Got a Combi boiler and recently the water upstairs has been a bit weird.

If we put the bath tap on even remotely high, it doesn’t get up to temp. but the shower (mixer tap) and the sink both work perfectly well.

Until recently it’s worked perfectly…

It’s not a problem as such.. but a minor inconvenience.

Is this the first year that it’s happened? My understanding of combis is they they add a fixed amount of heat to the incoming water so if the water in the main is very cold then the water coming out if the boiler will be a bit colder as well. The bath will also be fed with a larger diameter pipe than the whb and shower which means the water doesn’t stay in the boiler long enough to get really hot. Turning the tap down quite a bit solves the problem for me. I’ve also put a narrower section of pipe on the hot tap supply to my bath to give the same effect.

I had a problem with the mixer on the shower allowing cross feed of cold into the hot, when the shower was off. So with the bath hot tap on full, it was drawing cold water via the shower, as well as hot. Quick solution was one way valves on the shower feeds, apparently that’s good practice anyway.

Yes, first time it’s happened. We’ve been in the house nearly 8 years now. Totally understand the pipe/amount logic but struggling with why just now and not ever before.

valve not closing correctly somewhere in one of the other devices is my guess.

Try turning them on (in turn) at the same time as the bath tap and see if the bath tap water runs hot.

wwaswas – Member

valve not closing correctly somewhere in one of the other devices is my guess.

Try turning them on (in turn) at the same time as the bath tap and see if the bath tap water runs hot

Sorry fella, I’m not following. you mean in the sink then ?

Obviously I can’t do it with the shower as the taps are mixed by a flip lever, I can’t do it with the bath tap either for the cold as that’s mixed, so only leaves the sink.

sorry, didn’t realise that the shower was on the same tap as the bath.

I was trying to find a way of proving greybeard’s theory that the hot supply is pulling in cold somewhere due to a sticky valve.

My guess is also that the shower/bath mixer is allowing cold in. If you can get access to the hot water pipe running to the tap try holding it near the tap and see if it’s getting as hot as the hot supply to the sink.

OK, I’m with you, will require a little dismantling, but should be in theory possible.

I’d have thought if this were the case though the water flow would be increased ? the hot in the bath tap has always run a little slower than the cold (I assume this is normal), and even now, the flow doesn’t seem to have increased at all.

But does sound like you’re on the right lines guys as all the other taps in the house are roasting hot.

Why would it stop pulling in supply from the other one though when we turn down the tap ? Once we do that, it’s lovely and hot again.

Maybe it just needs sufficient ‘suction’ from the hot supply flow to trigger it?

wwaswas – Member

Maybe it just needs sufficient ‘suction’ from the hot supply flow to trigger it?

That’s cool mate, thanks for all the advice to you and all on thread, it’s given me a direction to go in.

I’ve just called my neighbour who happily IS a plumber, he’s going to shoot round tonight and have a look.

if there is ‘cross flow’ – flow from hot to cold or (in this case) vice versa it will be because of a difference in pressure between the two systems. your combi boiler might have a flow limiting valve that restricts the flow rate through the combi to a sensible amount to ensure you don’t run it too fast.

This has the effect of increasing resistance in the hot circuit leading to a slightly higher pressure at cold outlets as there are fewer restrictions on the cold. Not ‘suction’.

As previous posters have suggested, if it is very recent then the culprit is most likely a mixer device near the bath tap – the shower mixer.

Other culprits for anyone else include the washing machine – only ones with a cold and hot fill though, older mixer taps that mix in the body rather than at the end of the spout.

HTH