Boiler size

[Jonnie1066]

Hi can any one tell me what size combi boiler I will need for a 5 bed 2 bathroom sized house. Also has utility room and downstairs wc. Was told above 30 kw boiler . If anyone can help thanks

 

[Skelly87]

Good luck getting advice on that

 

[AlexGas]

I would not recomend a combi with your set up.

 

[Ray Stafford]

Hi Jonnie

There are two issues here, space heating and hot water. It sounds like a big house, and although a 30kw+ combi may well be fine for the heating, if it is really large, badly insulated, detatched and on a windy hill in scotland, then it may well not be fine. You need to talk to your gassafe registered installer who will be able to do the heat loss calculations for you.

The hot water is not really related to the size of the property, it is about the number of hot water outlets, and how many of them you want to use concurrently. Even the highest output combis will struggle to run two baths at the same time. The best that a top-end combi will achieve is moderat-ish flow from two 1/2 outlets (like basin taps). The vast majority of combis are designed to run a single outlet.

If you have plenty of flow and pressure coming into the building on your cold main, then your best solution is a boiler properly sized for the property, and a largish (say 250ltr or 300ltr) unvented indirect cylinder.

If either the flow or the pressure of the cold main is lacking, then you will need to store some cold water in a tank in the roofspace, with an ordinary indirect cylinder and again a boiler properly sized. If you want more pressure, say for a shower, add a pump, but be prepared to upgrade your cold water storage - at least 50 galls, preferably more if pumped.

This is very difficult to do over the internet - you really need to get a good engineer to visit the property and advise you.

 

[Guest123]

Welcome to the forums, but please read the big bold announcement at the top of the plumbing forum page:

Announcement:Please do not post anything gas related or heating related on here!

Read more: Plumbing Forum

Your post has been moved to a more suitable location in the Central Heating Forum and as long as your questions are not gas related, you will get answers there.

 

[TBServices]

Hi Jonnie

There are two issues here, space heating and hot water. It sounds like a big house, and although a 30kw+ combi may well be fine for the heating, if it is really large, badly insulated, detatched and on a windy hill in scotland, then it may well not be fine. You need to talk to your gassafe registered installer who will be able to do the heat loss calculations for you.

The hot water is not really related to the size of the property, it is about the number of hot water outlets, and how many of them you want to use concurrently. Even the highest output combis will struggle to run two baths at the same time. The best that a top-end combi will achieve is moderat-ish flow from two 1/2 outlets (like basin taps). The vast majority of combis are designed to run a single outlet.

If you have plenty of flow and pressure coming into the building on your cold main, then your best solution is a boiler properly sized for the property, and a largish (say 250ltr or 300ltr) unvented indirect cylinder.

If either the flow or the pressure of the cold main is lacking, then you will need to store some cold water in a tank in the roofspace, with an ordinary indirect cylinder and again a boiler properly sized. If you want more pressure, say for a shower, add a pump, but be prepared to upgrade your cold water storage - at least 50 galls, preferably more if pumped.

This is very difficult to do over the internet - you really need to get a good engineer to visit the property and advise you.

Great advice from Ray here.

 

[ringi]

I am a computer programmer not a plumber, but am going though the same exercise for if/when we replace our boiler.

Firstly lets think about heating, the energy saving trust has details of how to calculation you’re heating needs on their website; however you need to know how good your insulation is. Cavity wall insulation is not very predictable, as it depends on how clear your cavities were. If you’re current boiler gives you enough heat that provides some good information on your heat needs.

If the maximum heat output of your chosen boiler is too low, it will take a lot longer to heat up your home and you may not be able to maintain the temperature you want. However you may have a gas fire in your living room you can use to top up on the few really code days each year, or just leave the system running 24hr a day when it is really cold.

If the minimal heat output of your chosen boiler is too high, then it will not run as efficiently it may also be hard to maintain an even temperature, as the boiler will only be one for a short time before switching it’s self off.
Often you can find a boiler in the range with a higher maximum heat output but the same minimal heat output. Having a larger range the boiler is happy to operate over is the safe option. Then put a load compensator controller on, so the boiler flow temperature is automatically matched to your heating load along with oversized radiators.

As too hot water, there are other options then a largish unvented indirect cylinder. Have a look at Your Page Title, including the GasSaver and SuperFlow, there are several boiler makers that do systems like these. These are cost comparable with a large unvented indirect cylinder and take up less space and can be more efficient – all depending on your current building layout and pipe layout. Also look at “storage combis”.

 

[Dotty]

I am a computer programmer not a plumber, but am going though the same exercise for if/when we replace our boiler.

Firstly lets think about heating, the energy saving trust has details of how to calculation you’re heating needs on their website; however you need to know how good your insulation is. Cavity wall insulation is not very predictable, as it depends on how clear your cavities were. If you’re current boiler gives you enough heat that provides some good information on your heat needs.

