Anyone with plumbing knowledge around please?

[SandunesAndRainclouds]

I’ve got a bathroom problem!

We’ve got a combi boiler, which works fine. All taps in the house run hot apart from my bath and shower (shower is over the bath). They’re both running tepid water and I can’t work out why or how to fix it - I probably need to get a plumber in but I don’t have a huge amount of spare funds at the moment.

Can anyone shed some light on why this would happen? It was all working fine until a week or so ago.

Put a shout-out to PigletJohn over on the Property/DIY part of Homes and Gardens here on MN.

You’ll soon get an answer

I’ll do that, thanks!

If you turn it on to just over a trickle into the bath rather than full power is it still tepid or does it run hotter if it's a slower flow?

Still tepid. I can run a whole bath for the DCs with just ‘hot’ water, doesn’t seem to make a difference on how open the tap is.

It does take an absolute age for it to run hot elsewhere if that’s relevant.

the problem you describe occurs when you have the followingcombination of circumstances

• a combi boiler
• winter

-bathtap.

A combi can heat X volume of water by Y degrees in Z amount of time.

Let's suppose it's 10 litres, 20 degrees, in one minute.

The same amount of energy would heat 5 litres by 40 degrees in one minute,

Or 20 litres by 10 degrees in one minute.

In winter, the incoming water from the watermain will be colder than in summer. let's suppose 2C in winter and 12C in summer.

so with our formula, you can see that the water going into the boiler is 10 degrees colder, and although it is still heated up by 20 degrees, the water coming out of the taps will also be 10 degrees colder.

Because the water coming out of the big bath taps is delivered at a faster flow that the water from the small basin taps, the bath delivers cooler water from the bath taps than from the basin taps.

Sadly that's how combis work.

The tip to turn down the bath tap to reduce its flow will work, but the bath will take longer to fill.

If you want to check what's happening, you will need a watch with a second hand, a bucket, and a thermometer; and you need to measure the temperature of the incoming water at the kitchen cold tap; the temperature of the water delivered by the hot bath tap; and how many litres per minute the hot bath tap delivers.

There are some defects that can make the problem worse, but we won't know without measuring what you currently get.

Hot water cylinders don't work the same way.

Sorry.

oh, and post a photo of your bathtap and any shower mixer, please.

Thanks PigletJohn

I understand, and will try reducing the flow rate today and see what happens.

Other than this happening a couple of weeks ago and self-rectifying it’s never happened before, and we’ve had the same boiler for a few years now.

Would servicing the boiler make a difference?

Bath has two standard taps, shower is a Moretti bar. Will take pics in daylight.

Thanks again.

Can't tell without knowing your numbers and seeing the mixer.

Taps and mixer pictures hopefully.

Are the bath and shower furthest from the boiler by any chance? Could be that the other taps run hotter because the water isn't travelling through as far so not cooling on the way. Can you turn up the thermostat on the hot water control of your boiler? You might need to put it up during the winter, then you can drop it back down once spring has sprung.

You could also try turning off the radiators at the timer for long enough to go cold, then seeing if any of them start to warm up when you run a hot bath. Most likely the ones nearest the boiler, and you would feel it on the radiator pipe before the whole rad warmed up.

The boiler is in the loft conversion, bath is on the floor beneath. The en-suite in the loft definitely gets hot the quickest, and the kitchen on the ground floor is super slow. We had a Baxi back boiler which was in the lounge - the piping wasn’t moved from there so it has a long way to travel from the top to the kitchen.

I find it strange that this is the first time it has happened. It was fine until a couple of weeks ago, the shower and bath have never run cool before.