Cold water supply to shower from boiler inlet, no cold in shower

[Tibicar]

The cold water supply to our shower comes off the combi boiler inlet pipe, up into attic, along and then down into shower. (Don't ask why, very long story.)
Shower only runs hot even on full cold.
When the shower was fitted the cold water supply pipe was checked and gushing. The man then fitted the shower.
When shower is on, therefore boiler is on supplying hot there is no cold.
When we shut the power to the boiler off so it couldn't supply the shower with hot, the shower then ran cold.
My question is could the boiler be siphoning all the water and not letting any cold up to the cold water supply for the shower?
Would we be better changing the cold water supply to coming off the bathroom sink instead?
This is our 2nd new shower in the year since we moved in, and at £100ish each one plus building work destroying and fixing the shower cubicle wall to get to everything this is getting very expensive.
We could control the temp of the shower via turning down the boiler but I don't want to risk that damaging the innards of the shower, thermostatic valve etc, think that's what happened to the last shower.

 

[PrenticeBoyofDerry]

When you say combi inlet pipe, I assume you are on direct mains pressure from the rising main?
It seems strange that the cold water supply would take the route you have described, is this the same route that other cold water outlets are supplied in the bathroom.
I doubt that the boiler has anything to do with the lack of cold water to the shower, it sounds more like a mixing valve/thermostatic problem with the shower, what type of shower is it, they do generally come with at least a 1 year warranty and manufacturers support.

 

[Tibicar]

Yes, with good pressure generally around the house. All other taps in the bathroom have their own pipes, the shower is teed off the inlet supply to the boiler. The thing is the shower runs cold when the boiler is switched off and the pipe gushed cold water when it was tested before the shower was fitted and the boiler kicked in for hot supply. This is the second brand new shower and third new thermostatic valve, something in our shower set up is obviously screwed up. At £350ish each go, this is ridiculous.
It's a Thermostatic Recessed Dual Control Shower valve. This is the third one including the one we originally had replaced, this one was only put in on Friday.

 

[Tibicar]

it just seems odd that when we turn off the power to the boiler to stop it kicking in, then the shower gets cold.
Another thing, when boiler is on and no cold water, the shower flow can be on or off but no adjustment, when boiler is off and I have cold shower then the flow can be adjusted a bit.

Should say shower is way too hot. have boiler set at 55 as I like really hot water for washing up. The instructions for the shower say the intake temp should be at 10 degrees above required shower temp.

 

[DaveHerns]

I can't quite get my head around it but what if the shower was piped up with the hot pipe going to the cold inlet and vice versa?

 

[Tibicar]

It isn't but even if it was there would still be hot water and cold water.

 

[Newboy]

it just seems odd that when we turn off the power to the boiler to stop it kicking in, then the shower gets cold.

Well surely that's logical - if the water flowing through the boiler isn't being heated then the shower will then run cold?

It sounds very much like H&C feeds plumbed in the wrong way - are the feeds to the shower valve exposed? If so which side is the hot pipework connected too?

 

[Daveydub]

If you take the cover plate off your valve you will probably see H and C stamped on to body of valve. If you run shower, does it now get hot where it is marked H?

 

[Tibicar]

When he ran water through the pipe to check the cold water supply wouldn't that have come out hot if he had the wrong pipe?

 

[Tibicar]

If you take the cover plate off your valve you will probably see H and C stamped on to body of valve. If you run shower, does it now get hot where it is marked H?

How can I tell? touch the valve?

 

[Daveydub]

Yes or the pipe connection to the valve.

 

[Daveydub]

It does seem unlikely that someone could fit 3 shower valves and not notice that hot/cold are reversed but it remains the most likely answer to your problem.
If this is not the problem then can you turn off the cold supply to the boiler rather than the power to the boiler. Make and model of boiler if you need help with this.
Don't understand how someone fits valve which does not work any better than the previous ones, does not notice, gets paid by customer who also does not notice. Wish I could earn a living this way!

 

[fezster]

IME, if the hot and cold to a thermostatic valve are the wrong way around, the shower will continuously run extremely hot and then extremely cold, and loop. This does not sound like your problem.

The other thing a thermostatic valve should do is if there is no cold water coming, then flow should be reduced on the hot, effectively reducing the shower to a drip. Again, this does not seem to be happening. Is it possible both hot and cold are piped up to the boilers output?

How are your other cold water outputs piped up? Why not change the pipework to the shower cold inlet to be from the same as one of the others?

 

[Tibicar]

IME, if the hot and cold to a thermostatic valve are the wrong way around, the shower will continuously run extremely hot and then extremely cold, and loop. This does not sound like your problem.

The other thing a thermostatic valve should do is if there is no cold water coming, then flow should be reduced on the hot, effectively reducing the shower to a drip. Again, this does not seem to be happening. Is it possible both hot and cold are piped up to the boilers output?

How are your other cold water outputs piped up? Why not change the pipework to the shower cold inlet to be from the same as one of the others?

 

[Daveydub]

If you take the cover plate off your valve you will probably see H and C stamped on to body of valve. If you run shower, does it now get hot where it is marked H?

How can I tell? touch the valve?

How did that go then?