Potterton Puma 80 really low hot water pressure, ext. pump?

[smullaney]

Hi there,

we have the above mentioned combi and the hot water pressure is woeful, the cold mains is great. We recently had our shower thermostat replaced and upon experiencing this pressure the plumber suggested an external pump.

So I did some searches on this site but cannot tell if you can or cannot have an external pump just for hot water. I have read yes and no for pumps and combies.

Also, any ideas what could be causing such a low hot output pressure - is there something that could be servicable on the puma?

Thanks

 

[PEDANTICVINDICTIVEMAN]

Check in the boiler installation instructions to find out how many litres a minute the combi should be giving out, bucket jug and a stopwatch will do, it may be that the boiler is only designed for 9.5 litres a min, if the flow rate is less than specified you need to look in the serviceing part to find out if the boiler has a filter to be cleaned, worth checking that any valves on the cold inlet to the boiler are fully open.
Cant see how you could fit a pump to a combi to boost hot water pressure, but if anyone does let us all know.

 

[smullaney]

Interesting, and what is mains water pressure?

 

[kevplumb]

mains water pressure can vary between 1 and 6 bar . However water pressure is not your problem flow rate is read the last post again

 

[smullaney]

Sorry, I am getting my terms confused. I am curious about the flow rate you get from the mains compared with what comes out of the combi. Shouldn't the combi be close to the mains flow given the water comes in from the mains, through the heating element and then out of the combi?

 

[ChrisR]

"Shouldn't the combi be close to the mains flow given the water comes in from the mains, through the heating element and then out of the combi?"

NO
If it came out that fast it would only be luke warm. The flow is restricted such you get reasonably hot water when the invcoming water is at its coldest.
80.000 btu/hour goves you about 9.8 litres per minute at a 35º temp rise.

Or 19.6 l/min at 17.5 - which would be usueless. By the way we all shower at about 38 - 40.
So do as you're told and measure your hot water flow, using a 10 litre bucket, for example.

You (almost) never use a pump with a combi - you certainly would not if your mains is good. Could be that the boiler is faulty, or scaled up such that you aren't getting all the HW you should.

COuld also be that you're expecting too much from a combi. eg my bath tap - dead conventional system, gives 22 litres/min at whatever temperature I choose to store it at , perhaps 60º !

 

[kevplumb]

if my memory serves there is a flow restrictor on the incoming cold side
used to be a pain for blocking with crud from the main
im sure somebody will tell me im wrong but you cant pump off the mains water supply

 

[smullaney]

Well there most probably is something wrong with the combi and I don't think I expect too much from it. And I did test the flow and it was only about 4 litres per minute.

Time the heating guy I think. Thank you for your replies, including the straight to the point ones!

 

[ChrisR]

4 l/min - so crud in input filter/flow restrictor sounds most likely. Some boilers have bits which scale up inside to restrict the flow but I don't think yours is one of them on the whole.

Would be interested to hear the outcome.


You can pump the mains if you're getting less than 7 litres a minute of cold at your kitchen tap and the pipework is not at fault. Then you can only pump 7 l/min . In other words forget it! (Oh and regions vary)

 

[kevplumb]

stand corrected old dog new tricks etc

 

[vangard63]

there's a thermostatic valve on the dhw outlet wich shuts down if the temp. is too cold , sounds like its stuck

 

[ChrisR]

I have learned since last post that there is a flow restrictor, a pain to get at, which has a rubber O ring in it. The idea is that as the flow increases the ring gets pushed into a space so as to reduce the flow. Hence a stabilisation. Trouble is the O ring distorts with age and fills the space when it shouldn't.

Kevplumb was right - it's rearwards, left of centre.