Spec me a combi boiler

eidolon

eidolon

eidolon

Soldato
Joined
31 Oct 2002
Posts
13,892
Location
Newcastle upon Tyne
My new place is a late 1800s property so before I move in I'm going to get a new boiler and radiators fitted as the current ones are around 10yrs old.

I've used an online calculator to work out the BTUs for each room and I'm looking to fit column radiators which comfortably match or exceed the maximum BTUs advised.

Hallway 2675-3200BTU
Living Room 6980-8225BTU
Dining Room 5975-7150BTU
Kitchen 5000-6000BTU
Bathroom 2225-2675BTU
En-suite 2225-2675BTU
Front Large Bedroom 4175-5000BTU
Front Small Bedroom 2025-2425BTU
Rear Large Bedroom 4375-5250 BTU
Loft Room 6875-8250 BTU

Is over-speccing the radiators a good idea? The price isn't much more in the grand scheme of things so is it a case of bigger is better?
Secondly, can anyone recommend a good quality combi which can feed the radiators plus two showers. Ideally I'd like to be able to use both showers at the same time but I understand that might not be possible without fitting a water tank. (Which I don't really have space for)

Would something like this do the job or is a boiler this big just a waste of money?

https://www.ebay.co.uk/itm/272590763008
 
Two showers = don't waste your time with a combi.
You need a system boiler.

Regards radiators bigger is better as long as your using thermostat valves.
They will be sized for a Delta 50c if you went of the published figures which is really too high for a modern condensing boiler, calculate using a Delta 40 or lower.
 
robj20 said:
Two showers = don't waste your time with a combi.
You need a system boiler.

Regards radiators bigger is better as long as your using thermostat valves.
They will be sized for a Delta 50c if you went of the published figures which is really too high for a modern condensing boiler, calculate using a Delta 40 or lower.
Click to expand...

So if I over spec based on delta 50 then I should be about right based on delta 40?

For example if the calculator recommends 2500-3000BTU for a room and I get a radiator that is advertised at 4000 (which I assume is based on delta 50) then that should be about right?
 
Shouldn't be far off, but it's amazing how it ramps up. My own I have used Delta 35c, it requires uprising by around 70%.
Its easy to find the figure you need in the radiator literature it would mean your uprising by the same ratio in each room rather than just adding a bit.
 
robj20 said:
I think most people either have a system boiler or combi boiler these days, keeps things tidy.
Click to expand...

I don't, I have a regular boiler. The expansion vessel is integrated in to a system boiler so as I had room in the airing cupboard I was told there was no benefit to having a system boiler.
 
We are currently looking at getting our family bathroom and ensuite replaced and have a 2 year old combi boiler. Rather than replace the combi we are looking at having 1 shower off the combi (in the ensuite) and then the highest kw electric above the bath in the family bathroom. Apart from replacing the boiler and installing a hot water tank I can see no alternative. If I was starting from fresh, I'd add a tank back in but I don't think I can justify spending £3-4k replacing a boiler that is only 2 years old.
 
Felix said:
We are currently looking at getting our family bathroom and ensuite replaced and have a 2 year old combi boiler. Rather than replace the combi we are looking at having 1 shower off the combi (in the ensuite) and then the highest kw electric above the bath in the family bathroom. Apart from replacing the boiler and installing a hot water tank I can see no alternative. If I was starting from fresh, I'd add a tank back in but I don't think I can justify spending £3-4k replacing a boiler that is only 2 years old.
Click to expand...

You know it's very easy to add a tank to a combi.
 
I
mattyg said:
Until the water gets to the shower. Then the shower will reduce the flow in order to heat the water up.

Electric as a very very very last resort.
Click to expand...

I was pointing out that a cylinder at mains pressure is better than an electric at mains pressure, which is the option currently on the cards.
 
mattyg said:
Until the water gets to the shower. Then the shower will reduce the flow in order to heat the water up.

Electric as a very very very last resort.
Click to expand...
We currently have an electric in the ensuite. But swapping it with the family bathroom. It's not awful but not as powerful as I would like.

Surely if 2 showers are used at once with a cylinder at mains pressure, the flow will drop? Which is the same issue as running 2 showers off a combi?
 
Felix said:
We currently have an electric in the ensuite. But swapping it with the family bathroom. It's not awful but not as powerful as I would like.

Surely if 2 showers are used at once with a cylinder at mains pressure, the flow will drop? Which is the same issue as running 2 showers off a combi?
Click to expand...

No, boilers are rated worth a 35c rise, so say 16lpm.
But you more than likely have it at around 50c. So let's say 12lpm. So 6lpm per shower of hot water.

What's your mains flow roughly lets say 20lpm that's 10lpm per shower.
But there is nothing stopping you from pumping it so it can be what ever you want.

Edit: It just occurred to me that you might think I mean to have the boiler fill the cylinder.
To add a cylinder to a combi you add it as a zone to your central heating, when the cylinder calls for heat a zone valve would open and it would heat the cylinder. It would be fed direct from the cold mains or via an expansion tank.

You would still feed your kitchen and bathroom basin from the DHW of the boiler but bath and showers could use the cylinder.
 
Last edited:
Adam said:
Good advice but you could also consider a regular boiler and an unvented cylinder.
Click to expand...

Got this and seen the bills come down from the old boiler originally in there, we had it installed 7 years ago would buy again if i moved house.

I had trouble with a combi at the old house back in the day so not a fan.
 
Had a plumber out to have a look at adding a Unvented Hot water cylinder. He is suggesting to basically turn the boiler into a system boiler and heat the water that way and would feed all taps. He did look at adding it as a zone to the heating but there would be a lot of pipe work involved getting it to the bathrooms. Water pressure on the incoming mains is aroudn 22l/m. He also recommended changing the incoming mains to the boiler to 22mm from the current 15mm.
 
I'm not an expert but that sounds fair enough to me, I'm sure we're upgrading our water pipe to 22mm as part of our heating replacement too.
 
You must log in or register to reply here.
Back
Top Bottom