How to correctly size combi boilers

Over the years there have been many discussions raised in this NG about the merits and demerits of combi boilers. I don't want to go through them again except to note that the message about combi's that seems to come through loud and clear is that a poorly performing combi system is usually the result of not being spec'd correctly for the job.
The problem is that every plumber I have spoken to about replacing my existing standard, vented system always starts out with "Have you though about a combi". Never with "Well what do you want from your hot water supply".
This leads me to believe that they want to foist something onto me that generates the most profit. Therefore I mistrust combi systems because I can't be sure that it will meet my needs rather than the plumbers needs.
But that is not right at all. Anyone who has read the threads on this NG will see that there are combi installations that are bad (usually undersized for the job) and those that are good (those sized correctly).
The point I'm trying to make is where can I go to get a good, impartial recommendation about the right kind of system, and what specification that system should be? It's no good talking to a plumber because all you will get (in my experience) will be "You want a combi, gov! Sort you out a treat it will!" before they even know how many bathrooms I have, or whether I have a washing machine and a dishwasher, or how many people live there.
I don't mind installing a combi, or an unvented system, or a conventional system as long as it is sized to do the job. Can anyone tell me where to get fair, balanced advice about what will meet my requirements?
Thanks,
Dean.
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On 24 Dec 2003 03:53:19 -0800, snipped-for-privacy@bluerose.freeserve.co.uk (Dean) wrote:

It's not that difficult and you have already outlined the issues.
I would work backwards from what you want to begin with.
Assuming that you already have a system and the number of bathrooms with baths and showers that you intend to have, then you can do the measurments.
With a measured container and stop watch measure the flow rate of hot water at the bath. Are you happy with it? Does it fill the bath quickly enough for you? Do the same for a shower. As a rule of thumb, 15 litres/min of water at 40 degrees is a starting point for a shower in my opinion - I prefer 20-25. Others are happy with 12-15, and yet others with an electric shower delivering 4.
Bear in mind that in the winter, with the mains water at 5-8 degrees, the flow rate that you will get from a shower at 40 degrees matches the standard 35 degree temperature rise that a combi is specificed to produce. Are you happy with that in the light of the above?
Do you have two bathrooms and need to have them operational simultaneously? If so you need to account for that.
Would you like to have greater flow rate and pressure on anything in the bathroom than you have today?
These are really the major consumers of hot water. You can virtually ignore the washing machine because most are cold fill or where they do have both, the hot is only used for the hotter programs which are fairly rarely used by most people. The same is true for dishwashers. If they do still use hot water, it is quite little, as is what people typically use at the sink.
Once you've worked this out, you need to assess whether the mains supply can deliver an adequate flow rate. If it can't then the choice is to upgrade the supply (can be expensive or not technically possible) or to use a vented roof tank system.
You can run some combis from roof tank supplies if needed.
If the water supply is adequate, then the next step is deciding how to heat it. You can buy pretty powerful combis of 40kW or more which will deliver water at a temperature lift of 35 degrees at a rate of 20lpm or more. Manufacturer's data sheets will help you here.
There is a limit on most domestic gas supplies of approximately 60kW, so if the flow rate that you need implies a combi or combis exceeding that then you would have to use a storage system in part at least.
You could use more than one combi boiler in a very large house or use one to provide the downstairs supply and a storage cylinder to provide the bathroom supplies.
Storage systems do not fundamentally have a flow rate limit, especially if fed from a vented roof tank, but there is a finite amount of stored water. On the other hand it is stored, then that is at 60 degrees and more cold water is mixed with it than with a combi.. If you use a thermal store, then the stored water is at 75-80 degrees and warms the mains water via an efficient heat exchanger. Thus more energy is stored in a given amount of space. The other factor with any storage system is how quickly the boiler can replenish the heat. As soon as you start to use it, the boiler can start adding a lot of heat, but ultimately the stored energy will have been used up, so what comes out equates to what goes in.
If you start from your requirements and work from there, it is not difficult to arrive at what will match your needs. There is really no magic in any of this - it ultimately comes down to the simple physics of heating water and for that the formula
Energy gained = mass x specific heat x temperature rise
gives you what is theoretically possible.
You can also easily work out the resulting temperature of mixing water of different temperatures together, Again you can decide whether you want to design for worst case, or whether you will accept a lower flow rate from a combi in the winter should you choose one that will deliver the goods for the rest of the year.
I haven't considered space use, because you have indicated that you have no preference between types of system, and I assume that you do have space for tank and cylinder if that is the most appropriate for your needs.
.andy
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In an earlier contribution to this discussion,

