# What size condensing oil boiler?



## lfcjfc (28 Jul 2008)

Just at the heating installation stage of a new build. House is about 3000sq ft and planning to run UFH and DHW off a condensing oil boiler. Plumber originally quoted to install a 118,000 BTU (26KW) boiler, now the time has come, he has bought a 88,000 BTU boiler. I'm concerned that the boiler is too small for the job. A couple of UK websites that have boiler-size calculators on them, tell me I should have around 27 - 32KW. He also plans to instal a 200L water tank which I also have concerns about (will be running 3 showers and a bath off it). Any plumbers out there that would be able to tell me if he is way off the mark or not? I dont want to be ripping out stuff at a later stage.


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## DavyJones (28 Jul 2008)

lfcjfc said:


> Just at the heating installation stage of a new build. House is about 3000sq ft and planning to run UFH and DHW off a condensing oil boiler. Plumber originally quoted to install a 118,000 BTU (26KW) boiler, now the time has come, he has bought a 88,000 BTU boiler. I'm concerned that the boiler is too small for the job. A couple of UK websites that have boiler-size calculators on them, tell me I should have around 27 - 32KW. He also plans to instal a 200L water tank which I also have concerns about (will be running 3 showers and a bath off it). Any plumbers out there that would be able to tell me if he is way off the mark or not? I dont want to be ripping out stuff at a later stage.



Sounds a bit small alright. It you want to do a rough calculation for heat requirement, Mutiply W x L x H x 5 of each room then add them up and then add 10%. If you use feet you will end up with BTU's. Divide by 3.412 for watts. there are 1000 watts in a kilowatt.

When the plumber fits the correctly sized boiler make sure it is set right. will save you money in the long run.


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## sydthebeat (28 Jul 2008)

Most calculations of boiler sizes do not take into account the standard of construction (u walues of construction, airtightnes, etc) so the boiler sizing might be ok.


This is an excerpt from an email i recieved on this very issue.

"........_The boiler size will depend on the house size and the build quality. A new house with an A2 Building Enegy Rating will require a much smaller boiler than an existing house with a B3 rating._

_The best way to select your boiler is to assume a heatloss of 50 watts/sq.m. This would be typical for a B1/B2 house. A 3000 sq.ft fouse is 280sq.m. This give a heatloss of 280 x 50 = 14.0kW._
_What this means is the the building will loose 14kW of heat every hour. As most houses in this country have radiators fitted which means that the heating is turned off for long periods and the house has then to be heated from a cold start._
_If we were to fit a 14kW boiler it would take in 60 to 90 minutes for the house to each its design temperature - this would be unacceptable._

_For this reason we would size the boiler for twice the heating load i.e. 28kW and add 5kW for domeestic hot water to give 32kW or 110 BTU's._
_The recommended boiler for this application would be 30kW_

_For new build with an anrticipated energy rating better that B1 we would use 40 watts/sq.m."_

so ifcjfc, have you any idea what rating your dwelling will have?


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## DavyJones (28 Jul 2008)

interesting email. The math is a bit off but otherwise the original plan for ifcjfc's 120 btu boiler seems to be correct. I rather be over then under.


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## sydthebeat (28 Jul 2008)

DavyJones said:


> interesting email. The math is a bit off but otherwise the original plan for ifcjfc's 120 btu boiler seems to be correct. I rather be over then under.


 
well, unless ifcjfcs dwelling is A rated id say the 26kw boiler is slightly undersized. I would be like you are well and prefer to over size, just in case.

however ive seen some installers put 35-40 kw boilers into smaller, well rated, dwellings. i supposed its something that needs to be looked at by building regs.


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## DavyJones (28 Jul 2008)

Sorry, 26KW is a miscalculation by ifcjfc. 118000 btu is just shy of 35KW. I always oversize, boilers can be adjusted to give the correct heat requirement (within reason). So for example if a client would like to build extension , add radiators etc this would be allowed with a small adjustment at the burner.
The most important thing is that the burner is commissioned by a compentant person. The days of just hooking up a boiler and switching it on should be behind us.


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## sydthebeat (28 Jul 2008)

The plumber is looking to put in a 88,000 btu (26 kw) boiler. I would up that to 30k myself. What do you think davy?


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## yob (28 Jul 2008)

Having 3 showers and a bath doesn't tell us anything of usage.
my house came with 225 ltr tank,it caused more arguements,theres 5 of us,and never any hot water,i've since installed 300ltr alongside the old one,loads of hot water.
i must stress we play alot of sports,so at least 2 -3 people would shower twice a day.so thats 7 or 8 showers a day.


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## DavyJones (28 Jul 2008)

I see now. I think that 26kw boiler might have been meant for another job that didn't work out or the plumber is trying to save a few quid. I would nearly go for a rated boiler up to 120,000 btu, Slightly over 35kw. this gives plenty of scope.


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## lfcjfc (29 Jul 2008)

Folks - thanks for all the information. 

Davy - your thoughts on the boiler being meant for another job is exactly what went through my mind. 

Syd - I'm not sure what rating the house will achieve - I have to admit at the time we designed it, it wasnt really a factor (would be if we were doing it now though). However we spent as much as we could on insulation - the house is standard cavity build pumped with polypearl, we drylined all external walls with 38mm rigid insulation plasterboard. Roof pitch insulated with Rafterloc with thermoboard to the underside of the rafters. The insulation I am most concerned about is the floor - where height restricted us to only 50mm. Windows were quoted at a u-value of 1.2 - best we could get without going the extra few thousand for triple glazing. The back of the house is south facing and so benefits from solar gain during the day. When I questioned the plumber on the boiler size, his answer was that with all the insulation, we wouldnt need anything bigger - that would be based on his own gut instinct though rather any particular knowledge of the energy rating.

I think based on what I've heard here that I will ask him to upsize the boiler to 35KW. Can the boiler be too big for the job and therefore be inefficient?


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## DavyJones (29 Jul 2008)

lfcjfc said:


> Folks - thanks for all the information.
> 
> 
> 
> I think based on what I've heard here that I will ask him to upsize the boiler to 35KW. Can the boiler be too big for the job and therefore be inefficient?




If you get a 90(000) - 120 (000) boiler, it can be adjusted to any output in between. your plumber should know how to do this. If not, injector size and oil pump pressure dictates output. I can send you a data sheet if required showing outputs using these two factors. 
If your home gets bigger in the future a simple adjustment up's the output.


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