Combi - low hot flow when cold on

[mallangong1]

Hi,

I have had a good search through the many combi flow related posts but can't seem to find any answer - sorry if I missed it.

Our new house has a Potterton Puma 80 combi - I have never had a combi before. Cold water flow is 16 l/m. Hot alone is around 8 l/m. However when both the hot and cold are run together the hot flow reduces to below 4 l/m.

At this flow the boiler keeps cutting out (presumably as the flow is too low), so not only does the hot water end up being low but it goes in waves of temperature.

Is this just the nature of combi boilers? I can't believe that people just run one tap at a time or wait for the toilet to re-fill etc.

Thanks,
Jim

 

[tamz]

Check the main stopcock is fully on.
If it is you have low pressure/flow. How new is your new house?

Combis aren't great (especially an 80) but they are much better than you have.

 

[mallangong1]

Thanks tamz,

Yes the stopcock is fully on (first thing I checked). Is it worth checking the Outside water company one? - am I allowed to do this?

House built circa 1900, combi installed circa 1995 (I have a letter showing the guarantee was about to run out in 1997, and presume the original Potterton warranty was for 2 years).

Thanks,
Jim

 

[45yearsagasman]

Jim you may well have flow/pressure problems in your property.That combi as you mention is 15 yrs old and was only designed for single point operation.9/10 litres per min is the best you can expect from it.Maybe its time for an upgrade.

 

[JohnD]

House built circa 1900,

like my old mum's house, where I recently replaced the old lead incoming pipe with a new blue poly pipe. This increased the water flow and pressure at the open taps considerably, because the new pipe was bigger bore.

If your incoming water main doesn't deliver much, you can't get any more out of your combi and taps, even if you buy a bigger boiler.

Polypipe costs about £30 for a 25-metre roll, and, being flexible, is fairly easy to put in your trench, under your floor etc without needing any joints. You can fit a 25mm pipe, the internal diameter is about 22mm (the internal diameter of the old lead pipe I took out was about 3/8", say, 9mm, and it had various joints in it, and might have been squashed a bit in places underground. You could also buy 35mm plastic pipe for not much more, which would do no harm.

However:
Before you start this, look and see if you have a lead pipe. If you do, ask your water company to test the drinking water at your kitchen tap for lead content. If it is higher than a certain amount, they will have to replace their part of the pipe free (this will probably start under the pavement, and either go up to the boundary, or, if you have a stop-cock just inside your front gate, they will probably replace up to that if you ask nicely and give the workmen tea and biscuits, and especially if you have already dug your trench and put the new pipe and stopcock in it ready to be connected up.

The advantage of having this done first, is that if they test it including your internal lead pipe, the lead content will probably be high enough for then to do the external work. They may offer a subsidy to replace your own internal old pipe. If they are replacing their bit of pipe with modern plastic, it will be a lot easier for you to fit a new full-bore 25mm stopcock and to connect your own plastic pipe to it

In my own case (1) by the time they arrived to test for lead I had already changed the consumer's pipe, so the lead content was quite low and they would not do any work and (2) I had to connect my new pipe to a 100-year old stopcock inside the front gate, that was connected to lead pipe and was in a size that did not fit modern pipes.

If I had asked them to connect my new plastic pipe to their main or water meter, by digging up the pavement, it would have cost me £500 as my lead test was OK.

Incidentally if you have an old steel pipe, it will need replacing anyway because it is likely to have rust pinholes and leaks by now, especially at joints or elbows, and if not, it soon will have.

 

[Agile]

The mains water supply into your houre is typical!

The answer is that you will only get a total flow of 16 li/min as it all comes throught the same supply pipe.

If you want more hot flow ( up to 9 li/min ) then the obvious answer is not to use so much cold at the same time.

I adjust cold outlets to the minimum useable flow rate in this senario.

Tony