Combi boilers - why?

Hi,
just catching up on the NG after a few days away.
Currently there seem to be several threads about problems providing hot water at a decent flow rate with combi boilers.
Now I can see that with a good high water pressure and flow rate, and a limited number of outlets, the combi boiler allows you to install a domestic hot water system with a much reduced component count, saves space previously used by header tanks and hot water cylinders, and can give you a nice hot shower without needing a pump.
However with low water pressure and/or a large number of outlets (and according to the TV shows a house is not a house without at least one en-suite in addition to the family bathroom) the combi starts to struggle.
Fixes suggested include installing a header tank and pump for additional cold water pressure/flow and adding a hot water storage tank to provide enough hot water at peak times.
Once you start down that route I must ask "Why was a combi fitted in the first place"?
Seems to me that at the moment the automatic design for a new system is to use a combi boiler.
Surely this is best for flats and small houses, and any reasonable sized house should be designed with hot and cold water storage because the peak demands cannot be supplied by heating water 'straight from the main'.
Or am I missing something :-) [Hmm...cheaper option for the developer?]
Cheers Dave R
[Also noted comments about 'high output' combis which need so much gas that they need a larger gas pipe and can compromise pressure for other customers. In this situation a slower heating coupled with storage of hot water seems a more sensible option. Peak flow of gas and water must limit to the amount of hot water you can supply through an 'instant heater'. Then again, how about a pumped gas storage tank to supply extra gas at peak demand without compromising the main supply - design for a mini-gasometer anyone? Hmm...compressor fed from the mains to fill a high pressure tank with the outlet pressure reduced back down to mains gas pressure but through a large bore pipe?]
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On Fri, 29 Aug 2003 11:15:36 +0100, "David W.E. Roberts"

True, but there is nothing magically new - it is really the 21st century version of the Ascot. The principle, benefits and limitations are the same.

I think that that is one point, as is the ability to use less skilled labour because the pipework is simplified to an extent.
Provided the mains water and gas supplies are adequate (which should be possible on a new build and the boiler is specified properly, there should be no problem, although as you've spotted the amount of heat required to raise the temperature of a flow of water 'X' by 'Y' degrees can exceed the ability of a domestic gas supply if X and Y are high and within the requirements of a medium to large house.
The developer can claim to have provided the hot water service in the same way that they can claim to have fitted enough power sockets. They are there but may well not be adequate for the purposes of the occupants.
The more problematic area is with replacements in houses already there. In mine, which is only 18 years old, the water supply would not be adequate for a worthwhile mains fed HW system without upgrading the communication pipe from the main - a cost of about 5k. People are sold mains systems, and especially combis on the argument that they can have high pressure and flow showers etc. etc. without the obvious service checks being made. Then they find that the results are not as good as they had with their storage system.

.andy
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wrote:

18 years old and you don't have decent cold mains supply. That is bad.

This is true. And the same is so when people install a power shower pump and exhaust the cylinder of hot water in a few minutes. Then a larger cylinder is required and the heat up time may be sluggish as the boiler may be too small as well. A professional should assess the customers requirements and the suitability of the services available.
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Surely that is the point. There must be a level of demand (mainly judged on house size and number of bathrooms) which cannot be satisfied by the average mains water supply (and possibly gas).
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Once into very large domestic houses you are in a grey area of commercial systems. Many large houses do have Commercial gas supplies and water/heating systems.
BTW, there are commercial combi boilers, which give 40 litres/minutes and above, etc. Some are fitted in banks. The reason they are used is that the users have a continuous demand for hot water. Take a health centre for e.g., people are coming and going all the time, so having stored water would entail a "very" large space consuming cylinder and boiler(s) for the showers. Having combi's which will produce 80,100-120 litres min for the showers is ideal as they never run out of hot water. the control system can be arranged to bring in boilers as demand rises. The ideal solution for that problem.
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Is that because they are laundering the money?

