as you are a total plantpot.

If you can do so and want to do so, try to replace the existing radiators with ones with fins. This should allow you to get a slightly better result from the new boiler.

They will condense as long as the water returning to the boiler is cool enough. IIRC 50-58 degC is the range you are looking for. AFAIK they will modulate down to try to get the return temperature low enough.

Your cylinder should have a power rating for the heat exchanger. If you have a system where you can heat the central heating and hot water at the same time then you could add up the total of the radiators and the hot water coil. Otherwise just take the maximum of the two figures. Add on a bit incase you swap or add any extra radiators/change cylinder.

I guess strictly you should size your boiler based on the total heat loss from your house rather than from the radiators currently fitted.

In most[1] cases the hot water would require a larger amount of power than the CH.

Anything below 58C return water temperature.

Depends on the control system to the boiler. They are not all the same. Some modulate to maintain the set flow temperature.

Best a quick recovery coil cylinder. Or fit a bronze pump and plate heat exchanger and the cylinder will heat in no time and boiler condense all the same.

How do you define "good mains pressure"?

Whilst I like the heat only what you need aspect of a combi, which hopefully saves on the fuel bill, we use economy 7 to heat the cylinder overnight. We would lose this feature with a combi. I thought that economy 7 was cheaper than oil for heating water? It also gives us a back-up system so that if the boiler breaks, we still get hot water. So I think we would like to stick with a regular boiler this time.

Thanks.

Thank you. We shall try to replace the rads one by one as we work through the house redecorating.

I am not sure if economy 7 is actually cheaper. If you need backup insert an instant electric water heater in the combi line - about £100. The DHW runs though. If the combi is down then turn on the water heater. Gives a get wet shower and one tap for backup.

You save on standing heat losses with a combi.

And if the combi needs faults seen to that include the water supply to it?

Please eff off as you are a total plantpot.

Only a total wanker would suggest a backup system which might not work when the main one fails and is under repair. Thank god you're not in a position to specify such things.

Please eff off as you are a complete plantpot.