Boiler turning off when using the shower

[totalitarian]

I have a Baxi Duo Tec Comdensing Combi and a Thermostatic shower.

For 6 months it is has been running fine but recently the shower runs hot and then cold. I can hear the boiler switching on and off every 30 secs.

I've removed the shower head, adjusted the boiler temp from high to low, cleaned out the filters and the thermostatic mixing valve , the problem stays the same. All the other HW taps in the house work fine with a steady temperature.

What could be the issue with the shower?

 

[TeaTime]

What did you do to the thermostatic valve?

Did you replace the cartridges or just clean the valve?

The thermostatic cartridge may be on it's way out, which is why the boiler is having a sporadic demand on the DHW.

 

[totalitarian]

I opened it up. The cartridge looks pretty much new so I just flushed it under a tap and reassembled it.

 

[TeaTime]

The internals may be damaged, stopping it from operating correctly. How long have you had it?

 

[totalitarian]

About 6 months. But no receipt so I can't get it replaced under warranty

 

[sooey]

Does it run hot and cold at other taps or just the shower?

 

[totalitarian]

Just the shower

 

[TeaTime]

If all your other outlets work and they make the boiler operate as it should on a hot water demand, then it must be an issue with the shower set up.

Since it has worked well in the past, I can only assume that the thermostatic cartridge is acting up, allowing too much cold over the hot water.

What's the make and model of the shower?

I assume you haven't been playing with the pipework recently?

 

[totalitarian]

This is the shower

http://www.bathstore.com/products/metro-thermostatic-bath-shower-mixer-deck-mounted-2479.html

I've not managed to find a replacement valve for it.

In terms of the pipework, I have fitted a dishwasher but that shouldn't effect anything?

 

[totalitarian]

Could the issue be the pressure sensing valve in the boiler at all?

 

[TeaTime]

I doubt it. You are receiving good hot water pressure at all other outlets.

Plus, you have had 6 months of trouble free showering, so if the shower / DHW pressure were not compatible, you would have realised it immediately.

Have you removed the cartridge and had a look at it?

I assume the service valve to the hot water supply isn't partially closed or anything silly?

 

[totalitarian]

Yes I stripped it down and there was not debris inside of it.

Where is the service valve to the HW supply?

 

[TeaTime]

How do you know if there is any debris inside it? The thermostatic cartridge is a sealed unit!

The service valves are the valve you use to isolate the hot / cold water. Shutting off the supply so you can 'service' the parts upstream.

 

[totalitarian]

How do you know if there is any debris inside it? The thermostatic cartridge is a sealed unit!

The service valves are the valve you use to isolate the hot / cold water. Shutting off the supply so you can 'service' the parts upstream.

The cartridge itself was fitted in a sleeve. You are right, I couldn't see inside the that, but everything else looked free from scale.

I haven't ever isolated the water apart from at the main stop cock, so I doubt there will be a problem with the HW service valve

 

[TeaTime]

If it is the cartridge, you could get in touch with bathstore and ask them about the replacement part and where to get it from. It should only be about £35 and be an easy fix.

In the mean time, you could soak the cartridge in hot water (just off boiling) and vinagar for a while to loosen the scale inside and see if that helps.