Combi questions

phantomclipe

Looking for some advice, i'm replacing an old gas boiler (Baxi unit) and on the fence between a combi boiler and conventional condenser boiler. The house is a small 81sqm 3 bed terraced house with 1 bathroom with an electric shower fitted in the bathtub. I have read that combi boilers struggle to supply the flowrate required for a bath, Is this just an issue of being slow? I’m okay with that if so as the shower will be the primary source with only a couple of baths pa anticipated. Furthermore I understand a combi does away with the hot water tank which is un-lagged, of unknown condition and would free up space in the house. Are there any other considerations to make when choosing? I'm aware of the mains pressure requirement and scailling and do not believe either to be an issue? Are there brands worth going for over others and finally are there any SEAI grants available (currently only control in place is timer switch for whole house).
Any tips recommendations or advice from those in the know would be greatly appreciated
Cheers

Dtp1979

Finally someone that’s actually suited to having a combi. Filling a bath will be a bit slower. That’s ok.

Combis are designed for mains pressure. Which is great if you live in England and have 3 bar night and day. But this is Ireland I’m afraid. Install your combi but put a submersible pump in your attic tank to provide the pressure needed. My preferred choice of boiler would be Ariston. Oh yes, and make sure your water is soft.(no limescale)

phantomclipe

Thanks for the reply, the house is fed from Vartry which i understand to be considered soft. Interesting your submersible CWS tank pump suggestion, Do combi boilers not have any pumps within them?

Dtp1979

phantomclipe wrote: »

Thanks for the reply, the house is fed from Vartry which i understand to be considered soft. Interesting your submersible CWS tank pump suggestion, Do combi boilers not have any pumps within them?

No not pumps for pressurising the plumbing system

Andyinter

They would only struggle to fill the bath if it was feed from the mains and it was low pressure. As DTP1979 says, stick a pump in to guarantee the pressure, plus in Ireland we are not allowed connect direct to mains, only the cold tap in the kitchen and the tank in the attic. Just make sure if you go with the pump that you dont connect the toilet to it as the last thing you want is the pump kicking in at 2 in the morning, although a submersible is slightly quieter its still noisy. Personal preference is the Intergas, particularly when it comes to a combi, less parts, no plate heat exchanger, no divertor valve, no valve motor so less to go wrong, plus unlike normal combi boilers that use a plate heat exchanger the Intergas will condense in Hot water mode as well as central heating

jeepcj

I have a gloworm flexicom 35 combi boiler, 3 bed standard dublin semi detached house. It's been the best upgrade I've ever did to the house. Absolutely no problems filling the bath, in fact it's the complete opposite, it has too much hot water flow, I've a rainfall shower head and the amount of water it uses is crazy. I have a large pump between the attic tank and the boiler(boiler is moved to the attic as well). But like others say don't put the toilet after the pump as it will come on in the middle of the night if you flush it. The 35cx boiler puts out 14.5 litres of hot water at 60 deg C

John.G

jeepcj wrote: »

I have a gloworm flexicom 35 combi boiler, 3 bed standard dublin semi detached house. It's been the best upgrade I've ever did to the house. Absolutely no problems filling the bath, in fact it's the complete opposite, it has too much hot water flow, I've a rainfall shower head and the amount of water it uses is crazy. I have a large pump between the attic tank and the boiler(boiler is moved to the attic as well). But like others say don't put the toilet after the pump as it will come on in the middle of the night if you flush it. The 35cx boiler puts out 14.5 litres of hot water at 60 deg C

I think Gloworm are a bit optimistic re their claims of 14.5 LPM @ 60C, if one assumes that its output is 35KW? then it will raise the temperature of the water at this flowrate (14.5 LPM) by 35C and if one assumes average cold water supply temperature of 10C then you end up with a still very satisfying temperature of 45C, it will raise 10 LPM from 10C to 60C. Is it possible that they have a very small storage/buffer tank which will give 14.5 LPM @ 60C in combination with the boiler but only for a limited time until the storage tank temperature falls?.
35 KW gives a very decent hot water flow but from a space heating point of view it would/might be considered very over sized, 20 KW would probably be adequate for a 3 bed semi and with a turn down ratio of 5:1 will modulate to 4 KW which in a lot of cases will mean that the boiler will continuously fire for longer than the combi which will only modulate down to 7 KW or does a combi boiler have a higher turn down ratio than a system boiler?.
A oil fired combi of 35 KW which has no turn down would be cycling quite a lot under the above conditions.

