30Kw v 35Kw running costs

[AJ16]

Will a 35Kw boiler use a lot more than a 30Kw? Only two bathrooms in 4 bed house but need a good hw flow to fill a bath each day.

 

[chris watkins]

Are you after a combi ??? because the heat load should be calculated & boiler selection based on that if not.

 

[TBServices]

You should leave sizing the boiler to your installer and listen to his recommendations

 

[doitmyself]

You should leave sizing the boiler to your installer

Assuming he knows how to size a boiler correctly.

 

[wheeto]

Will a 35Kw boiler use a lot more than a 30Kw? Only two bathrooms in 4 bed house but need a good hw flow to fill a bath each day.

Well I agree you should leave the sizing to your installer, but to A the Q...

If it's a combi then in theory, ceteris paribus, they will use exactly the same amount of gas to warm a given volume of water - your bath! - the same amount but the 35KW one will do it faster. In practice they will use roughly the same amount.

For heating, it will depend how well they modulate down to supply the steady-state few KW that you house will need. This will be far below the peak output of either the 35KW or the 30KW so, again, I'd say it will be about the same.

Anyone disagree strongly?

 

[chris watkins]

Well I agree you should leave the sizing to your installer, but to A the Q...

If it's a combi then in theory, ceteris paribus, they will use exactly the same amount of gas to warm a given volume of water - your bath! - the same amount but the 35KW one will do it faster. In practice they will use roughly the same amount.

For heating, it will depend how well they modulate down to supply the steady-state few KW that you house will need. This will be far below the peak output of either the 35KW or the 30KW so, again, I'd say it will be about the same.

Anyone disagree strongly?

Spot on !!
Just to add that we are finding a lot of people putting in these large (heating boilers) thinking that all will be alright because they will modulate down only to find that the control within the boiler mean it is shutting down & holding of firing (anti-cycling) because the system can not carry way the heat. There is no excuse not to select the correctly sized boiler for the job, apart from anything else they cost more !!!! Does anyone else see this problem with the larger boilers ???

 

[Kemble]

Firstly... I'm very impressed with the latin Wheeto !

Correct boiler sizing is important, and I think it's totally right for the OP to ask these questions and NOT blindly leave it to the installer... if the suggested sizing is correct the installer should have no problems in explaining it to the OP. That's what I would expect to do anyway...

 

[chris watkins]

Well I agree you should leave the sizing to your installer, but to A the Q...

If it's a combi then in theory, ceteris paribus, they will use exactly the same amount of gas to warm a given volume of water - your bath! - the same amount but the 35KW one will do it faster. In practice they will use roughly the same amount.

For heating, it will depend how well they modulate down to supply the steady-state few KW that you house will need. This will be far below the peak output of either the 35KW or the 30KW so, again, I'd say it will be about the same.

Anyone disagree strongly?

Oh, that's what it was !!!! I only just about managed English at my school, they wouldn't let us do foreign languages. LOL

 

[Kemble]

I've always wanted to use "obiter dicta" in a posting... but I never seem to get the chance !

 

[Ray Stafford]

My favourite is "quidquid Latine dictum sit altum videtur"

 

[Dotty]

My signature pretty much covers all occasions!

 

[kirkgas]

English is fast becoming the second language in our schools

 

[chris watkins]

English is fast becoming the second language in our schools

& even Scottish is reduced to third place. LOL

 

[AJ16]

Are you after a combi ??? because the heat load should be calculated & boiler selection based on that if not.

Yes it will be a wall mounted combi, I would hate to settle for the smaller output if at some stage later I found that it was a bit slow to fill the bath, that's why I asked the question really.
I was recommended a 30Kw some months ago but just want to be sure.

 

[AJ16]

You should leave sizing the boiler to your installer and listen to his recommendations

Thanks but it's good to know the reasoning, it's not a complex issue so it's seems reasonable to take an interest particularly as we will be the end users

 

[Nostrum]

What do you class as slow and how big is the bath? The true answer is there somewhere. As already said, take into consideration the minimum output of the boiler for heating. when you have a small heat load after half the radiator valves have shut down you may find the boiler even at minimum burn cannot dissipate the heat provided quick enough.

