Oversizing Modulating Boilers

celticwarrior1

Hi,

I have calculated a boiler requirement of 13kw for my house so I was going to go with the WB Greenstar 15i.

However the 18i is a lot cheeper. So as it self modulating is it ok to oversize by this much?

esox28

Do you mind me asking how you came to the heat requirement 13kw?

celticwarrior1

esox28 wrote: »

Do you mind me asking how you came to the heat requirement 13kw?

Hi, I used a radiator sizing calculator for each room and gave an extra 3 kw allowance for the hot water cylinder.

esox28

Perfect. Yea sorry didn't mean to come across like I was questioning your result, just needed to establish the bases for heat loss.

Boiler manufacturing companies usually state you should match the heat load of the system as close as possible to the boilers maximum rated output especially when installing condensing boilers. In general terms your boiler needs to be 'chasing the temp' over sizing a condensing boiler would be Undoing any savings you would have made with the A-rated condenser.

esox28

esox28 wrote: »

Perfect. Yea sorry didn't mean to come across like I was questioning your result, just needed to establish the bases for heat loss.

Boiler manufacturing companies usually state you should match the heat load of the system as close as possible to the boilers maximum rated output especially when installing condensing boilers. In general terms your boiler needs to be 'chasing the temp' over sizing a condensing boiler would be Undoing any savings you would have made with the A-rated condenser.

It's the other way round you match the boiler output to the heat load requirement. Most manufactures give this information in the MI and would be part of the commesioning process and is handy if you have two rads and you want to fit a 38kw combi.

Over sizing a boiler is like trying to put a elephant in to a rabbit hutch, ideally a RGI would make adjustments to the boiler to match the heat load be it 18 kw boiler or 15kw.

esox28

Ha ha gary but not really the other way round you phrased it right but mines same but different. :P

celticwarrior1

gary71 wrote: »

It's the other way round you match the boiler output to the heat load requirement. Most manufactures give this information in the MI and would be part of the commesioning process and is handy if you have two rads and you want to fit a 38kw combi.

Over sizing a boiler is like trying to put a elephant in to a rabbit hutch, ideally a RGI would make adjustments to the boiler to match the heat load be it 18 kw boiler or 15kw.

Don't the new High Efficiency gas boilers self modulate through various outputs rather than manual adjustments made during commissioning as you say?

celticwarrior1

celticwarrior1 wrote: »

Don't the new High Efficiency gas boilers self modulate through various outputs rather than manual adjustments made during commissioning as you say?

Elephant rabbit hutch;)

Over sizing boilers leads to problems which modulation will not help you with.

A properly commissioned boiler fitted by a RGI will work as the manufactures intended without any messing, most of the work I do is generated by installers not reading the MI, I'd like to pretend I'm really really clever but the only difference is that I have opened the manual.

celticwarrior1

gary71 wrote: »

Elephant rabbit hutch;)

Over sizing boilers leads to problems which modulation will not help you with.

A properly commissioned boiler fitted by a RGI will work as the manufactures intended without any messing, most of the work I do is generated by installers not reading the MI, I'd like to pretend I'm really really clever but the only difference is that I have opened the manual.

That is what I suspected.

So is there a general rule of the maximum % a boiler can be over sized where as modulation can take over without any loss in efficiency?

If I go with the 15 kW boiler I would be over sizing by around 15%

And if I use the 18 KW boiler I would be over sizing by close to 40 %

PHIJOE

celticwarrior1 wrote: »

That is what I suspected.

So is there a general rule of the maximum % a boiler can be over sized where as modulation can take over without any loss in efficiency?

If I go with the 15 kW boiler I would be over sizing by around 15%

And if I use the 18 KW boiler I would be over sizing by close to 40 %

Not sure about greenstar, but some of the other makes glowworm for example can be range rated down as low as about 9kw on a 18 kw boiler.
so buying 18 kw and bringing it down to 13kw is possible, plus you have an 18kw boiler if you ever extend.
Finding output for your boiler is better to size your house .

Modern gas boilers are very intelligent and will use fan and pump modulation to run as efficiently as possible, so it's not just a question of efficiency, it's will it work then followed by how efficient is it now it works.

If you only have a damand 13 kw and a boiler rated to 18 kw it can cause problems as the boiler will fire as a 18kw then modulate down but if the heat isn't absorbed by the system quickly enough the boiler will reach temperture and go off for 3-7 minutes before it repeats the process.

Personally I favor a W-plan wiring system as you can take the hot water requirement out of the boiler sizing and have less issues with over sizing for zone controls on the heating circuits.

Dtp79

gary71 wrote: »

Modern gas boilers are very intelligent and will use fan and pump modulation to run as efficiently as possible, so it's not just a question of efficiency, it's will it work then followed by how efficient is it now it works.

If you only have a damand 13 kw and a boiler rated to 18 kw it can cause problems as the boiler will fire as a 18kw then modulate down but if the heat isn't absorbed by the system quickly enough the boiler will reach temperture and go off for 3-7 minutes before it repeats the process.

Personally I favor a W-plan wiring system as you can take the hot water requirement out of the boiler sizing and have less issues with over sizing for zone controls on the heating circuits.

Would you mind explaining w plan Gary? I've heard of it but not overly familiar with it

Dtp79

Dtp79 wrote: »

Would you mind explaining w plan Gary? I've heard of it but not overly familiar with it

W-plan is the given name to a heating and hot water system that utilises a 3-port valve controlled by a programmer and stats to determine the flow of heating water. The difference is with a W-plan the hot water has priorty and allows no flow to the heating circuit untill the hot water stat is satisfied allowing the installer to fit a smaller output boiler than if the cylinder was being heated the same time as the rads.

http://www.honeywelluk.com/products/Systems/Wired-Sundial/Sundial-W-Plan/

Dtp79

gary71 wrote: »

W-plan is the given name to a heating and hot water system that utilises a 3-port valve controlled by a programmer and stats to determine the flow of heating water. The difference is with a W-plan the hot water has priorty and allows no flow to the heating circuit untill the hot water stat is satisfied allowing the installer to fit a smaller output boiler than if the cylinder was being heated the same time as the rads.

http://www.honeywelluk.com/products/Systems/Wired-Sundial/Sundial-W-Plan/

Grand thanks Gary.

DGOBS

http://wiki.diyfaq.org.uk/index.php?title=Central_Heating_Controls_and_Zoning