Dtp1979

John.G wrote: »

I think Gloworm are a bit optimistic re their claims of 14.5 LPM @ 60C, if one assumes that its output is 35KW? then it will raise the temperature of the water at this flowrate (14.5 LPM) by 35C and if one assumes average cold water supply temperature of 10C then you end up with a still very satisfying temperature of 45C, it will raise 10 LPM from 10C to 60C. Is it possible that they have a very small storage/buffer tank which will give 14.5 LPM @ 60C in combination with the boiler but only for a limited time until the storage tank temperature falls?.
35 KW gives a very decent hot water flow but from a space heating point of view it would/might be considered very over sized, 20 KW would probably be adequate for a 3 bed semi and with a turn down ratio of 5:1 will modulate to 4 KW which in a lot of cases will mean that the boiler will continuously fire for longer than the combi which will only modulate down to 7 KW or does a combi boiler have a higher turn down ratio than a system boiler?.
A oil fired combi of 35 KW which has no turn down would be cycling quite a lot under the above conditions.

Why’s everyone using 60 degree water. Good god.

John.G

Dtp1979 wrote: »

Why’s everyone using 60 degree water. Good god.

Very good but I suppose its a standard comparison condition.

Dtp1979

John.G wrote: »

Very good but I suppose its a standard comparison condition.

True yes. You’re the maths guy. How much cheaper is it to run the combi at 40 degrees for 100L compared to storing 100L hot water at 60 degrees allowing for heat loss

John.G

John.G wrote: »

I think Gloworm are a bit optimistic re their claims of 14.5 LPM @ 60C, if one assumes that its output is 35KW? then it will raise the temperature of the water at this flowrate (14.5 LPM) by 35C and if one assumes average cold water supply temperature of 10C then you end up with a still very satisfying temperature of 45C, it will raise 10 LPM from 10C to 60C. Is it possible that they have a very small storage/buffer tank which will give 14.5 LPM @ 60C in combination with the boiler but only for a limited time until the storage tank temperature falls?.
35 KW gives a very decent hot water flow but from a space heating point of view it would/might be considered very over sized, 20 KW would probably be adequate for a 3 bed semi and with a turn down ratio of 5:1 will modulate to 4 KW which in a lot of cases will mean that the boiler will continuously fire for longer than the combi which will only modulate down to 7 KW or does a combi boiler have a higher turn down ratio than a system boiler?.
A oil fired combi of 35 KW which has no turn down would be cycling quite a lot under the above conditions.

The commissioning process for a gas boiler requires the installer to adjust the output of the heating to match the heat load which has no impact on the hot water performance so you could have 35kw on the hot water side and 4 kw on the heating side if you wanted.

60c is a stored hot water requirement to prevent legionnaires which is not a factor for Combi boilers.

John.G

Dtp1979 wrote: »

True yes. You’re the maths guy. How much cheaper is it to run the combi at 40 degrees for 100L compared to storing 100L hot water at 60 degrees allowing for heat loss

My 150 litre cylinder loses ~ 0.45C/hr but some manufacturers claim far less losses, if one assumed 0.3C/hr then over a 24 hr period the loss might be 0.84 Kwh/100 litres, realistically, say 1.0 Kwh/day?.

John.G

gary71 wrote: »

The commissioning process for a gas boiler requires the installer to adjust the output of the heating to match the heat load which has no impact on the hot water performance so you could have 35kw on the hot water side and 4 kw on the heating side if you wanted.

