Conflicting advice from corgi installers

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I want to replace our ~20 year old boiler with a condensing combi.

I'd like to install one that allows for 1 bathroom (with shower) and a shower room, so basically be able to handle 2 people having a shower and someone washing up in the kitchen simultaneously.

Property is 3 bed flat with 6 radiators in London.

My mains pressure is ~3 bar and I wanted a 15ltr/min boiler ecotec plus 837 (job to include replacing two radiators and disconnecting a gas fireplace)

1 British Gas installer said that's too big, a 9.8ltr boiler would do, "it's all about the mains pressure coming in" (charging ~£5500 inc vat )
2 Independent A installer said 15ltr/min is fine (£2990 inc vat 837 boiler)
3 Independent B says 837 is too large and to go for 12ltr/min (waiting for quote) - and he says the larger boiler is for when you have lots of radiators.
4 Independent C (over the phone quote) said 9.8ltr/min (£2700 inc vat 824 boiler)

Who is right?

Are Drayton TRV valves better than Honeywell?

Many thanks in advance!
 
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be able to handle 2 people having a shower and someone washing up in the kitchen simultaneously.

No combi can run two showers and a sink at the same time, you need a system boiler and unvented cylinder for that. A Vaillant ecotecplus 937 or a glow worm Xtramax He are about the best performing combi's (for hw flow rates) on the market imho. The heating load of the radiators is almost irrelevant as the main gas usage of the boiler is to heat the water. Providing you have an adequate cold mains supply I'd opt for the 937 myself. Another option you could look at is the glow worm ultrapower sxi 170.
 
You always size a combi for your hot water usage.

If you have mainly showers the 831 will be fine. If you have mainly baths and your pressure/flow rate is good then the 837 will be better.

From your post go with the 831, avoid the 824/9.8 l/min at all costs.

Take heed of DM's post though.
 
Any combi will keep your radiators hot all day with no problems, its the hot water supply that needs careful thinking.

No combi will be able to supply 2 mixer showers and a tap at the same time. Period!

I like the Danfoss RAS-D more than Honeywell or Drayton.
 
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Thank you for your replies!

Pressure vs Flowrate?

I assumed that flow rate per shower would be 15 ltr/min divided by 2, is that correct? And both showers would get 3bars (mains pressure)? Or have I got it the wrong way round?
 
Pressure has little to do with it. All (almost all) combis use a benchmark of 35 degree C temperature rise. If incoming cold is at 5 degrees C, hot water delivery will be at 40 for the stipulated flowrate (9.5 for 24kwatt model, 10.5 for 28kwatts being examples).

If a boiler delivers 12 l per minute, and showers are on different levels, do not expect both showers to get equal division of hot water. And if someone runs another hot tap, expect to be disappointed.

Another thing to consider is incidence of boiler breakdown. Combis keep me busy throughout the year. I would install unvented in this case to even a 24kwatt vaillant. Vaillant supplies water to the kitchen and cylinder to showers and bath. Unvented then makes good use of 3bar water pressure. I hope 3bar is not static pressure.
 
HW temperature rise is at 30 degrees. I could set a boiler up to give 80l per minute which would mean noting as temperature rise might be 15 useless degrees. Good figure but of no use.
 
I've read this thread and I can't see the incoming flow rate (litres per min) into the property. That will decide what rating HW boiler is needed.

Any quoter that didn't measure it is a chancer and should be removed from your list. If you have a water flow reading, take off around 20% for cold water and what is left is your ideal boiler output. Therefore if you have 15l/m incoming, a boiler output of 12l/m is about the maximum you will get the use of.

Bear in mind that nearly all boilers are set to 42C temp rise on their output restrictor; the 35C rise flow rate figure is for comparison only. In other words, if a Vaillant shows an output of 12 litres at 35C rise, at 42C rise it will give about 10 litres/min.

Honeywell valves are preferred by us because the lockshield body is the same one used by a TRV. That means that you can take a TRV and fit it on a different radiator if you wish, in a few seconds. Or you can upgrade a room to TRV control at any time without calling a plumber or draining down.

Drayton valves are however a similar cost and quality. But the lockshields and TRVs are different.
 
Simond, good post but it has raises a point which your view would certainly clear.

I go to a boiler, burner pressure is as per the data badge, hot water delivery is as per the weir gauge. All I ever see is about 37 degrees temperature rise. Have also checked with clamp thermometers which shows around 37 degree temp rise.

Water in Glasgow today is at 15 degrees C. Serviced a 28Kwatt boiler which delivered hot water at 51 degrees C. Flowrate and burner pressures as per the data badge.
 
Thanks for the replies!

How can I measure my mains pressure and flow rate myself?
 
If BG are telling you to go for a 9.8 litre boiler then it's fair to say your pressure and flow are poor.
 
If BG are telling you to go for a 9.8 litre boiler then it's fair to say your pressure and flow are poor.

Or that they are trying to keeping the quote price low as they already overcharging extortionately :rolleyes:
 

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