Boiler issue? Hot water downstairs, tepid water upstairs

[Bufferingkisses]

Does anyone have any ideas why this may be the case? Basically we have a combi boiler. The kitchen tap provides very hot water, the bath tap provides tepid water (hot enough for a shower so long as you don't use any cold but too cold for a bath).

The boiler is old so we need to replace it but ideally we need to get another year out of it. I'm wondering if we have to live with this or if there may be a solution - or even if its not a boiler issue at all?

Thanks for any ideas

What is your shower set up? Is it run off the bath tap and you pull a lever to make the water come out the shower head? Is it a mixer tap?

if you reduce the bath tap flow does it get hotter? If so it could be boiler related - either not powerful enough, has lost pressure or if in hard water area have a clogged heat exchanger. Is there a sink in the bathroom that does get hot enough?

Otherwise is the bath tap actually a tap or part of a shower/mixer fitting - if the latter these can have their own thermostatic safety valve to limit maximum temperature.
These can fail/stick or just be adjusted too cool - easy replacement but hard to get parts :(

Does this happen with the central heating on and off ?

Oh gosh, I totally forgot I had posted this thread, I'm so sorry!

To answer questions, it is run off the bath tap which is a mixer. We are ensuring the cold is fully off, definitely no thermostatic element.

@FrankieStein403 now you mention it yes the bathroom sink tap does get hot. If running a bath we turn off the tap (when it runs too cold) use the sink to get it running warm then switch back to the bath - this gives us a few more minutes of warm from the bath.

We set the hot tap so it keeps the boiler running but no more iyswim?

@tanstaafl yes it does

If you're sure there is not a thermostatic safety valve in the mixer tap then it sounds like the combi is not able to supply enough heat to the larger flow for the bath tap.

If you run the bath with the tap not fully open it should be hotter - if not then there must be a thermostatic safety valve in the mixer. Your description of getting the flow hot via the sink does imply there is no such safety valve - it's just the boiler not able to heat the full flow.

Bath taps used to be (still are? ) 22mm diameter whereas basin taps are 15mm.

All depends on what you mean by ‘tepid’ but I’m thinking the combi can’t keep up with the demand for hot water with the wider 22mm pipe.

I’ve experienced this where we had to run the bath hot tap by itself , only half open to reduce the flow so the combi could heat it. Took at least 45 mins to get enough hot water to add some cold too!
Lost the enthusiasm for having a bath by then.

Boiler service would only be worth it imo if the boiler used to be fine.

Add some cold to

Thanks all. I think you're right it is clearly just that the boiler can't cope with the flow. I hadn't thought about the sink tap also being hot, that pretty much clears it up I guess. Sadly there isn't a point on the bath tap between off and only just on that keeps the boiler running which doesn't overwhealm its ability to keep up.

I'll stick to showers I think