newcombi-boiler size in doubt

[KTM]

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...

 

[ShaunCorbs]

combis come with flow reg devices already installed so not too much flow

 

[KTM]

combis come with flow reg devices already installed so not too much flow

combis come with flow reg devices already installed so not too much flow

problem occurred when we run the taps opened them too much and the water temp dropped from 60 deg to 45 deg , the taps are very old.
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problem occurred when we run the taps opened them too much and the water temp dropped from 60 deg to 45 deg , the taps are very old.

 

[ShaunCorbs]

problem occurred when we run the taps opened them too much and the water temp dropped from 60 deg to 45 deg , the taps are very old.
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problem occurred when we run the taps opened them too much and the water temp dropped from 60 deg to 45 deg , the taps are very old.

what kind of taps ? mixer taps?

 

[KTM]

what kind of taps ? mixer taps?

yes mixer taps on bath /basin and kitchen sink . bath and basin are good 35yrs old
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yes mixer taps on bath /basin and kitchen sink . bath and basin are good 35yrs old

 

[ShaunCorbs]

yes mixer taps on bath /basin and kitchen sink . bath and basin are good 35yrs old
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yes mixer taps on bath /basin and kitchen sink . bath and basin are good 35yrs old

do they have iso valves on the cold ? as they cold could be mixing / forcing its way in

 

[KTM]

do they have iso valves on the cold ? as they cold could be mixing / forcing its way in

no isolation valves good point did think we may-be pulling cold through just concerned on size of boiler fitted in kitchen /bathroom upstairs that's where the flow is at worst
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no isolation valves good point did think we may-be pulling cold through just concerned on size of boiler fitted in kitchen /bathroom upstairs that's where the flow is at worst

 

[ShaunCorbs]

easy way to tell turn boiler cold water off eg on inlet valve then open a hot tap you shouldn't get anything coming out after 10 seconds

also to confirm too much flow which i dont think the issue is turn the boiler iso down to half open

 

[KTM]

Good thanks I will try this . Do you think the boiler should be bigger than 24kw .

 

[ShaunCorbs]

Good thanks I will try this . Do you think the boiler should be bigger than 24kw .

tbh depends on load as it’s a flat should be fine at most it’s a shower and the hot taps

 

[SimonG]

Fit an iso on the hot water tap to bath that regulates the flow. Not rocket science as they say.

 

[Chuck]

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.

 

[ShaunCorbs]

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

 

[Chuck]

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]

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]

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

 

[centralheatking]

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

 

[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]

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.

 

[Chuck]

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.

 

[KTM]

hi great thanks will look at a small tank.

 

[king of pipes]

hello can I run a small unvented tank from a combi boiler .

Yes as i posted you can say keep your kitchen hot tap on the combi , and install a unvented to feed the rest of the house . Kop

 

[KTM]

Once again thanks will give this a go
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Why did you decide to fit a 24kw?
It was always going to be a disappointment compared to the old boiler

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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 .

 

[king of pipes]

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

 

[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