1 x 50kw gas boiler or 2 x 25-30kw gas boilers?

simplesimski

I've calculated the heating requirements for the house and it comes in at around 44kw, including 4kw for 300l dhw tank. (it's a big old 3 story house + basement)
Should we be fitting one semi commercial 50kw gas boiler or linking 2 25-30kw gas boilers...?
Any suggestions welcome...
Thanks.

antoinolachtnai

I am not a plumbing professional and I don't play one on the 'net but ...

I have a 2 story + basement georgian house in the middle of a terrace, basically 6 big rooms. I have a 26kW boiler (Viessmann 100W) I got recently. It seems to be plenty to heat it. The boiler doesn't really stay running on for more than a few minutes. It quickly modulates down to one-third power.

What I am really saying is that unless your house is massive (more than 3000 feet, say) and is exposed to the weather, it sounds like your allowance is very generous. I would have someone check it again. You can get 35kW domestic boilers and they sound like the sort of thing you need. But you really need good advice before spending this sort of money on a boiler.

What you want to avoid is having a boiler that is far too big for your needs. It will not be efficient. Also, if you have two boilers, the whole setup will be more complicated to operate and service. You would probably not need the two boilers working together for more than 15 days a year. i would imagine this would cause its own difficulties. But you really need to get advice from someone who actually puts in this scale of system on a regular basis.

I have insulated this house pretty well, including putting in secondary glazing behind the sash windows, which makes a big difference to comfort.

I would try to get a fast-recovery tank, which could draw 10 or 15kW. This will heat the water much faster and as a result your system will be able to deliver more hot water. The way the modern boilers work is that they heat the DHW at high temperature, and then switch over to heating the radiators at a lower temperature. This is to maximize the efficiency of the boiler (these boilers work most efficiently at lower temps, but DHW needs higher temps). (In this house, i still have the old slow-recovery tank. It works fine, but if there were more demand for hot water, it would not be great.)

simplesimski

Thanks antoinolachtnai for your comments, this house is 3 stories + Basement and all the rooms are between 3-4 m high which is very demanding on radiators for heating them... I have done my calculations 3 times, once with a Mears central heating calculator and twice with online heat requirment calculators and it ranges between 41 - 44KW total so i think a boiler over 35KW is definately needed...?

heinbloed

If all the house is going to be used as 1 household then a two boiler solution seems to be approbriate.
These are to be installed as a cascade.
One boiler would have a fixed output at about half the space heating load and the second boiler would be a modulating boiler.

Some boiler manufacturers offer a special service for free or at low cost for the design of the cascade.
Make sure your energy demand is calculated to EN12831, with this you can aproach the boiler manufacturers. Contact them via the home page, they have teams of heating engineers who would be able to help.

MiniGolf

simplesimski wrote: »

I've calculated the heating requirements for the house and it comes in at around 44kw, including 4kw for 300l dhw tank. (it's a big old 3 story house + basement)
Should we be fitting one semi commercial 50kw gas boiler or linking 2 25-30kw gas boilers...?
Any suggestions welcome...
Thanks.

Worcester-Bosch do a 40Kw CDi boiler.... I installed one in a house not dissimilar (by your description) to yours and it heats it without any problem!!
I will say that quality controls are a must and preferably with weather compensation or at the least optimum start control. Although you have calculated the heating load at 40-44 Kw the chances of it needing all 44 are remote..... trv's/zones etc.

DGOBS

Long term with 2 appliances your service and repair costs will be close to double

simplesimski

Thanks MiniGolf, How ru finding that Worcester-Bosch 40Kw boiler, is it a regular one or combi...? I've read some reviews about that brand which wern't the best but maybe its all about the installation...

antoinolachtnai

One thing - if you have done the measurements and everything is calculated in accordance with the EN, then that's really your figure and you really need to get those KW unless you have very strong advice to the contrary. It does sound kinda high, but if those are the numbers, then those are the numbers. It is obviously a very big investment and you have to get it right.

From my research, that is a perfectly good brand of boiler. It is in the upper tier for sure. There are certainly differences between the different top tier boilers, and you could argue all night about the pluses and minuses of each one. But they are all perfectly good boilers.

It is true that correctly installing these boilers is very important. The reason seems to be that the design of modern highly efficient heat exchangers can cause any dirt that is in the system to get stuck.

A combi boiler is unlikely to be the answer for you, for all sorts of reasons. Main one is that the house is too big, there will be too many people in it (potentially, maybe not actually) and the peak demand will be too high. The second one is that the mains pressure might not be enough to reach the top of the building on a sustained basis, so then you'd need to pressure-boost the pump. Third one is plumbing regulations. Running your hot water directly off the mains feed is not something that the Dublin plumbing regulations allow for, for sure. Boosting the pressure is something you will need the approval of the local authority for, at least in Dublin, and I wouldn't say you'd get it.

Another suggestion is that all these boiler companies have helplines and they may be able to suggest a course of action. But you really need good local advice before making a final decision.

MiniGolf

simplesimski wrote: »

Thanks MiniGolf, How ru finding that Worcester-Bosch 40Kw boiler, is it a regular one or combi...? I've read some reviews about that brand which wern't the best but maybe its all about the installation...

I am an installer so things are from my point of view!!
I removed two C & M BritonyII boilers from this installation - they were less than 5 years old! It had gotten riduculous the constant breakdowns and the cost of spares!!
The original installation was not too bad - apart from the choice of boiler - and the finishing touches were not pretty.
I fitted the WB about 4 years ago and it hasn't mised a beat .
It gets worked hard too as the lady works from home a lot and to quote her own words she is "a bit of a lizard". I come out of the house in my tee-shirt as I can't stand the heat!!
Its a "conventional" boiler by the way - not a combi - with external pump and expansion vessel etc utilising a DCD Controller with outdoor/indoor and cylinder sensors.

sullzz

MiniGolf wrote: »

and to quote her own words she is "a bit of a lizard".

JohnnieK

simplesimski wrote: »

Thanks MiniGolf, How ru finding that Worcester-Bosch 40Kw boiler, is it a regular one or combi...? I've read some reviews about that brand which wern't the best but maybe its all about the installation...

On-line reviews are pants:D. They are written mostly by angry end users and appear on further examination to be system or installation issues and not the boiler. Worcester CDI's are Worcester top of the range and are good boilers.
Alternatively would be the Viessmann Vitodens 200, now that's a cleaver boiler.