Why big boilers?

[HiQ]

Hello

Intro: -
I am talking system, not combi boilers here.
I am generalising somewhat, but my observation is based on 7 fairly recent experiences.

The question:-
Why do some plumbers recommend boiler sizes 50%-100% larger than the energy saving trust sizing tool indicates.

Reason for question:-
I want a boiler that will run at highest possible efficiency all year round, and I do not mind using supplementary heating, if required for a few hours a year.

Many thanks

 

[SimonG]

Need to ask those plumbers specifically.

 

[SimonG]

Also its not all about the size of the boiler but how the boiler is utilised.

 

[lame plumber]

A little correctly sized boiler will do better in the long run, as with most things in life size isnt the be all and end all.

 

[snowhead]

Why do some plumbers recommend boiler sizes 50%-100% larger than the energy saving trust sizing tool indicates.

I assume you've had 7 quotes and they all want to fit boilers bigger than you've calculated using the sizing tool which in common with all theoretical calculations uses averages.

The 7 plumbers however will be using real life experience of systems they've fitted previously.
If you want a system guaranteed to work, you'll have to go with the judgemenet of the experienced installer.

If you want a minimal sized boiler you'll have to take the risk of the system not being fit for purpose and it could be seriously out, so your supplementary heating may be on all year round.

I doubt you'll find a Professional to fit a boiler they consider too small.

The clue is you've had 7 people all agree.
Who's right?

 

[Ray Stafford]

Its because if you make a mistake on the too-large side, the downsides of marginal inefficiencies and slightly shorter appliance life are virtually indetectable.

Whereas if you make a mistake on the too-small side, you have a very real, very measurable complaint right the hell now.

Ergo there are more powerful incentives to oversize, rather than to undersize.

 

[LPC]

^ exactly that.

Oversizing is the matter of a few quid. Undersizing is the matter of a poorly performing heating system.

 

[lame plumber]

oversizing equates to short cycling and a poor performance, not just a few quid

 

[Ermintrude]

oversizing equates to short cycling and a poor performance, not just a few quid

10% ok 50% just wrong

 

[HiQ]

Also its not all about the size of the boiler but how the boiler is utilised.

Very true - but non of the plumbers asked me about how we would use the system.

I volunteered the information. When I talked about short cycling, using low boiler temperature, and balancing the rads I either got raised eyebrows or no comment.!!

 

[HiQ]

oversizing equates to short cycling and a poor performance, not just a few quid

That is my point. Over 10 years use, and with ever increasing cost of gas - will the inefficiency add a lot to the £300 extra cost of the boiler?

 

[HiQ]

Its because if you make a mistake on the too-large side, the downsides of marginal inefficiencies and slightly shorter appliance life are virtually indetectable.

Whereas if you make a mistake on the too-small side, you have a very real, very measurable complaint right the hell now.

Ergo there are more powerful incentives to oversize, rather than to undersize.

Thank you Mr Stafford - suspected that may be the reason.
Now I can have a more meaningful conversation with my plumber.

 

[Worcester]

Guess also it's the difference between a £3k investment and a £30k investment...

For our systems we do full room by room heat loss calculations and radiator sizing, plus take into account items such as intermittent heating and exposed areas.... - And we charge for that sevice. ergo our boilers are always sized within 10%, plus buffer tanks hence even the 10% is nullified.

You could always use the stelrad stars tool and do yourself a heat loss calculation and radiator sizing analysis and I bet you find at least one room with an incorrectly sized radiator.