Shower temperature - PRV, equalising valve or gate valve?

Combi boilers and thermostatic valves don't work very well. You have two control systems which are not compatible.

The pressure between your cold side and hot will be the same. Its just the flow rates differing because your combi has likely slowed the flow down so that it leaves the combi at the right temperature.

 

The problem is not on the cold side but on the hot side.
The thermo mixer has its own method of reducing the cold flow so valves are not required..

 

Is your thermo mixer combi compatible?

 

Would be better to contact the manufacturer of the shower valve and get their advice. Still reckon the problem is on the hot side but a bodge outside the manufacturers recommendations may work.

 

Something a bit odd with your system - the flow rates between hot and cold shouldn't be as large as you have. You would expect the hot to be reduced a bit but not halved as in your case.

I don't agree with Ryluer (sorry!) "Combi boilers and thermostatic valves don't work very well". They work perfectly, the whole point is that a combi cannot control the hot water temperature accurately enough for showering, so the thermostatic shower does the job, especially when someone else opens up a tap somewhere while you are in the shower.

Your problem is to do with those different flow rates, but the pressures may be the same. So you do need to check the pressures before we can advise further.

 

The problem with combis and thermo shower mixers is that you have two control systems in tandem. In some circumstances they react with each other to produce an effect callled 'hunting'. The output temperature from the shower varies from hot to cold and never settles.

This problem is more likely to arise if the temperature produced by the combi varies more than a certain amount forcing the mixer to start altering the flow rates to keep the shower temperature constant. This alteration is fed back to the combi causing a bigger variation in temperature as the combi tries to counter the effect of the changed flow rate. This causes the mixer to produce an even greater change to compensate and so it goes on.

 

I get your point but have never seen or experienced this. The combi actually reacts quite slowly compared to the mixer shower so you end up with an almost constant shower temperature.

Problem here I think is these wildly different hot and cold flow rates are somehow 'fooling' the shower mixer. So establishing the pressures is the first step. Going to need flow rates, temperatures and pressures from various taps round the house I'm afraid, also can the combi manage the 35deg temp rise test?

Got a feeling that the op may need to get someone in who can work out what is going on, not confident can manage it from my keyboard.

 

Certainly the Grohe 1000 I have just installed states that it IS suitable for combi boilers >24kW in the installation instructions.

It works just fine on my 32kW worcester

 

I have 5 years old WB boiler was working fine all that time, have descaler, etc, however now the hot water is not hot at all and I put the pressure up several times recently, it looks ok but as soon as the hot water is on downstairs the boiler pressure drops to "0"!!! Called the heating engineer, he said the pressure is low from the mains, but how can it be, the cold water tap flow is alright as usual! Don't want this man again, don't trust him, but is there any hint on what can be done? I had dripping radiator slightly, but that is sorted now, didn't change anything! Please help!

 

Callita - I suggest you start a new thread - you have a completely different problem. And, yes I agree you need a different heating engineer - sounds like that one doesn't have a clue. My absolute guess is you have a pressure vessel problem,but there are much more knowledgeable people than me on here who will help. Head your new thread "continual pressure loss on WB boiler"

 

I donlt even know how to start a new thread, quickly looked around...

 

click on the forum. Towards top right "post new thread"

 

done

 

I have a similar issue, but closing off the cold isolator valve slightly helped to stabalise the hot/cold shower mix. (In effect did a similar job to running cold bath tap).
I understood from other links that (a) It is not a good idea to use the isolator valve this way and (b) A PRV is the way to go.
However if I connect a PRV to the cold supply before the Combi, then my logic would say that I'd gain nothing as it would reduce the pressure to the whole system, including the Combi. So my thoughts were to fit the valve off the cold feed after the T junction that takes the cold into the home, leaving the untouched pressure to head for the boiler. Does this make sense to those who know what they are talking about out there?

 

I have to 'fess that that's what I have done too - slightly shut off the cold isolating valve.

My shower is meant to be thermostatic and has separate temp and flow controls. However, the temp control makes sodall difference - no effect whatsoever - and it usually ran a bit chilly from the date of installation.

One thing I did notice in the installation instructions is that the hot water supply should be at a min of 60oC. Not sure mine is - it's certainly 'hand hot' but not scalding (that's how I set it at the combi boiler).

Anyhoo, I chust tweak the isolating valve on the cold feed slightly shut and this gives me a good shower temp. A couple of teeny tweaks are required over the year to compensate for changing water temps.