Bath hot only like warm...

[Toni Clarke]

Hello,
I wonder if anyone can help me please?
We have a Bosch combi boiler (approximately 10 yrs old and serviced regularly) and a thermostatic mixer unit for bath/shower.
The shower gets 'just hot enough' for showers but the bath on same unit and heat setting, only runs luke warm.
All other hot taps run perfectly hot in the house.
We've already had several plumbers out that replace parts in boiler though we believe it is functioning correctly as the replacement parts seem to make no difference.
It's been suggested that we might now need a 'non return valve' on the hot into the bath, believing the cold is to forcing the hot back down the pipe.
Does this sound plausible? Or would any of you lovely people be able to suggest anything else please?
Pulling our hair out as already spent a fortune on plumbers but still no joy.
Thank you all in advance.

 

[Nozzle]

Yeah, that sounds perfectly plausible if you have gravity fed hot water and mains cold water. Though really an NRV isn't addressing the real problem, but if anything it might make it worse. What needs to happen is for there to be a pressure drop on the cold side or a pressure increase on the hot side. Where the two come together there is a variable pressure drop (the thermostat) that balances pressure drops from the hot and cold legs the net result of which is to give you temperature regulation.

You have two choices: to increase pressure on the hot side, fit a pressurising pump (expensive)
to decrease pressure on the cold side, fit a regulator or a simple valve that is slightly closed (cheap)

Fitting the NRV achieves a similar thing, but unfortunately leads to throttling on the hot side - and the greater the differences in pressure between hot and cold sides, the more it closes (throttles).

I think the reason you see a difference between the bath and shower is because the shower head holes are tiny and are a throttle plate, whereas the bath tap is a larger opening and so the higher pressure cold side more easily overwhelms the flow from the lower pressure hot side.

Nozzle

 

[Nozzle]

Oh, and another thought - it might already have NRVs built in to the shower mixer (for water regs) - so adding another wouldn't have any benefit. Your thermostatic mixer has some degree of adjustment in it, so you can set how hot the hotest temperature is. It normally involves removing the knob/lever and adjusting a screw. This is effectively allowing more hot water through against the higher pressure cold water (by throttling the cold more).

Nozzle

 

[bernardgreen]

Most likely that the flow rate on the "other" hot taps is lower than the shower or bath and thus is something the boiler can cope with and that the higher flow rates for the shower and bath are more than the boiler can cope with.

Un-likely to be a serious pressure difference because as the boiler is a combi there won't be a gravity fed hot water tank..

 

[Toni Clarke]

Thank you both for your comments and advice. So am I right in understanding if we can increase the hot pressure on the thermostat unit and reducing the cold flow it should equal out.
Otherwise we need to pop a reducing valve on cold if little difference in trying the first option.
Thank you again, much appreciated.

 

[Toni Clarke]

or reduce flow rates on both if too high for boiler to cope. how do we reduce the flow rates? is this also hopefully a via screw within thermostat unit?
Thanks in advance.

 

[Madrab]

If you only have a combi boiler, no tank for hot and cold water, then a NRV will not alter anything as you have a balanced supply and the cold should not be acting against the hot. Unless of course you have really high pressure cold mains and the cold feed to the combi is wound down by a pressure reducing valve (PRV).
If not it may be the case that the combi boiler is not large enough to feed the bath outlet adequately. Do you know the boiler model?
Can you turn the mixer to full hot and turn the tap down a bit, does it get hotter? Is this a recent issue or has it always been like that?

 

[Toni Clarke]

it is a glowworm ultra 30cx
I have ran hot very slowly and it's the same dribbling barley Luke warm water. Does that help in diagnosing the problem?

 

[Toni Clarke]

this is very similar to what we have installed

View item:
Concealed Thermostatic Bathroom Bath Shower Mixer Tap Valve - Round 3 Dial 2 Way

 

[Toni Clarke]

I know the top one has three outlets to select either bath, shower and third is closed off.
Middle is to open it up. bottom is thermostat.
Thank you in advance.

 

[Toni Clarke]

just found this also....

5 Years Manufacturer's Guarantee

Triple Round Thermostatic Shower Valve with Diverter
High Quality Chrome Finish
Portrait Version
Recessed
Minimum Pressure - 1.0 Bar
Dimensions: H 280mm W 130mm
Operation

• Top Valve - Water control with diverter for three separate outlets
• Middle Valve - On/ Off
• Bottom Valve - Temperature Control

Approved to TMV2 Standards - High Standards of thermostatic protection and suitability for use in the home
High Durable and Effective
ANTI-Scalding

 

[Toni Clarke]

Find your Plumbing Systems
Hot Gravity & Cold Gravity / Mains
Low Pressure Systems (LP 0.1-0.2 bar)

• You have a cold water storage tank in the loft
• You have a hot water cylinder
• You do not have a shower pump installed

If any of the above apply to your home, you can choose any product marked with LP.
NOTE: A gravity fed system provides MP (0.5 bar) to fittings downstairs in a 2 story property where the cold water tank is in the loft.
Combination Boiler
Mesium Pressure Systems (MP 0.5-1 bar)

• You have a combination boiler that heats water on demand
• You do not have any storage tanks
• You do not have a shower pump installed

If all of the above apply to your home, you can choose any product marked with LP, MP or HP1.
Mains High Pressure
High Pressure Systems (HP 1-3 bar)

• You have an unvented cylinder with an expansion cylinder
• You have no cold water in the loft

If either of the above apply to your home you can choose any product marked with LP, MP, HP1, HP2, HP3.
Pump Assisted
High Pressure Systems (MP to HP4)

• You have a booster pump fitted

If so, then you can choose any type of product.

 

[Madrab]

it is a glowworm ultra 30cx
I have ran hot very slowly and it's the same dribbling barley Luke warm water. Does that help in diagnosing the problem?

If that's the case and you have hot water (not luke warm) everywhere else, I'd be more inclined to be looking at the filters or thermostat setting/cartridge on the bath/shower mixer as the first port of call.

 

[polesapart]

it is a glowworm ultra 30cx
I have ran hot very slowly and it's the same dribbling barley Luke warm water. Does that help in diagnosing the problem?

If that's the case and you have hot water (not luke warm) everywhere else, I'd be more inclined to be looking at the filters or thermostat setting/cartridge on the bath/shower mixer as the first port of call.

+1