Combi problem: bath sinks tepid, basin taps boiling hot: WTF?

[Lobster]

I've just been contacted by a tenant who rents a house from me, saying that the bath tap water is tepid, whereas the basin water is boiling hot, and can I sort it please?

Sounds a bit odd to me, as they are both in the same room and fed from the same combi boiler... only possibility I can think of is that the bath has 22mm taps against the basin's 15mm: could the flow rate just be too high in the bath for the water to be heated properly? Can't come up with any other scenario really. I've been letting the property for about 8 years and nobody'd ever mentioned it before, though; including during the really cold spell last winter. I'm going round in the morning, adjustable spanner in hand, so would be grateful for any input first!

Thanks David

[Syd]

you got it in one, just slow the flow at the boiler till the waters running hot at the offending tap or tell them to run the tap slower

[John Rumm]

Assuming there are no faulty blending valves in the system it sounds like the typical combi setup where the maximum flow rate through the bath tap is higher than the boiler can hope to heat[1]. This has caused a world of hurt for stupid people who can't work out how to not turn a tap full on!

You can fix it by throttling the cold main inlet valve on the boiler a bit to limit the maximum flow to something it can heat...

(the tenant will then complain it takes longer to fill the bath)

[1] this is a place holder for some dozy muppet to interject that "high flow rate combis are available" and "you should have bought a

[polygonum]

"you should have bought two brand of the month? boilers instead"

[Newshound]

When I had this problem I put in a pressure regulating valve on the combi supply: more expensive than a gate or stop valve, but quieter and nice and easy to control.