.andy
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wrote:

ROLF. You must watch Last of The Summers Wine.
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In David W.E. Roberts typed: : Hi, : : just catching up on the NG after a few days away. : : Currently there seem to be several threads about problems providing : hot water at a decent flow rate with combi boilers. : : Now I can see that with a good high water pressure and flow rate, and : a limited number of outlets, the combi boiler allows you to install a : domestic hot water system with a much reduced component count, saves : space previously used by header tanks and hot water cylinders, and : can give you a nice hot shower without needing a pump. : : However with low water pressure and/or a large number of outlets (and : according to the TV shows a house is not a house without at least one : en-suite in addition to the family bathroom) the combi starts to : struggle. : : Fixes suggested include installing a header tank and pump for : additional cold water pressure/flow and adding a hot water storage : tank to provide enough hot water at peak times. : : Once you start down that route I must ask "Why was a combi fitted in : the first place"? : : Seems to me that at the moment the automatic design for a new system : is to use a combi boiler. : : Surely this is best for flats and small houses, and any reasonable : sized house should be designed with hot and cold water storage : because the peak demands cannot be supplied by heating water : 'straight from the main'. : : Or am I missing something :-) : [Hmm...cheaper option for the developer?] : : Cheers : Dave R : : [Also noted comments about 'high output' combis which need so much : gas that they need a larger gas pipe and can compromise pressure for : other customers. In this situation a slower heating coupled with : storage of hot water seems a more sensible option. Peak flow of gas : and water must limit to the amount of hot water you can supply : through an 'instant heater'. Then again, how about a pumped gas : storage tank to supply extra gas at peak demand without compromising : the main supply - design for a mini-gasometer anyone? : Hmm...compressor fed from the mains to fill a high pressure tank with : the outlet pressure reduced back down to mains gas pressure but : through a large bore pipe?]
As a house owner with an old boiler, header tank and all the rest of it, I'm terrified about changing the thing to a "combi". But all the plumbers I talk to want to sell me one, & did notice that they only want to sell a Combi. It does ratherm remind me of the endowment sales of the 1980's.
The problems asked about Combi's in this NG must be the highest thread count here? It makes me wonder what the rest of the population who don't post here suffer from with CH and Combi's? It's certainly enough to make me never want one. They do seem to be easy fit/fat profit devices, but I'm having a cynical day so do excuse me if I'm talking shite.
I still don't ever want one of 'em. -- ctc Once, I couldn't spell 'engineer'. Now I are one.
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"Crippen" wrote | As a house owner with an old boiler, header tank and all the | rest of it, I'm terrified about changing the thing to a "combi". | But all the plumbers I talk to want to sell me one, & did notice | that they only want to sell a Combi. It does ratherm remind me | of the endowment sales of the 1980's.
Or Ted Moult telling you "you only fit double glazing once"
| I still don't ever want one of 'em.
I don't want one because there's been more than one occasion I've been very grateful for having a tank of stored hot(ish) water, whether when the boiler fails (or in my case got Condemmed by Mr Corgi) or the electric goes off.
Owain
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Absolutely. Maximum profit for minimum work.
If you have a storage system which works well stick with it - the hard work and expense has already been done. With a clean sheet it might be worth investigating the options, though.
IIRC, if 'normal' boilers become difficult to get when yours needs replacing, there's no reason why you can't use a combi as one - and say have a feed direct to one hot tap in perhaps the kitchen. Then you'd have the best of both worlds.
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Dave Plowman snipped-for-privacy@argonet.co.uk London SW 12
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Look at my posts on combi's, they dismiss old wives tales on them. Over one million boilers per year are fitted in the UK. 60-70% are combi's. In a few years time they will be the majority of the installation base. The problem with combi's, well it is not a problem, is that people do not fit the correct models to suit demand. There are high flowrate models around that will satisfy your needs in the average house.
People go on about them being complex. this is tripe as they as similar to a basic system boiler inside, with a water section added. Many are just adapted system boilers. Combi's are very cheap for what they are. So cheap it is feasible to install two to double the flowrate, zone off the CH system and provide backup.
Like any other product on the market, there are better makes than others. If the flowrate meets your needs, then the advantages far outweigh the disadvantages.
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