 

[chris watkins]

Yes it will be a wall mounted combi, I would hate to settle for the smaller output if at some stage later I found that it was a bit slow to fill the bath, that's why I asked the question really.
I was recommended a 30Kw some months ago but just want to be sure.

I think you get a flavour of the problems on the heating side from what has been posted so far (most combination boilers can be adjusted down for the heating) as far as the hot water is concerned you pays your money & takes your choice, the extra 5kW's will not effect the efficiency too much, but I would be checking with your plumber how much it will be costing to up-rate your gas supply to the new boiler & wether your mains water supply will provide the boiler's stated flow rates (which could be used to calculate how long it will take to fill your bath). Me personally, I would not be fitting a combi into a property such as yours, I prefer to store my hot water, IMHO combi's are only suitable for smaller houses & flats.

 

[AJ16]

I think you get a flavour of the problems on the heating side from what has been posted so far (most combination boilers can be adjusted down for the heating) as far as the hot water is concerned you pays your money & takes your choice, the extra 5kW's will not effect the efficiency too much, but I would be checking with your plumber how much it will be costing to up-rate your gas supply to the new boiler & wether your mains water supply will provide the boiler's stated flow rates (which could be used to calculate how long it will take to fill your bath). Me personally, I would not be fitting a combi into a property such as yours, I prefer to store my hot water, IMHO combi's are only suitable for smaller houses & flats.

Many thanks for that, we have 6 bar mains pressure and a new gas pipe needs to be fitted along the outside wall anyway from the meter box round the outside wall and through to the new boiler position. In some ways I would like a storage tank but they are more expensive, take up useful floor space and of course you have to keep them heated really I think. It sounds as if a 30 or 35Kwh would do the job easily but a 35 would heat the bath water a bit faster. The price difference is not that much for the boilers so just wondering if the running costs would be about the same?

 

[AJ16]

What do you class as slow and how big is the bath? The true answer is there somewhere. As already said, take into consideration the minimum output of the boiler for heating. when you have a small heat load after half the radiator valves have shut down you may find the boiler even at minimum burn cannot dissipate the heat provided quick enough.

Yes I see what you mean about the heating, it just comes back to the bath filling thing then

 

[TBServices]

Yes I see what you mean about the heating, it just comes back to the bath filling thing then

Where is the new boiler going to be located? If out if sight you could try a Vaillant 937 which has an inbuilt 15l storage tank which really improves HW delivery and performance.

 

[AJ16]

Where is the new boiler going to be located? If out if sight you could try a Vaillant 937 which has an inbuilt 15l storage tank which really improves HW delivery and performance.

Interesting, just checked it out but probably a bit too bulky to go on the wall in the utility room. I've seen a couple of comments on the forum suggesting that combis are not necessarily the best choice these days. Is there an alternative?

 

[TBServices]

Interesting, just checked it out but probably a bit too bulky to go on the wall in the utility room. I've seen a couple of comments on the forum suggesting that combis are not necessarily the best choice these days. Is there an alternative?

Honestly the best way to go is system boiler and unvented cylinder, but floor space and price will be an issue, unvented is sometimes difficult to find a place for due to water main size/flow/pressure requirements, primary flow and returns and the discharge pipework route.

If you're starting a project from the ground up, it's often a viable option, if your house is fully decorated, sometimes the disturbance puts people off.

 

[chris watkins]

I think what AJ16 wants really, is the performance of an unvented stored hot water system with the convenience & space saving size of a combination boiler & I bet he only wants to pay the same price & the bloke in the one bed flat.

Now come on all you lot out there, you must be able to come up with a suitable product as a solution ??????

PS can you let me know if you can cos I have loads of Architects, house designers & diy'ers who have the same problem.