Combi Boilers

Hi,
I've read a few posts about the positive/negative points of combi boilers. I'm just selling my house which had an almost new combi boiler when I moved in and to be honest I have been less than impressed by it. I did speak to my neighbour who was a plumber and mentioned this to him, he seemed to think that the Ariston that I had was not particually good anyway.
I did not like that when only the hose pipe was on, its pressure was useless. If I took a shower and either the washing machine or dishwasher was on then the shower kept cutting out. Also quite often it failed to start up.
Anyway the point of my post, we have found a house we like but it has a combi boiler installed (in the loft ?), dont know what make it is but my question is, is it more than likely to still have the same problems as this is how combi boilers work ? Also its currently a 3 bed house with 8 rads, we will be extending to make a fouth bed room and bigger kitchen so that will probably be 10 rads in total. Would this be to much for a combi boiler ?
If so then what are my options of removing the combi boiler and replacing it with a conventional system ? Could everthing be placed in the loft still ?
Sorry for the long post, many thanks.
Marv.
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On 1 Nov 2003 06:25:40 -0800, snipped-for-privacy@btopenworld.com (Marv) wrote:

There are good,bad and indifferent combi boilers as with everything else. If you really must have lashings of hot water ,on demand and at mains pressure then consider fitting a pressurised hot water system which is heated by a high efficiency conventional boiler.
joe

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On 1 Nov 2003 06:25:40 -0800, snipped-for-privacy@btopenworld.com (Marv) wrote:

It is typically not the heat output to radiators that is the problem,
There are three potential downfalls of a combi:
- Inadequate hot water production rate for the requirements of the bath(s) and shower(s) in concurrent intended use. They are normally specified with a rate in litres per minute for a temperature rise of 30 or 35 degrees. In cold weather when the water is at only a few degrees above freezing, a shower (40 degrees) will be limited to whatever the combi can do. Small ones with 9-11 lpm are going to be disappointing.
Larger size combi boilers are available with outputs of 20 lpm or more, although upgrade of the gas supply to the boiler may be required if the pipe length is long or too small.
- Inadequate flow and pressure from the main, especially in an older property. This can be measured at the kitchen tap. If you are getting less than 20 lpm or so then results in general will be poor. There are then two alternatives. a) Pay for the main to be upgraded, which can be expensive. b) Install a conventional system with tank in roof.
- Poorly implemented internal plumbing. This can be through re-use of plumbing from an older roof tank/ gravity system providing inappropriate distribution of water to the various taps and appliances. It may be possible to correct this with flow restrictors but often involves some replumbing or additional plumbing so that feeds to certain locations are brought back to a common point.

Yes it could. The appropriate solution would rather depend on the limitations if any of the system.

.andy
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There are few negative sides to a combi. the dissatisfaction arises when they not sized or installed correctly. Fitted with a poor flowrate or with an inadequate mains will not nor perform to expectations.

If your cold water mains can't cope, combi's are not the ideal solution as they are mains fed. It appears your mains pressure/flow is poor. And that goes for any mains fed system like Megaflows and thermal stores. Although with poor mains pressure you can run a combi off a tank and pump in the loft and save cylinder space in the tiny house below - sorted. This may be an interim solution until your mains is updated.
They can go up to 22 litres/minute if you want. The average combi's are the 10 litre/minute cheapo's. For "most" households they the best solution in simplicity and importantly, far less space taken up in tiny British homes. Around 550,000 fitted per year says it all. For the majority of British homes combi's are a panacea. They also heat the house up "real fast". They are the ideal choice for DIYers when selecting a fully specced idiot proof job like the Ariston Microgenus (far better than the earlier Ariston models). BTW, A Microgenus 2 has just come out with 3 outputs of 24kW, 27kW and a 13+ litres/minute 31or 32kW job .

Why not test it. Runs a few hot taps at once. It should have "decent" flow with the kitchen and basin tap running at normal flow (not all full on).

A combi will cope with the heating side. They are over-powered for most heating needs, as the instantly heating hot water takes a lot of power, so they are sized for the hot water side.

A conventional systems in the loft is difficult, and probably not achievable, and will entail a lot of upheaval. You would be better having a new larger bore plastic water main from in from the street.
Depending on the cold mains pressure/flow. If this is fine and the combi is not man enough for its intended hot water use. The best solution may be to install another combi. Yes, another combi. Combi's are cheap. Have one do downstairs heating, one upstairs, on two different time clocks. This means upstairs can be off while downstairs is on, or vice versa. Then split the hot water taps between the two combi's (have two showers on two different combi's). A new gas supply will have to be run from the combi back to the meter.
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>

Hi,
Thanks for the feed back, that makes sense. Must be a poor cold water mains that I have. The model I have here is a Microgenus. I have since visited the new house and the water is 10 times more powerful than I have, thats restored my confidence in Combi's. The idea of 2 combi boilers one for upstairs and one for downstairs is also very interesting, especially when the washing machine/dishwasher is on. Just a thought here and before you say I'm daft I work in computers :), I was wondering if it were possible to join the 2 systems together on some sort of switch, for example if the 2 systems were joined but shutoff from each other in the event of one breaking down would it be possible to open the switch to make them one system ?
Anyway thanks again for your help and advice.
Marvin.
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This model operates on low water pressure, a reason why it may have been chosen. It is also small and reliable.

Yes. But! "most" makers don't like you to combine the outputs, as they don't want their technical depts answering queries all day from poor installations. Technically there is no reason why you can't using non-return valves.
One combi can serve a section of taps and the other another section to spread the load independently and provide backup if one fails. But you may need both combi's outputs to say provide a good flow to fill a bath and/or shower.
Say you have two bathrooms and a shower in each. One combi does one bathroom the other the other bathroom. Simple. After the washbasin and before the bath and shower draw-offs of each bathroom join the pipes using a few tees. "Before" the tee offs and after the basin draw-offs in each bathroom, fit spring loaded non-return valves.
The way it operates is that the taps before the non-return valves operate off separate combi's. Obvious. When any of the two baths or showers are opened "both" combi's operate giving high flow.
If one combi fails then at least heating is available on another floor. One bathroom will work off the remaining combi. This way is foolproof and requires no user intervention.
For backup heating joining the two combi's heating circuits and using full bore valves to open one and close the other is the way. But you need to know what valves to open and close.
You could have both boilers on the heating circuit and split the circuit to upstairs and down using zone two 2-port zone valves. A single pole double throw wall switch (like a light switch)can have either boiler on run and the other off. One drops out, press the switch and other operates. This gives full backup, but the expensive of two extra zone valves.
In your case, adding a combi is not a great expense or upheaval. Just make sure you do not exceed the meter loading. You are allowed 212 cu foot per hour. divide the BTU/h of the combi by 1000 to get the cu ft/hour. A 80,000 BTU/h combi consumes 80 cu foot, so two is 160. Give enough for your gas hob too.
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