If the maximum heat output of your chosen boiler is too low, it will take a lot longer to heat up your home and you may not be able to maintain the temperature you want. However you may have a gas fire in your living room you can use to top up on the few really code days each year, or just leave the system running 24hr a day when it is really cold.

If the minimal heat output of your chosen boiler is too high, then it will not run as efficiently it may also be hard to maintain an even temperature, as the boiler will only be one for a short time before switching it’s self off.
Often you can find a boiler in the range with a higher maximum heat output but the same minimal heat output. Having a larger range the boiler is happy to operate over is the safe option. Then put a load compensator controller on, so the boiler flow temperature is automatically matched to your heating load along with oversized radiators.

As too hot water, there are other options then a largish unvented indirect cylinder. Have a look at Your Page Title, including the GasSaver and SuperFlow, there are several boiler makers that do systems like these. These are cost comparable with a large unvented indirect cylinder and take up less space and can be more efficient – all depending on your current building layout and pipe layout. Also look at “storage combis”.

Or you could just get a few quotes from your gsr's of choice who would have the opportunity to have eyes on your system and advise you of your possibilities.

Internet educated have a go heroes just muddy the waters.

 

[chris watkins]

I am a computer programmer not a plumber, but am going though the same exercise for if/when we replace our boiler.

Firstly lets think about heating, the energy saving trust has details of how to calculation you’re heating needs on their website; however you need to know how good your insulation is. Cavity wall insulation is not very predictable, as it depends on how clear your cavities were. If you’re current boiler gives you enough heat that provides some good information on your heat needs.

If the maximum heat output of your chosen boiler is too low, it will take a lot longer to heat up your home and you may not be able to maintain the temperature you want. However you may have a gas fire in your living room you can use to top up on the few really code days each year, or just leave the system running 24hr a day when it is really cold.

If the minimal heat output of your chosen boiler is too high, then it will not run as efficiently it may also be hard to maintain an even temperature, as the boiler will only be one for a short time before switching it’s self off.
Often you can find a boiler in the range with a higher maximum heat output but the same minimal heat output. Having a larger range the boiler is happy to operate over is the safe option. Then put a load compensator controller on, so the boiler flow temperature is automatically matched to your heating load along with oversized radiators.

As too hot water, there are other options then a largish unvented indirect cylinder. Have a look at Your Page Title, including the GasSaver and SuperFlow, there are several boiler makers that do systems like these. These are cost comparable with a large unvented indirect cylinder and take up less space and can be more efficient – all depending on your current building layout and pipe layout. Also look at “storage combis”.

It's nice to have input / views from others outside the industry who may have read or seen things about a subject they do not have a full working understanding off, that reminds me must get on that computer forum & publish my paper on how to write better code !

 

[ringi]

It's nice to have input / views from others outside the industry who may have read or seen things about a subject they do not have a full working understanding off, that reminds me must get on that computer forum & publish my paper on how to write better code !

Let’s say you need to automate some system sizing calculation. If you ask my advice you are likely to end up with a custom written program that runs on your windows laptop. Ask another programmer you may get a smart phone app or a website. Ask my wife and you will get a lot cheaper solution in the form of a spread sheet that does not have a custom UI with fancy images of radiators, but is may do the job well enough. (PS my wife is an accountant)

I see the same issue with a gsr, there is now such a range of options and most people recommend what they are most used to – just look at how long it took for unvented systems to become normal. So to get some “gsr's of choice”, the customer needs to know the options to cover then choose 1 or 2 gsr that specialize in each of them to quote.

 

[Dotty]

Let’s say you need to automate some system sizing calculation. If you ask my advice you are likely to end up with a custom written program that runs on your windows laptop. Ask another programmer you may get a smart phone app or a website. Ask my wife and you will get a lot cheaper solution in the form of a spread sheet that does not have a custom UI with fancy images of radiators, but is may do the job well enough. (PS my wife is an accountant)

I see the same issue with a gsr, there is now such a range of options and most people recommend what they are most used to – just look at how long it took for unvented systems to become normal. So to get some “gsr's of choice”, the customer needs to know the options to cover then choose 1 or 2 gsr that specialize in each of them to quote.

Already exist, in use and working well.

Designed by folks in the trade.

Difference between your job and ours is that we deal in extremes. Heating is a physiological necessity as explained in Maslow's Hierarchy of Needs.

Whereas you're just attempting to Reinvent the wheel.

 

[dancinplumba]

wonderful that john

 

[AWheating]

Hi can any one tell me what size combi boiler I will need for a 5 bed 2 bathroom sized house. Also has utility room and downstairs wc. Was told above 30 kw boiler . If anyone can help thanks

your house is likely to large for a combination boiler. A 30kw combi will give approx 10-11 litres per min hot water output, share that over a couple of taps including the cold then your get problems.

Ray and other have given good advise. Get quotes from a few local engineers to she whats best.

 

[Dotty]

wonderful that john

I like to surprise people now and again Steve lol