Neither have you considered space *heating* requirements. There seems to be a general assumption with combis that if you size them to deliver sufficient hot water, they will automatically heat your house ok. Nevertheless, you really need to validate this - as for any type of boiler - by calculating heat losses for each room and adding them up to arrive at the total space heating requirement. If this turns out to be bigger than the water heating requirement, you will need an even bigger combi!
--
Cheers,
Set Square
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On Wed, 24 Dec 2003 14:30:50 +0000, Set Square wrote:

IMHO when a home is so big that a 24kW (smallest combi)[1] boiler is inadequate for space heating it will invariably be wrong for a combi to be installed.
Every aspect of the house must match the overall size and status of the building and that includes the plumbing. It's a bit like putting vinyl seats in a Roller/Merc/BMW/Jag.
[1] OK there are/were some 18kW units on the market.
--
Ed Sirett - Property maintainer and registered gas fitter.
The FAQ for uk.diy is at www.diyfaq.org.uk
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wrote:

Agreed. I hadn't tackled that aspect nor did I go into the merits of condensing boilers since I wanted to cover the ground with hot water without diluting the content too much.
.andy
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"Andy Hall" wrote | I wanted to cover the ground with hot water | without diluting the content too much.
Very good :-)
Owain
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Remember that it's not just the size of the boiler that matters, but both the gas and water supplies into it - and the water feeds out. And these can cost *far* more to update than the difference in price between boilers.
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*Growing old is inevitable, growing up is optional

Dave Plowman snipped-for-privacy@argonet.co.uk London SW 12
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Very sensible questions. I eagerly wait on seeing a few "nonbiased reply's
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I've given you one. But consider it one of *replies*.
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*If PROGRESS is for advancement, what does that make CONGRESS mean?

Dave Plowman snipped-for-privacy@argonet.co.uk London SW 12
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On Wed, 24 Dec 2003 03:53:19 +0000, Dean wrote:

I'm not sure that has been said at lest by a majority of posters. Probably something more like: "In all but the very biggest homes a combi boiler will be adequate for the heating but you are well advised to buy a quality unit."

This is not the best way to run a heating business IMHO.

Yep, less work to install and much less external controls to wire up.

As a regular poster that has not been my impression, but it is possible that I have skipped over the threads that looked like plain "to combi or not to combi", since that is well covered in the FAQ.

Try the main FAQ of uk.d-i-y
-- Ed Sirett - Property maintainer and registered gas fitter. The FAQ for uk.diy is at www.diyfaq.org.uk Gas fitting FAQ http://www.makewrite.demon.co.uk/GasFitting.html Sealed CH FAQ http://www.makewrite.demon.co.uk/SealedCH.html
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I have not read previous posts on the subject (but I have read the faq). So here are my thoughts:
The size of a boiler needs to be matched to the size of the radiators etc to which it will be attached.
The heat loss from each radiator depends on its area. If you cant find this in a radiator catalogue or else where try a factor of 170 Btu/sq ft /hour
You can also work out the heat loss from the pipe work. Say 10 Btu/hour per foot lenght for 15mm piping.
Do you have an indirect cylinder and will you be keeping it? A ball park figure for an indirect cylinder is 12000 Btu/hour
Calculate the total heat loss and then maybe add on a safety marging say 20% and this will give you an idea of what size boiler to use. The look to see what size boilers you can buy and what the difference in price between a small one and a larger one is. This will give you and idea of what to do. Also how big is the boiler that you have now? does it run continously on a very cold day or does it spend time switched off whilst the radiators cool the water/boiler to the point where the boiler will start gain.
Alternatively get a few quotes and look what size of boiler is specified.
You might also want to consider a high efficiency condensing boiler. Do you want to dispense with your expanion tank and hot water tank )which I assume that you have now)
Happy Xmas Michael Chare
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On Wed, 24 Dec 2003 18:28:40 +0000 (UTC), "Michael Chare"

It needs to be rated to exceed the sum of the radiator outputs plus a reasonable margin - typically 20% unless the house is exposed, the ceilings are high or the heating runs for short periods There are uplift correction factors for this.

