Combi Boiler Size

[PaulC 13]

Hi All
I'm a newbi to this forum and would like your advice. I'm planning a new Combi central heating system. I have worked out the heat requirement for the house (A 1890's 3 bedroom end terraced house). The heat requirement is 34,000BTU or 10 KW. I have been told that I do not have to add any extra BTUs for water heating in a Combi system. If this is correct the boiler appears rather small. Have I got something wrong?
Thanks
Paul

 

[mojorisin]

Hi Paulc I'm not a Plumber, but just thought this might help. If you take your room sizes, window sizes ( in that particular room), wall construction how many doors or fireplaces in the room down to you local builders' merchants' they should be able to calculate your exact requirements for boiler size and radiator size, thats if your having radiators!
Cheers
Ps I don't know if you have a Grahams near you, but I found the one in lincoln to be cracking on Prices.

 

[so-solar]

There are lots of other factors to consider when sizing a boiler for a particular property

design internal temperature of 21°C (included in location factor).

design external temperatures, dependant on the location of the property (included in the location factor).

an allowance of 10% for intermittent heating (included in the location factor).

an allowance of 5% for pipe losses (included in the location factor).

a ventilation rate of 0.7 air changes per hour (included in the 0.25 ventilation factor).

an allowance of 2 kW for heating hot water.
TABLES

Factors used

Window Factors

Detached 0.17
Semi-detached 0.2
Mid terrace 0.25
Flat 0.25

Window U-values

Double glazed wood/plastic 3.0
Double glazed metal frames 4.2
Single glazed wood/plastic 4.7
Single glazed metal frames 5.8

Wall U-values

Filled cavity wall 0.45
Unfilled cavity wall 1.6
Solid wall 220mm 2.1

Roof U-values

Pitched < 50mm insulation 2.6
Pitched 50-75mm insulation 0.99
Pitched > 75mm insulation 0.44
Flat uninsulated 2.0
Flat 50mm insulation 0.54

Location Factors
North & Midlands 29
Northern Ireland 26.5
Scotland 28.5
South East & Wales 27
South West 25

This is a quick guide to the factors.

It is far better to oversize because if you put a boiler undersized in a property it will be working flat out and therefore the cost of running will be very high

hope this helps

 

[Dotty]

In addition to what So-Solar has said, you also need to consider mains water supply flow rates and pressure.

Get a few quotes from registered engineers in your area, get them to size and recommend a boiler for you. And make sure you give that guy the job!

Nothing p***ed me off more than when i'd put together a quote (including talking to insurance companies in a couple of cases!) only to find the custard had taken my advice (and the insurance money) only to get their family to fit the thing!

And that's why I gave up my personal PLI and no longer do private work!

 

[so-solar]

I am finding this is becoming more common croppie, i try and keep my cards a little closer to my chest and do not give to much information away, it also seems alot of customers are supplying there own boilers,rads,pipe and fittings etc, and just want a quote for labour only, but the customer thinks that they have all of the fittings only for me to tell them they need to spend a couple of hundred more on bits, they then ask what bits! it goes on and on, in the end you are running around for them and they moan when you invoice them for extras!

 

[PaulC 13]

Hi
Thanks for your prompt replies. Usefull list of U values from So-Solar. I got mine of the net but I'm sure others will find them usefull. Sorry to see Croppie's comment. I like to think I'm always up-front when asking advice from porofessionals. And my experience has been that most are more than willing to give advice for free even if they know that's all you are wanting. And I will have a look at Graham's, mojorisin. The boiler I'm going for is a Glowworm Betacom 30C Combi Boiler. Found one for £509 inc Vat which appears good value.
Thanks
Paul

 

[plumbster3456]

I am finding this is becoming more common croppie, i try and keep my cards a little closer to my chest and do not give to much information away, it also seems alot of customers are supplying there own boilers,rads,pipe and fittings etc, and just want a quote for labour only, but the customer thinks that they have all of the fittings only for me to tell them they need to spend a couple of hundred more on bits, they then ask what bits! it goes on and on, in the end you are running around for them and they moan when you invoice them for extras!

tell people like this where to go I get them all the time can i supply my own boiler? no i dont do this as I dont fit those cheap and nasty biasi boilers that they think are a good one when i could probably get a *A* RATED VOKERA BOILER for the same price or stick another couple hundred and go with a worcester boiler.
I sometimes go to fix a boiler and tell the customer for example it needs a new p.c.b. how much are they ? 200 pound oh no thats too expensive.
I say well what do you want me to do then? they say just leave it i charge 45 pounds they then go and i know this for a fact and get another gas engineer out to give a second opinion in the hope the job can be done cheap and he will say the same haha and charge them again to be told the same thing this always makes me smile as in the end up they are out the 200 pound for the part plus 2 engineers call outs plus whoever will fit the new p.c.b. for thems labour happy days serves them right for being so tight fisted I actually enjoy this (which maybe makes me sick and twisted lol)

 

