Combi/thermostatic shower going hot/cold

[JohnnyDollar]

Hi All,
Got a query on combi boilers and thermostatic shower mixers. Had two separate cases of a thermostatic shower mixer with a combi boiler where the shower keeps going hot/cold.
I've changed cartridges on both showers so definitely not that.
What I know is happening is the boilers fire up and give hot water then after a minute or two shut off, (insufficient flow?) and the showers gradually go cold. The boiler then fires up water heats up and the process keeps repeating itself.
Both the boilers are about eight/nine years old. but have been serviced and are in good condition. The first shower I fitted and was a low pressure capable shower as the water supply was at the 1 bar threshold.
It seems the showers are part responsible as at all other hot taps it stays a constant hot.
Any help greatly appreciated.
Jon

 

[justlead1]

Hi What happens when the shower rose is removed?

 

[JohnnyDollar]

Sorry, hadn't thought of trying that. Will do that next, however, the flow coming out did not seem to be powerful enough that the rose would be restricting it.

 

[JohnnyDollar]

Hi again,

Took the rose off but no difference, was wondering what your theory behind it was as maybe there could be something else that could be affecting it that way?

Many thanks

 

[Plumbstar Tom]

have you got the flow limiters fitted to the inlets of the shower?

 

[JohnnyDollar]

They weren't fitted to the first one. The second I'm not sure, unfortunately it's all tiled over and even taking the plate off there's limited access so without making the shower unusable I can't tell. I know that may have to be the case but just trying to see what can be done first.

Many Thanks

Jon

 

[Plumbstar Tom]

SO my second question of non return valves, will that be unanswerable?

Is the shower not fitted with access for servicing to the filters etc? access from behind the plate

Sounds like a bit of hacking away maybe needed

 

[JohnnyDollar]

Thanks Tom, couldn't see non return valves but think you're right, likely means having to hack away a bit.

Just had a thought and you can shoot me for not mentioning it earlier although doesn't explain 1st shower's same problem.

The boiler was set up with 15mm gas pipe that would only allow it to be run on minimum settings. Original complaint was the shower ran feebly. To increase shower flow the pipe was upgraded to 22 to allow higher burner temp so hotter water with more cold.

Thought it ran fine when first tested but guessing didn't run test for long enough, will go back and investigate further, see if anything else has been affected.

 

[JohnnyDollar]

Thanks for all your help guys. Think I've got it licked. Thinking it through it had to be something to do with the shower as the taps work fine. Took off the rose as said before, no difference, took off the hose and shower worked fine with the boiler not over heating at all. So now to get a wide bore hose and high flow shower head and retest.

 

[Toddyplumb]

It could be that the thermostatic controls on the shower and boiler are fighting against each other .

 

[JohnnyDollar]

Thanks Toddyplumb, that was my original thought, anyway of proving/disproving that theory?

Tried with 11mm bore shower hose and was better but still cycled hot/cold when TSV was turned down. Was wondering if 13mm might make a difference, only 2mm more but that's nearly 20% bigger.

It was suggested that the boiler modulator sensor might be iffy and not sensing less flame needed so boiler would over heat and shut off, not got my gas ticket yet so can't play with that but does it sound likely before I get a gas man to come take a look?

 

[UncleVagrant]

Only just registered and realised this site is mostly for plumbers and not customers. My boiler was brand new last year and we only managed to get a thermostatic mixer shower put in a few months back. Hardly had any good performance from it so decided to get someone in when the boiler service was due. Big problems! The symptoms were almost exactly the same as the ones for this thread. Shower runs hot/cold and flow pitches up and down a bit. I'm really losing the rag with this as the original installers cut unnecessary corners and the first Tasman/plumber to attempt a fix was really difficult to book. We went with a third guy who identified that the sink still ran hot so he replaced the bar mixer and did a proper service on the boiler. Unfortunately, the symptoms have returned.

In short, the old system was treated to a new boiler last year and 2 shower kits (of wildly different costs) have not yielded any better results than the duff electric shower we got with the house. The boiler pressure still gradually drops over time but it's by tiny amounts and there's no neighbours going nuts below us! Would it be cheaper to replace the system too and save myself a lot of diagnosis and revisits?

 

[Plumbstar Tom]

Can you feel the inlets to the mixer of the shower? is the left side hot?

 

[Breeze]

Hi. I'm also having an issue with my thermostatic unit running hot then cold then hot again. Its a triple one so has a shower and bath tap and both outlets are affected so I know its not the hose or the rose. There is a button that you have you depress on the temperature control to put it up over 38 degrees. The boiler has been checked out by the manufacturer and the pump and heat exchanger replaced, they have rules out the boiler and say it is one of the taps/showers. In total we have five mixer taps throughout the house but it is only this triple unit that has the safety button on the temperature control. After the boiler was eliminated from the line up I called various plumbers who either turn the job down or will only investigate on an hourly rate. I have spent a few hundred pounds now on various plumbers none who have changed any parts or come up with anything decisive although a couple thought it was the boiler. Any suggestions? The difference with my issue to the others I have been reading about is that every tap in the house if affected not just one.

 

[Plumbstar Tom]

Sounds like the feeds to it are the wrong way round. ie cold water is going into the shower where it should actually be hot. Can you get to the back of it?

 

[lame plumber]

what is wrong with paying an hourly rate to look at a fault and resolve it for you?