It also depends on the type of radiator, single/double panel, fins or not and the model. You can't just pluck numbers out of the air - they will probably be wildly inaccurate, BTUs are really a deprecated unit. It is far easier to work in watts, metres etc.
The heat losses for each room should be calculated and radiators sized to match those losses, assuming -3 degrees outside temperature, choosing the appropriate derating factor to account for the flow and return temperatures from the boiler.

This is generally not a significant factor and if the pipes are within the building envelope it simply adds to the radiator outputs. Outside the envelope or in places where heat is not wanted, the pipes should be lagged with insulating material of at least the same thickness as trheir diameter.

That's old practice and at 3.5kW will lead to very poor hot water recovery performance. Best practice nowadays with an indirect cylinder is to use one with a fast recovery heat exchanger and then to use a diverter valve or switched zone arrangement from the boiler. When heat is required to reheat the water, the boiler is switched over and runs at full output for a shortish time. The cylinder can absorb the full output which may be 20-30kW. This way the boiler runs more efficiently, the hot water is recovered more quickly and the boiler is off line from the heating for a minimum period.

Quit a lot of modern boilers, both condensing technology and not, are able to modulate their output dynamically to match the heat demand rather than switching off entirely. This makes matching to heat loss less critical as long as the boiler is not grossly oversized,

.andy
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in
I didn't I took it from a reference book, and I suggested using the figure as the less preferred alternative.

Still surprisinly common in the heating industry. The new Potterton boiler catalogue that I happen to have gives the boiler ouput in Btuh/hr with the kW figure in brackets!!
Michael Chare
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On Thu, 25 Dec 2003 00:54:05 +0000 (UTC), "Michael Chare"

It's very less preferred. If you compare a single panel with a double convector (e.g. in Myson's Premier range) the range is 3:1.

Mmm. One noted for its conservative ways. Many of its practitioners do not seem to be able to able to do arithmetic anyway.
I recently helped a neighbour with selecting an installer for a replacement heating system. It had been badly neglected over the years, and one or two radiators had sprung a leak. The Glow Worm boiler was at the end of its useful life (if it ever had one in the first place. Essentially it was replace all the system except the pipework.
We put together a requirements document to give to the contenders including a number of points:
- Description of the house and exisitng system - List of requirements for heating operation - List of requirements for hot water, including number of bathrooms, pattern of use.
we asked contenders to provide:
- References of installations that we could contact - Details of proposed systems, vendors used - Evidence that the proposed systems could meet the specifications.
The six responses made entertaining reading, two contenders having decided not to quote.
Two of them totally ignored the requirements document and proposed a fairly small combi boiler - 15lpm. This was totally inadequate for the hot water needs and they hadn't bothered to check that the mains cold supply is poor where we are. They had both proposed large double convector radiators in places where it was not necessary.
Another two proposed a pressurised hot water cylinder, but didn't check the mains supply. They did bother to do so form of heat loss assessment but one got the numbers wrong and proposed radiators in a couple of rooms which were too large and the pipework would have been inadequate to supply. They did get the boiler size about right though.
The final two proposed a vented storage cylinder, with fast recovery, larger than the existing one to meet the demand. They both presented detailed heat loss figures for each room, a radiator proposal and pointed out that in one large room the pipework would need to be upgraded to support the radiator's requirements. Both produced references.
In the end he selected one from the third group, the lower price of the two, and because the person was able to do the work a little earlier as well. These two were not the most expensive nor were they the cheapest. The most expensive proposal was in the first group and was the least suitable. It was from a very well know national firm.

I'm not surprised. Potterton has been noted for trading on its past glories and producing junk products. A search through Google Groups will reveal some of their more recent horror products.

.andy
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In an earlier contribution to this discussion,

I'm sure it's good for marketing hype too! Since 1 kW is equivalent to about 3.4 kBThU/Hr, you get a number that's 3.4 times as big (or even 3400 times as big - depending on how you express it) if you quote the output in old units.
For example some may be misled into believing that 68,000 BThU/Hr is better than 20 kW!
--
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Set Square
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Maybe because a double convector (i.e. with some type of fin arrangement in the middle) has a correspondingly larger surface area when compared to a single panel no fins radiator.

snip
Interesting story the OP should note!
And these installers can probably work on your plumbing etc without using a Building Control Service because they are members of the Institute of plumbers where as us DIY people can not. (Depending on the exact job).
Michael Chare
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Unless its a Baxi Barcelona which performs perfectly (touch wood) for 9 months then corrodes away and has to be chucked away
<snip>

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Andrew

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