[PaulC 13]

Oh dear, you realy don't like people who try to find a less expensive option, and that must include me as I fitting my own system. Although the real plus is not saving money, but that helps, the real buzz is creating something tangable. In my day job I don't create anything that at at the end of the day I can look at and say, "I made that". You guys don't realise how lucky you are. If I had my time again I would have taken the path of a trade, but I'm too old for that. So I buy the tools, read the books and make things in my spare time. As I said before, sometimes I need the help and advice of professionals and thankfully in most cases it has been given willingly.
Paul

 

[metaldust9]

personally i think your going a bit deep into it here if you go of the btu rant above you could end up putting in a 24 kw and not be enough id just go for something along the lines of a 28 kw to 33 kw ie baxi duo tec which will be ampal the 28 kw will give you plenty of heating and good hot water 33kw will give you well over for the rads but also brill hot water and scope to add on at a later date if you was to change a bathroom or something thats what i do with my mate and we never go far wrong

 

[doitmyself]

I'm planning a new Combi central heating system.

The size of a combi boiler is determined by the Hot Water requirement; this is because the water has to be heated instantly, raising the temperature from about 10C to 45C. You can calculate the HW requirement if you know the cold water flow rate into the house (check at the kitchen sink). You need 2.5kW for every litre per minute to raise the temperature by 35C. So, at a flow rate of 10 litres/min, you would need a 25kW boiler.

I have worked out the heat requirement for the house (A 1890's 3 bedroom end terraced house). The heat requirement is 34,000BTU or 10 KW. I have been told that I do not have to add any extra BTUs for water heating in a Combi system.

How did you calculate the heat loss? The Boiler Size Calculator is the recommended method.

So you will get two figures, e.g: Hot water - 25kW; heating 13kW. Now comes the tricky bit: choosing the correct boiler. You have to look at the HW performance and the heating performance of the boiler. It's easy to find a boiler which provides 25kW for HW and 13kW for heating but the heating requirement is not constant, you only need 13kW in the coldest weather. Most of the time you will need less. So you have to look at the modulation range of the boiler, stated as, say, 10-25kW. The lower the boiler can modulate compared to your heating requirement, the better.

Taking your 13kW requirement as an example, a boiler which modulates from 5kW to 15kw is better than one which goes from 12kW to 30kW.

 

[hihowudoin]

The boiler I'm going for is a Glowworm Betacom 30C Combi Boiler. Found one for £509 inc Vat which appears good value.
Thanks
Paul

Building Regs Part L from 1st October 2010
For existing housing stock, the new regulations are even more prescriptive as installers are no longer able to specify B-rated condensing boilers. However, installers can continue to fit band B boilers until April 2011 providing the order was taken before 1 October 2010.
Betacom is Sedbuk B rated so you may have trouble finding a GSR engineer to fit and notify it.

 

[Blackcatgas]

Building Regs Part L from 1st October 2010
For existing housing stock, the new regulations are even more prescriptive as installers are no longer able to specify B-rated condensing boilers. However, installers can continue to fit band B boilers until April 2011 providing the order was taken before 1 October 2010.
Betacom is Sedbuk B rated so you may have trouble finding a GSR engineer to fit and notify it.

Its ok that doesnt apply to him as hes doing it himself.

 

[Avatar]

Unless you are experienced in fitting heating systems, and plumbing you are likely to come unstuck in a hundred different ways on this job, which is not something you can do out of a 'book'.

I did a full city and guilds craft certificate, but still it was a couple of years before I felt cvonfident enough to fit heating systems, and thats working alongside a gas safe guy, who fitted the boiler ect!

What you might save paying a plumber to do a good job, which will be leak free for years to come, you may well spend just on repairs due to water damage alone, in the worst case . . .

Give it some thought!

 

[boreas]

Following this forum from the other side of the Channel, sometimes I have a feeling that you are undersizing heating. The coastline of Holland is comparable to SE of UK and lets now talk about renovation or refurbishing:
Boiler capacity: total volume in cubm x 80W if indirect cylinder, min 15-20% if combi.
Radiators: sleeping, landing, passways 70W/cubm, living, kitchen 80W/cubm, bathroom 100W/cubm, as well always the next in the capacity range.
As as engineer I am absolutely not against a correct heat transfer calculation and did it as manually as with the computer when it became available in late 80's, early 90's, but sometimes the customer was not really satisfied with the living/open kitchen or bathroom. A new development is of course different, all insulation values are given, no doubt about it, but it was a returning job after the first winter, in new neighborhoods to change radiators for a larger capacity. When the condensing boilers came to the market, the larger radiators with lower water temperature also provided even more efficiency.
Always advised customers if there was any discussion about rad sizes:
I can install you the smaller one, you'll save 25 euros and your cabinet fits correctly, but if suddenly 20 degrees below zero outside and you feel cold, I'll certainly do not come back for 25 euros with a larger one!
What is your opinion? Are we oversizing here or you are undersizing there?