Variable Shower temperature - HELP!

I hope someone can help me as my shower temperature is slowly driving me mad.

I moved into a property in March this year that has a potteron puma 80or 80e combi boiler (I forget which one), which is plumbed into mains water pressure and is at the end of the line of the cold water system.

Unfortunately since moving in we have had problems with the hot water, where it comes out scalding hot with no cold water and then as soon as the cold water tap is turned on the boiler switches off (just to the pilot light) and then I go from a scalding hot shower to a freezing cold shower. Even though we have no other appliances running or any other taps running.

We contacted a professional who said we need a non-return valve, which we duly fitted, but this did not make a difference.

After many hours learning about combi boilers and speaking to friends we believe we know what the problem is however we do not know how to solve it and I would be grateful for any help

We think the problem is that we turn on the hot water, which sends the cold water to the boiler, which pushes the diaphragm in the boiler and starts the boiler up and produces the hot water. But when cold water is requested in the bathroom, water is diverted to the bathroom this then drops the water pressure going to the combi boiler which thinks that it no longer require hot water and goes back to just the pilot light.

I have now been living with this for nearly 6 months and I would be grateful if anybody can suggest what I can do to over ride this problem so at last I can have a decent shower.

I would say you probably have more of an issue with the shower not being "combi Friendly" and not balanced correctly

Many Thanks for your quick reply.

We do think it is a shower problem however we do not want to waste money buying things which don't work and not solve the problem.

Can you suggest what we can put at the shower side to resolve the problem?

The boiler should stay on while passing only about 2.5 litres/minute. Try it on a different bathroom tap. A small cold drawoff wouldn't normally reduce the HW flow (which should be 8 litres/min or more) that much.

It certainly doesn't help that your boiler is on the end of the line. Is the mains pressure/flow low, say at the garden tap?

It seems that this problem is isolated to the shower and unfortunately we live in an upstairs masionette so we don't have a garden tap but the water doesn't seem to be especially low but some friends have suggested that the mains pressure could be too low.

Do you have any suggestions on how to over ride this problem?

Hi,

Sorry Last night I checked the other taps last night and I found that if I switched the hot water on and then switched the cold water on it dropped the water pressure coming out of the hot and switched off the boiler.

Does anyone have any ideas to what the problem is?

PS have checked with the water company they have said their water pressure in the street is 1 bar.

Many thanks in advance for any help with this.

Before we can help you further you need to measure the minimum flow rate from the hot at which the boiler will switch on. Should be 2.5 li/min as Chris says!

Tony

I checked the flow rates this morning

For the bath taps & shower the minimum flow rate for when the boiler switches on is 4.2L/min and with the maximum with hot tap fully open it is 5L/min

For the basin in the bathroom the mim flow rate is 4.8L/min and the max is 8L/min

For the taps in the kitchen the min flow rate is 5.2L/min and the max is 7.5L/min.

I also just in case measure the cold tap flow rate (the tap fully open) and with the hot tap off, and they were

Shower/bath taps - 15.4L/min
Bathroom Basin - 16.1L/min
Kitchen - 10L/min

Can anyone help?

Well it would apparently help if it were switching on at the right flow!
There is a flow detector (DHW flowswitch) which operates on the pressure drop in the tap when you open it. It operates a microswitch. Either of them may need replacing or adjusting.

Thank you so much, hopefully it will resolve the problem!