60c is a stored hot water requirement to prevent legionnaires which is not a factor for Combi boilers.

Gary, does this mean that you can have 35KW hot water output but can reduce the maximum space heating to say 20KW and still have a turn down to 4 KW?
Maybe I should have understood you to say that you will still have 35KW max space heating but can turn it down to 4 KW on modulation?

John.G

John.G wrote: »

Gary, does this mean that you can have 35KW hot water output but can reduce the maximum space heating to say 20KW and still have a turn down to 4 KW?
Maybe I should have understood you to say that you will still have 35KW max space heating but can turn it down to 4 KW on modulation?

You could have 4 KW Max feeding two rads in a take away and 35kw feeding the numerous taps.

On commissioning of a gas boiler of any type the installation engineer should match the boilers maximum KW output for heating to the calculated max KW requirements this is done irrespective of HE or SE, a HE gas boiler will use technology to modulate for efficiency much more precise than SE but a lot of SE would also modulate.

The settings/adjustments for heating have no impact on the hot water settings as these are controlled by the boiler and a separate entity within the boilers programming.

John.G

gary71 wrote: »

You could have 4 KW Max feeding two rads in a take away and 35kw feeding the numerous taps.

On commissioning of a gas boiler of any type the installation engineer should match the boilers maximum KW output for heating to the calculated max KW requirements this is done irrespective of HE or SE, a HE gas boiler will use technology to modulate for efficiency much more precise than SE but a lot of SE would also modulate.

The settings/adjustments for heating have no impact on the hot water settings as these are controlled by the boiler and a separate entity within the boilers programming.

Thanks again, I think I have a fair grasp now of "Rate Ranging" etc, as shown
for the Gloworm although it does seem to have a narrow modulation ratio since its minimum output is 10KW.
https://www.glow-worm.co.uk/glow-worm/flexicom-cx-157167.pdf

Vokera explain the benefits of a wide modulation ratio both in heating and hot water demand
https://www.vokera.ie/2012/02/17/benefits-of-high-boiler-modulation-ratio%E2%80%99s-explained/

John.G

Dtp1979 wrote: »

Why’s everyone using 60 degree water. Good god.

Just for the record, from the technical details, the 35CX gives a hot water flowrate of 14.4 LPM at 35C rise. (This will give a water temperature of 45C from cold water at 10C)

4ensic15

Can a combi boiler be over-sized? An installer insists that if the boiler is over sized it won't condense as much and so will be less efficient.

4ensic15

4ensic15 wrote: »

Can a combi boiler be over-sized? An installer insists that if the boiler is over sized it won't condense as much and so will be less efficient.

If the boiler is fitted correctly I can’t see it being a problem.

The boiler will adjust to lower demands and can usually have the heating output reduced by the installer to allow for smaller heating systems without effecting the hot water performance.

4ensic15

I have an installer wanting to install a combi in the attic from the mains. there are 4 taps and a washing machine and dishwasher on the main already before it gets into the attic. It sounds like madness not to pump from the attic tank.

4ensic15

4ensic15 wrote: »

I have an installer wanting to install a combi in the attic from the mains. there are 4 taps and a washing machine and dishwasher on the main already before it gets into the attic. It sounds like madness not to pump from the attic tank.

Depends on the oomph from the mains, the more oomph the better.

4ensic15

gary71 wrote: »

Depends on the oomph from the mains, the more oomph the better.

The mains will be weak if all of the taps and appliances run together. That would starve the boiler, surely.

4ensic15

4ensic15 wrote: »

The mains will be weak if all of the taps and appliances run together. That would starve the boiler, surely.

But that isn’t what usually happens most people adapt to the characteristics of a combi with good mains pressure, I inspected lots of mains fed combis in lofts and there never was a problem with good incoming mains .

Given a choice I wouldn’t pump a combi but when in Rome...