1 large boiler vs 2 smaller ones?

[unclebob1]

Good morning all,

I recently visited a property where there were two lower power boilers fitted rather then one higher power one (eg 2 30kw over 1 60kw)...

I also just had a look at the Vallient brochure today, and they also showed configuration plans for both a single and multiple (in this case 3) boilers.

All boilers connect up to the same loops for radiator and hwt so its not like one is designated for one floor, the second boiler for another floor etc, or likewise one isn't for hw while the other for heating etc...

Anyone come across this type of installation before (be it vented or unvented) and can anyone give me pros/cons over having one large boiler vs 2 small?

Thinking more along the lines of running costs, wear on the boiler etc I understand that if one breaks then there's still another to keep the hot water running, heating in the house etc (not that it would be a long term solution as the boiler isn't designed for that capacity!)

Any info/ideas?

 

[Dan Robinson]

If you have two, then keep the setup as it is more beneficial in terms of reliability and efficiency.

It would be worth looking at the control regime and make sure that is all it can be too.

 

[unclebob1]

Thanks for the response, I'm planning my refurb and this was one of the things that i'm looking into - what are the benefits and costs?
Does it cost more to keep 1 60kw boiler running or 2 30kw? (i know on the face of numbers it should be the same but we all know it doesn't work like that in reality!)

 

[doitall]

For most part of the year you only need say 30Kw with the 2nd boiler as back up when needed, so big saving can be had if installed correctly.

Will say that both boilers should supply heating and hot water with zones, manifolds etc.

Dan Robinson Covers a large area of London so if you're doing a refurb it may be an idea to get him on board.

 

[Agile]

The maximum power output is only required for about 5% of the year during the coldest part of the winter when its below freezing outside.

For most of the rest of the year less than half of the maximum power is needed.

A smaller boiler operating closer to its maximum power is usually more efficient.

A property really needing 60 kW is going to have an annual gas bill in the range of £3k-£6k !

Many installed boilers have been grossly oversized which also reduces efficiency.

Tony

 

[unclebob1]

A property really needing 60 kW is going to have an annual gas bill in the range of £3k-£6k !

Tony

Thanks Tony - I was using 30kw / 60kw only as examples! but essentially its still better to have 2 smaller boilers running at say 80%, then 1 big boiler running at 35-40%

 

[doitall]

It's better to have 1 small boiler running at 80% with the second boiler cutting in as required.

 

[Onetap]

Anyone come across this type of installation before (be it vented or unvented) and can anyone give me pros/cons over having one large boiler vs 2 small?

The only advantage is in installations where a failure of the one and only boiler would be catastrophic (care homes, schools, offices, hotels, etc.) and where the client is prepared to pay for built-in redundancy.

I wouldn't bother, unless you're in one of those categories. Get one quality boiler and ensure the water quality and inhibitor treatment is safeguarded.

It used to be quite common with natural draught boilers, where you could only get so many BTU/hr from a boiler and plant-space was cheap. You can now get a huge amount of kW from fanned flue boilers, plant space is valuable and boilers are (fairly) reliable.

You do not see many items of plant (pumps, DHW cylinders) with duty and stand-by sets. You could run a building for most of the year with, say, one out of two boilers giving 60 or 75% of the design (mid-winter) heat output.

The major problem you'd have with this would be in finding a control system that could operate two boilers simultaneously. It's not generally an off-the-peg requirement and few of the boiler manufacturers cater for it.

 

[Onetap]

It's better to have 1 small boiler running at 80% with the second boiler cutting in as required.

Not any more.

Condensing boiler efficiencies are fairly uniform over the entire range of modulation, providing the return water temperature is low enough to allow condensation.

 

[doitall]

Two boilers are still better in the larger market 40-60Kw for example, and even more so where hot water only is required.

 

[Steelmasons]

Many advantages when using multiple boilers (two in your case) over a single boiler.

Most boilers on the market operate on a 5:1 turndown ratio or even lower , if the two 30kw boilers in the OP's case were to be controlled using a series mod set-up then turndown ratio would be around 10:1 with a minimum output of 6kw , using the single boler would net a turdown ratio of around 5:1 with a minimum low modulation output of 12kw , the other advantage being if one boiler were to become redundant heating/HW is not lost.

 

[Dan Robinson]

The Coopra 80 twin was an excellent concept.... 80kW max output and could modulate down to 7.

Just two 40kW units in a single case with a conjoined PCB.

Takes some looking after though

 

[Steelmasons]

It used to be quite common with natural draught boilers, where you could only get so many BTU/hr from a boiler and plant-space was cheap. You can now get a huge amount of kW from fanned flue boilers, plant space is valuable and boilers are (fairly) reliable.
.

I'm not sure the term "fairly" should be used regarding the reliability of the premix condensing boiler. , a bank of six potterton diplomats will be more releiable than one of those band A condensers.

 

[Steelmasons]

The Coopra 80 twin was an excellent concept.... 80kW max output and could modulate down to 7.

Just two 40kW units in a single case with a conjoined PCB.

Takes some looking after though

Another great reliable boiler Dan. ....Not

 

[Dan Robinson]

You old cynic ,


Actually maintained properly they were fine. Just don't tolerate installation defects well.