combi boiler/ poor pressure

[brucej]

having had a combi boiler, we discovered it is unable to supply hot water at peak times due to low mains pressure .The installer is recommending adding a tank system with pump. what options are their. Is it possible to use a tank plus pump to supply water to the boiler at sufficient pressure, and then still work as 'hot water on demand'. Is this likely to cost hundreds or thousands of pounds?

 

[COG]

Find out what the min pressure required to opperate the flow switch/div valve (should be in the boiler tech and spec) then contact your water board they will let you know what pressure you have available at the street (working pressure ie when a demand is made) if this is well above what you require then investgate your supply pipe and stopcock etc

 

[ChrisR]

Do do that, also measure your mains pressure at various times (Gage costs a tenner). Hope for 3 bar or more.

Also measure your max flow, eg at the garden tap. Hope for more than 20 litres/minute.

If there's a problem you have loads of answers possible. Suggest you try plumbing pages forum where you'll get more pros answering.

It might even be your combi at fault

 

[brucej]

Thanks for the thoughts... the pressure supply is definitely where the problem lies... (unfortunately unly discovered this after the combi-boiler was installed). British Gas who installed the boiler, came to measure the pressures and flows after we were having problems. At 08:30 the pressure was 1 bar, with a flow of only 6l/min. since this is the tail end of the peak time, it is probably lower than this at the times we get ready for work (between 07:00 and 08:30). (boiler needs min 0.7 bar, and 1.2 bar for max flow). After kicking up a fuss British Gas are making a concerted effort to help, and have agreed to replace the service mains. I'm still not sure that this will do the trick, which is why I am looking at alternatives should the mains replacement fail to fix the problem.

 

[breezer]

it may be, sometimes old water pipes get "furred up" and changing the main incoming water pipe helps.

I went to a place which had just been converted into 2 flats , bg put a combi in the 2nd flat which wouldn't work when toilet in flat 1 flushed, water board said bg are always doing this, and he had to pay (can't remeber how much but it was a lot) for a new incoming water supply, so if bg are going to do it for freewell done to you, it usually works

 

[ChrisR]

Pressure should be measured at zero flow, so making the pipe bigger won't make any difference to that. 6 l/min is low for 1 bar though

Speak to OFWAT who will tell you what conditions need to apply for you to claim off your water provider. Yours is getting close to the limits!

Do speak to your water provider, you may find that they're doing work in the area so have turned their pressure down. Ask what they think your pressure should be.

With permission you can pump the mains direct, but not beyond a piddling amount (7l/min). Then you can as you have found use a "break" tank (which could be in the callar) followed by a booster pump, with the option of a pressurised water tank (eg/ similar to a "Megaflo". ie you can store pressurisd water, or store it at low pressure and pump it to use it. All of which as you say, can cost hundreds or thousands, depending on your specific requirements.

The "old fashioned" cistern in the loft can be used, with a small pump to give a modest but useful boost to your (hot) water pressure into the combi. Grundfos do a suitable small pump. Balancing presssures for a shower would need a shower mixer capable of working with unbalanced supplies.

http://www.grundfos.com/web/grfosweb.nsf/DocsBySearchName/60CD175E4B7C2C28C1256D56003FA7C9

 

[breezer]

i do not disagree with you but his supply pipe may be old and decrepid (b4 any one says it, I will) a bit like me. and changing it will at least eliminate this from the equation (besides, its free)

 

[COG]

Dont want to confuse the issue But you want a good pressure to lift your diverter valve etc,in other words working pressure when demand at min /max flow the standing pressure ie the pressure in the pipe when no demand is made is not that important as long as its below 10 Bar. What combi did british gas fit ?

 

[brucej]

Hi all, thanks for all the input.
The boiler is a Worcester Bosch, which from the reading I've been doing seems to be one of the better models, although until a few months ago I knew only what the plumbers told me. From some of your comments I do hope that the new mains helps, although I am still a bit dubious since British Gas's previous suggestion of replacing the internal (Cast Iron) piping seemed if anything to make the problem worse. I guess for now I just need to wait for the installers to do their bit, and see if the new mains helps.

I must say in retrospect, I am very surprised the potential problems were not emphasised more (which probably would have saved them a lot of money, since this is the argument I have used to convince them to do the necessary work).

regards
Bruce

 

[COG]

Hope you get lucky with the water main, if your boilers a Worcester then going by the spec you gave it must be a CDI .If anyone does a adjustment to the boiler and it seems to improve ,check they havenot just stycj a 5 pence coin to the div/valve plunger so it reaches the micro switches sooner (come accross this a few times).

 

[ChrisR]

Don't get me wrong I'm not saying don't change the communication pipe. Especially as BG's ineptitude has got them in trouble again!. I'm just saying that the pressure won't improve as a result. The flow undoubtedly will. 1 bar isn't much to play with so you might still not be happy.

Don't get confused with the diverter valve's function - coins stacked up will (for a properly working valve) bring the boiler on before the valve has diverted - ie not a good idea!

An aside - DV's are never serviced as part of an annual service. If you are short on pressure/flow then it would be a good idea to get it dismantled, greased and a new diaphragm every 2 - 3 years.