New boiler size which measurement to use

[Topcat33]

Looking at replacing an old boiler with a new combi, all the calculators say 24kw to 27kw for my house but is this the advertised size or the one for the central heating as when you delve into the specs of say a Viessmann 050 30kw the water is 30kw but the ch is 25kw so do I take the latter as the guide or the size showing on the main sales page. Thanks

 

[fixitflav]

Looking at replacing an old boiler with a new combi, all the calculators say 24kw to 27kw for my house but is this the advertised size or the one for the central heating as when you delve into the specs of say a Viessmann 050 30kw the water is 30kw but the ch is 25kw so do I take the latter as the guide or the size showing on the main sales page. Thanks

Is there any reason to go for a combi, other than the installer pushing it?

It would have to be a big, or badly insulated house to need 25kW CH.

 

[Topcat33]

my boiler is over 40 years old, ideal Mexico floor standing and a Medway multi point for hot water, so time for a change!, so a Viessmann 050 25kw would be well big enough then?, on offer atm under a grand with flue at city plumbing. House is a 1930,s medium semi, 9 rads 1 bathroom

 

[DP]

For a start Viessman would not be on my short list.
Mexico, client wanted hers replaced. Talked her out of it. She now has top end controls fitted that gives her a healthy 25% saving on fuel ( her figure not mine)
Fit a cylinder would be my option.

Alternative. Intergas as open vented as system my well not be able to sustain sealed option

Rather than look at the power rating, check the quantity of hot water delivery in a minute (over 15 seconds times 4)
If you are happy with that flowrate, pick a combi, if you must, with same flowrate.

Only downsize I see in retaining the Mexico is possible obsolescence of a vital part should it be needed. Built like a brick outhouse it is

 

[johnny2007]

When I replaced my combi boiler, the heating could've done with a 25kw, but hot water delivery to loft conversion would've been poor.
Ended up with an oversized 32kw which delivers very good flow at loft level and heats the house in 15 minutes.
Is it more expensive to run thank a standard boiler system?
I don't know, surely I'm saving a lot compared to before refurbishment, but this is also due to the repiping which now takes the shortest route possible and doesn't have any dead leg as well as all new radiators.

 

[Topcat33]

When I replaced my combi boiler, the heating could've done with a 25kw, but hot water delivery to loft conversion would've been poor.
Ended up with an oversized 32kw which delivers very good flow at loft level and heats the house in 15 minutes.
Is it more expensive to run thank a standard boiler system?
I don't know, surely I'm saving a lot compared to before refurbishment, but this is also due to the repiping which now takes the shortest route possible and doesn't have any dead leg as well as all new radiators.

Yes I am thinking the same

 

[Topcat33]

For a start Viessman would not be on my short list.
Mexico, client wanted hers replaced. Talked her out of it. She now has top end controls fitted that gives her a healthy 25% saving on fuel ( her figure not mine)
Fit a cylinder would be my option.

Alternative. Intergas as open vented as system my well not be able to sustain sealed option

Rather than look at the power rating, check the quantity of hot water delivery in a minute (over 15 seconds times 4)
If you are happy with that flowrate, pick a combi, if you must, with same flowrate.

Only downsize I see in retaining the Mexico is possible obsolescence of a vital part should it be needed. Built like a brick outhouse it is

The efficiency of my old boiler is very poor and gas costs seem high, I have a complete set of gas valve spares in the loft for the Mexico
Flow rate from my multipoint is 6l per min (rain shower works well) 25kw says 10 and the 30 says 12 so a lot better

 

[DP]

The efficiency of my old boiler is very poor and gas costs seem high, I have a complete set of gas valve spares in the loft for the Mexico
Flow rate from my multipoint is 6l per min (rain shower works well) 25kw says 10 and the 30 says 12 so a lot better

24kw combi will deliver 9.5l per minute at 35 degree temperature rise.

Smallest combi will heat two semis easily ( 16 radiators)* but struggle to deliver hot water to two taps.

* 1.5kW radiators.