Domestic vs Commercial Boiler in large house?

[Brewer999]

Hi - I am currently refurbishing a house and I'm replacing an ancient boiler with a new system. The problem is I'm struggling to get consistent advice from the plumbers I've had round to look at the job.

The house is roughly 3,300 sq ft with 3 bathrooms, downstairs wc etc. A number of plumbers have said we need at least 60kw+ from a boiler and so we would need to look at a commercial boiler.

Another plumber has come back and suggested we put in 2 domestic boilers instead. The first boiler is a Worcester Bosch 40 cdi to run the downstairs CH and heat a 300 litre unvented cylinder. The cylinder will provide the hot water to the bathrooms and WC.

The 2nd boiler is a WB 28cdi to provide the central heating upstairs. This will also provide hot water to the kitchen and utility room.

I'd like some advice on how to approach this and what is the likely price difference from a commercial boiler vs 2 domestic ones described above? Does a commercial boiler cost a lot more install, run and service than 2 domestic boilers? Is there any other way to deliver a system other than has been suggested?

Thanks in advance for any comments/advice.

 

[Phill Peck]

Regardless of what boiler model you choose...

Go with 2 domestic boilers and find an installer who is familiar with this type of work....

Maintenance will be cheaper. Comercial boilers will require a commercial engineer and parts are often more expensive.

If one breaks the other can run even if only to run part of the house (depending on setup)

2 system boilers on a header would be better than completely dividing the systems. Imo

With what is suggested above if one of the boilers failed you could be left with no hot water and a cold storey of the house.

 

[MJGas]

For that sort of money why dont you put in a 40kw system something like an andrews water heater.

 

[MJGas]

Also what size gas meter is at the property?

 

[shray]

I doubt the gas meter would be an issue at all. Also need to take into account how many people are likely to live in the house, how many bedrooms etc.

For me the best bet would be a top end system boiler with high output, each level zoned seperately and an unvented cylinder sized up to meet the likely demand of the occupants.

 

[Dan Robinson]

60kW for 3300 square feet?

Does the place have I've side open to the elements or something?

 

[D_Hailsham]

60kW for 3300 square feet?

Does the place have I've side open to the elements or something?

You can't rely on the existing boiler as a guide to what size boler you need as oversizing boilers was very common in the past (and still is ), when fuel prices were very low.

Use the Whole House Boiler Size Calculator to find out what size boiler you really need.

 

[Brewer999]

Thanks for the advice so far. The house has 5 bedrooms and all the plumbers were happy with a domestic system until they did the calculations based on the rooms. They then quoted the 60kw+ figure.

I've asked a commercial firm to come out to take a look for another opinion.

 

[D_Hailsham]

Thanks for the advice so far. The house has 5 bedrooms and all the plumbers were happy with a domestic system until they did the calculations based on the rooms. They then quoted the 60kw+ figure.

Have you used the calculator I recommended? Until you do that, you won't know if the plumbers' estimates are anywhere near the correct figure.

I've asked a commercial firm to come out to take a look for another opinion.

You're jumping the gun.

 

[Brewer999]

Thanks for the advice so far. The house has 5 bedrooms and all the plumbers were happy with a domestic system until they did the calculations based on the rooms. They then quoted the 60kw+ figure.

Have you used the calculator I recommended? Until you do that, you won't know if the plumbers' estimates are anywhere near the correct figure.

I've asked a commercial firm to come out to take a look for another opinion.

You're jumping the gun.

I used the calculator and it reckons about 30kw but does that take into account the number of bathrooms etc or does that make any difference?

 

[Agile]

Only 2 kW is added to cater for hot water provision.

Most five bed houses need 18 kW or 24 kW.

Of course of particularly large or badly insulated then perhaps 30 kW.

But rather than add boiler power its best to add insulation!

Is the annual gas bill really about £4000 ?

Tony

 

[D_Hailsham]

I used the calculator and it reckons about 30kw but does that take into account the number of bathrooms etc or does that make any difference?

30kW sounds more like it. I assume that includes the 2kW HW allowance. I got 37kW assuming it was a square house with poor insulation and single glazed wood windows in the North

As for the HW requirement, a typical bath holds about 80 litres. If the hot water is initially at 60C and the cold at 15C and three very hot baths (45C) are run, one after the other, without reheating the water, you would use up about 200 litres of hot water - assuming a 300 litre cylinder.

A typical 300 litre fast recovery cylinder, with a 25kW coil, will take about 30 minutes to reheat after 70% of the hot water has been used.

Now this does not mean that you have to provide an additional 25kW for the very simple reason that the boiler is not running at full 30kW output all the time. In fact, for most of the heating season it will probably be running at minimum output. A few kW extra will provide sufficient capacity.

If you configure the system so hot water has priority, you don't even need to allow any extra; the 30KW will do fine. Though I would install the next size up, just to be on the safe side.

Are youi replacing the radiators? If so, it's a good idea to oversize them by 20-25%. This will allow the boiler to run at a lower temperature, so it will condense more, so saving money.

Did your plumbers provide calculations showing how they managed to reach a boiler size of 60kW? It would be interesting to see them.

 

[Dan Robinson]

For a house like yours I would be assuming a 38kW Atag with hot water priority feeding a 250 litre cylinder.

Weather compensating the heating if insulation and draft proofing is reasonable.


Whoever has suggested 60kW should be laughed down the drive.

We have houses over 6000 square feet with 50kW heat loss and lots of hot water requirements and even then we have a 60kW boiler running the lot including hot water. Should the cylinder reach 50% depletion then most of the heating is shut off by the controller to allow the boiler to concentrate on hot water and the ground floor UFH.

3300 square feet and 60kW is nuts.

 

[SilentDancer]

Worcester have an offer where if you fit a boiler with one of their cylinders you can get a ten year guarantee on the boiler with 25 years on the cylinder.

 

[dcawkwell]

One 35kW boiler is going to be plenty.
If you go for a biomass boiler it is free!!!