Combi's and DHW Flow rate

[simonbalaam]

Hello,
We are in the process of replacing our old Boiler (Baxi 133 HE Plus) which is stated to have a DHW Flow rate @ 35°c 16L/min.
Now looking at the boilers that are available now we would really like to have one that can get close to this flow rate. We have been looking at the Worcesters you need to go for the 8000 40kw range. Is this about right?
Is there any boilers that you would recommend?
Sorry for any stupid questions.
Thanks
Simon

 

[Madrab]

Unfortunately Combi boiler manufacturers HW delivery figures are created in a wonderous place called Nirvana, where everything is absolutely wonderous and tailoured to each individual requirement at the time of testing and nothing every changes.

In the real world it's never like that I'm afraid and trying to get close to the Manu's specs can be really difficult unless all the requirements are met and never vary. To obtain those kind of figures then yes, invariably you need to go large.

Are you just trying to maximise the output or are you specc'ing a combi where the site requirements would be beyond most of them and stored HW would be the more suitable option, or at a push a combi with some inbuilt storage?

 

[bernardgreen]

Madrab's comment about manufacturer's claims for their boilers is valid.

 

[gas112]

baxi 636 / 836 will give you 15L great reliable boilers head and shoulders above your present 133

 

[simonbalaam]

Unfortunately Combi boiler manufacturers HW delivery figures are created in a wonderous place called Nirvana, where everything is absolutely wonderous and tailoured to each individual requirement at the time of testing and nothing every changes.

In the real world it's never like that I'm afraid and trying to get close to the Manu's specs can be really difficult unless all the requirements are met and never vary. To obtain those kind of figures then yes, invariably you need to go large.

Are you just trying to maximise the output or are you specc'ing a combi where the site requirements would be beyond most of them and stored HW would be the more suitable option, or at a push a combi with some inbuilt storage?

I agree about the claims but they have to get figures from somewhere i guess.
We dont really have the option of stored water anymore as the house has evolved over the last 18 years to have no space anymore for a tank. Really just wanting as high flow as possible but not over speccing the Central heating side. Can you de-rate the Central side so that even if the boiler is over specced on the central that can be reduced by the installer but it allows for the hot water flow to be high?
I hope that makes sense!!

 

[Johntheo5]

Hello,
We are in the process of replacing our old Boiler (Baxi 133 HE Plus) which is stated to have a DHW Flow rate @ 35°c 16L/min.
Now looking at the boilers that are available now we would really like to have one that can get close to this flow rate. We have been looking at the Worcesters you need to go for the 8000 40kw range. Is this about right?
Is there any boilers that you would recommend?
Sorry for any stupid questions.
Thanks
Simon

Combi boiler DHW flowrate is often quoted at a DHW temperature rise of 35C. So 33kw will give a flowrate of 13.5LPM with a dT of 35C or a HW temperature of 41C in winter with a mains temp of 6C or say 50C in the summer with a mains temp of 15C.
If one assumes 40C as a reasonable showering/washing temperature, then the flowrate can be easily calsulated from..... LPM= (boiler output in kwX14.33)/(40-mains temperature) or put in your own required HW temperature.
So LPM = (kwX14.33)/(HWTemp-MainsTemp), A 35 kw boiler will give, 35x14.33/(40-6), 14.75LPM from mains at 6C to HW at 40C. and so on.

 

[bernardgreen]

I agree about the claims but they have to get figures from somewhere i guess.

Publicity from one manufacture claimed their new boilers were 127.6% efficient.

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It is not possible to have an efficiency of greater than 1 or an efficiency percentage greater than 100%. This would mean that more energy is being transferred than is being supplied, which would mean that energy is being created. This would break the law of conservation of energy.

 

[vulcancontinental]

To the cynics; if the manufacturers are lying then call them out.

With the correct amount of gas and a stated flow rate the combi boiler will raise that flow rate by the temperature claimed, this is confirmed not in nirvana but a lab and then certified independently (there is a tolerance but the machines aren't built to the lower tolerance).

If the machine does not fulfill that performance there will be a shortcoming in the installation which can be proven onsite with digital gauges and equipment.

If Baxi say they can achieve those figures they can. The installation has then to prove it can fulfill its side of the bargain.

If you can disprove this sue the manufacturer, if you can't do this I guess it's just trash talk, a curse of our age. Competitor manufacturers monitor each others machines minutely so there'd be a swift reaction to any false claims.

Trying to link a water flow issue with claims that are confusing obfuscating net and gross combustion efficiencies does not help nor support valid evidence.

With the boiler in standby run the hot tap. With a digital thermometer measure the cold water temperature running through the machine. When the temperature is stable set the flow rate to 16l/m, start the boiler and note the temperature rise. If it isn't 35 degrees gas rate the appliance.

 

[simonbalaam]

I think what i'm trying to work out is Modulation.
Can the system run say at maximum to gain the hot water flow but be modulated down on the central heating side?
So say for example a Worcester 8000 Life 35kw, for the DHW let run at maximum but the CH be modulated down to say 25kw?

 

[vulcancontinental]

I think what i'm trying to work out is Modulation.
Can the system run say at maximum to gain the hot water flow but be modulated down on the central heating side?
So say for example a Worcester 8000 Life 35kw, for the DHW let run at maximum but the CH be modulated down to say 25kw?

Don't know about Worcester or Baxi, most combi's have a hot water 'power' and a lower heating output.

Better combi's can be range rated on the heating output. Range rating means the maximum heating output is limited eg a three bedroom semi built in the 30's with solid walls is very unlikely to have a heat loss over 10kW and usually quite a bit less.

Modulation is how low the boiler will run when heat loss is not at max but minimum. eg if the property loses 10 kW at -5 degrees it'll be around 5kW at +10 degrees. All boilers will cycle, modulation limits cycling.

 

[ianmcd]

I think what i'm trying to work out is Modulation.
Can the system run say at maximum to gain the hot water flow but be modulated down on the central heating side?
So say for example a Worcester 8000 Life 35kw, for the DHW let run at maximum but the CH be modulated down to say 25kw?

All modern boilers do this

 

[Johntheo5]

Yes.

 

[simonbalaam]

Thanks everyone