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is the flow rate too much for the boiler
Traditional taps deliver between 10 (basin) and 15 (bath) litre / minute and a bit more if you put low pressure taps on a high-pressure system! Physics tells you that if you have 24kW available you can raise the temperature of about 8.6 litre/min of water by 40°C. Uprating to 35kW combi would increase that to 12.5 litre/min. A direct measurement of the flow coming out of the taps will confirm your diagnosis of the problem.

In my opinion, 24kW is cutting it a bit fine unless the flat has an economyl shower head fitted or occupants who are happy to wait up to 10 minutes for their bath to run.
 
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ShaunCorbs

Staff member
S. Mod
Plumber
Gas Engineer
Traditional taps deliver between 10 (basin) and 15 (bath) litre / minute and a bit more if you put low pressure taps on a high-pressure system! Physics tells you that if you have 24kW available you can raise the temperature of about 8.6 litre/min of water by 40°C. Uprating to 35kW combi would increase that to 12.5 litre/min. A direct measurement of the flow coming out of the taps will confirm your diagnosis of the problem.

In my opinion, 24kW is cutting it a bit fine unless the flat has an economyl shower head fitted or occupants who are happy to wait up to 10 minutes for their bath to run.
that doesn’t explain the temp drop at the taps tho

if it was a complaint of low flow out of the taps I could see your point but not having the required temp at the taps
 
that doesn’t explain the temp drop at the taps tho
I think it does, but evidently my reasoning needs unpacking a bit more.

The OP wrote that "when we run the taps opened them too much and the water temp dropped from 60 deg to 45 deg". Assuming his water comes in at 10°C at this time of year, the 24kW boiler will be able to deliver up to 6.9 litre /min (i.e. a partially opened traditional basin tap) at 60°C. If the tap is opened further by the time 9.8 litre / min is flowing the boiler will only be managing to deliver at 45°C.

So, the OP's suggestion that the the 24kW combi is underpowered as the root cause is very plausible. Personally, I'd want more than 'some guy on a website says it's plausible' before blaming a new boiler, hence my suggestion to confirm the cause by measuring the flow rate directly, e.g. with a jug.
 

king of pipes

Esteemed
Plumber
Gas Engineer
Advent Win
24 kw combis are very poor in the winter months at providing hotwater the lower the incoming temperature of the water then the more energy is needed to heat the water as it passes through the heat exchanger to a point where you have to limit the amount of water going into the boiler to get a decent temperature out for a bath, the fact that you have traditional brassware on the bath will make it seem even worse, theres no way to get more tha 10 litres a minute from a 24 kw combi you will get even less in winter , if its in a rental property you could fit a pressure reducing valve to the incoming cold supply to limit the amount of water that can pass through the hotwater plate heat exchanger but in reality if you really need more hot water then either more kws is needed larger boiler or fit a small unvented cylinder to store some hotwater for bathing . Regards kop
 

scott_d

Plumber
Gas Engineer
Why did you decide to fit a 24kw?
It was always going to be a disappointment compared to the old boiler
 

centralheatking

Esteemed
Plumber
just fitted a gloworm 24 kw into a flat with 6 rads one bathroom previous boiler were a 42 kw floor mounted ancient Worcester .

client wanted wall mounted new boiler, only problem now is the flow rate too much for the boiler and full the water is luke warm did think of reducing the flow but the client likes a bath . would fitting 35kw combi be the answer. Help...
Remember a customer changing to a combi system from vented is a lifestyle choice which the installer ought to make the customer aware of. It’s all a question of managing their expectation and I would
Centre on the energy savings. Esp if they are elderly sell them a shower then everbody will be happy Centralheatking
 
OP
K

KTM

24 kw combis are very poor in the winter months at providing hotwater the lower the incoming temperature of the water then the more energy is needed to heat the water as it passes through the heat exchanger to a point where you have to limit the amount of water going into the boiler to get a decent temperature out for a bath, the fact that you have traditional brassware on the bath will make it seem even worse, theres no way to get more tha 10 litres a minute from a 24 kw combi you will get even less in winter , if its in a rental property you could fit a pressure reducing valve to the incoming cold supply to limit the amount of water that can pass through the hotwater plate heat exchanger but in reality if you really need more hot water then either more kws is needed larger boiler or fit a small unvented cylinder to store some hotwater for bathing . Regards kop
hello can I run a small unvented tank from a combi boiler .
 

Stu-B

Advent Win
hello can I run a small unvented tank from a combi boiler .
Yes you can if properly done with zone valves and auto bypass etc, but you will still need to have at least one hot tap from the dhw from the combi.
 
Yes you can if properly done with zone valves and auto bypass etc, but you will still need to have at least one hot tap from the dhw from the combi.
If the boiler is in the kitchen, so the direct feed can go to the nearby kitchen sink, it can a nice arrangement IMO.
 
OP
K

KTM

Once again thanks will give this a go
Post automatically merged:

Why did you decide to fit a 24kw?
It was always going to be a disappointment compared to the old boiler
Post automatically merged:

Why did you decide to fit a 24kw?
It was always going to be a disappointment compared to the old boiler
I am beginning to wonder myself would have been wise to either go to system boiler or jump up to 35kw combi your thoughts .
 
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king of pipes

Esteemed
Plumber
Gas Engineer
Advent Win
Its a flat at the end of the day plenty of them with 24 kw combis in ? the mistake was what you have replaced it with doesn't match up to the hotwater flow rate of the old boiler the way i see it you have 2 options replace it with a higer output combi at least 35kw or add a unvented to the what you have its going to cost about the same to do so your choice really. Kop
 
OP
K

KTM

Its a flat at the end of the day plenty of them with 24 kw combis in ? the mistake was what you have replaced it with doesn't match up to the hotwater flow rate of the old boiler the way i see it you have 2 options replace it with a higer output combi at least 35kw or add a unvented to the what you have its going to cost about the same to do so your choice really. Kop
Hi agreed school boy error should have gone bigger or unvented cheers
 

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