Boiler Sizing

iamlegend2008

Hi

Im currently researching which boiler model to use for an 1800 sqft dormer. The oil boiler will be used as backup since the house will hopefully be airtight and has 50mm insulated slabs on external walls etc.

Using the online guides I have calulated my heat and water requirements to be only 16KW (including 2KW water)

The nearest firebird condensing boiler is the C26 which is 15-26KW (50,000-90000 BTU) preset to 21KWH
The nearest grant condensing boiler is the HE50/70 which is 15-21KW (50000-70000 BTU)

I believe that either of these boilers would suit my needs but wouild like to know which is better suited to mu heat needs and why ?

Any help would be appreciated.

Cheers

sinnerboy

Forum rules will prohibit a straight answer to a query like that. Get you plumber first - be rigorous in this process , look for references . Seek his advice

sinnerboy

i.e. how do you size a gas fire boiler ?

I sent this text below to a lot of the "big names" . Few responded .
Of the 2 that did - one said "we don't size boilers - ask your installer"

Now i consulted one or two trusted plumbers and they said they always followed the advice of the boiler manufacturer ..... ( merry go round ) .

The second manufacturer that answered advised .... a rule of thumb .
For a B3/2 rated house - 100 watts/m2 - to provide a heat-up-from-cold time of 30mins . For B1 + rated house 80 watts/m2 . Then + 20% for DHW ( I was grateful for this - at least they gave an answer )

Other guidance can be found here - based on simplified data inputs

http://www.energysavingtrust.org.uk/.../boilersizing/

http://www.idhee.org.uk/calculator.html

But I failed to obtain an answer anywhere to the question I have made bold below .

So ...... if such a lack of clarity exists with the familiar old gas boiler - how do we ( i.e. non heating engineers ) size boilers ?

Dear Sirs

Up until recently I have relied on (volume ft3 x 5 + 20% for DHW ) = BTU to arrive at a domestic boiler size

I think we need to get condensing boilers sized better than that for them to condense

Is there a ( non heating engineer friendly ) method to calculate ?

Or ...

If DEAP* tells me ( proposed B2 rated house , 300m2 , well insulated , i.e. to TGD L 2007 ) -

"energy required for main space heating"
13200 kwh/yr
and
"energy required for main water heater"
5945 kwh/yr

From that , can I select a ( gas ) boiler size ? How do I get from ( combined energy demand ) of 19145 kwh/yr to "X" Kw boiler ?


Any ( relevantly qualified ) poster able to assist ?

sydthebeat

iamlegend2008 wrote: »

Hi

Im currently researching which boiler model to use for an 1800 sqft dormer.

1. The oil boiler will be used as backup since the house will hopefully be airtight and has 50mm insulated slabs on external walls etc.

2. Using the online guides I have calulated my heat and water requirements to be only 16KW (including 2KW water)

The nearest firebird condensing boiler is the C26 which is 15-26KW (50,000-90000 BTU) preset to 21KWH
The nearest grant condensing boiler is the HE50/70 which is 15-21KW (50000-70000 BTU)

I believe that either of these boilers would suit my needs but wouild like to know which is better suited to mu heat needs and why ?

Any help would be appreciated.

Cheers

1. back up to what?

2. boiler sizing must take into account power to heat quickly. therefore, whilst required output may be low'ish' responsiveness must also be taken into account.

you must also take into account your energy heat losses through the fabric of the building, and count these into heating requirements.

iamlegend2008

Hi

Sinnerboy: Sorry, did not mean to break the forum rules (but just to be clear Im not asking who makes the best boilers, Im asking the question that you pointed out in more details which is how do I map my 16KW requirement to the correct boiler size as sold by manufacturers.

I has used both of the URL's you mentioned to get the 16KW figure.

Plumber advice: Go with cheapest (but surprised that requirement was so low)

Boiler Manufacturer A advice: Ireland has a history of oversizing boilers which adds expenses to the running cost (Im not concerned with the extra few hundred euros to purchase a large boiler, but the ongoing extra waste cost of using an oversized boiler).

Boiler Manufacture B advice: Get the boiler commisioned correcttly.

Sydthebeat: Backup to having a house that is well insulated (60mm xtratherm in cavity, 50mm insulated slabs, airtight walls and airtight roof membrane, HRV and Solar panels for DHW, MF stove/boiler in living room (ie trying to move to a situation where turning on the boiler is not required)

Ideally, I would not install an oil boiler at all but feel that some backup is required in case the house feels cold in daily use (unfortunately while only at first fix, my house is over 4 years old which means there are few limitations on what I can hope to achieve (whereas as new house builds will incorporate the new regs into their initial design).

I hope this sheds a little more light on the background to the question.

Cheers

iamlegend2008

mind the bump

What methods have people on the forum used to map their heat requirement to the appropriate boiler ?

Cheers.

Carlow52

from the second link provided by sinnerboy
[Also, if the control system gives priority to the hot water, then this number could be reset to zero.]

which means that, with proper zoning, you can drop the HW requirement, particularly as u are going solar.

I have already mentioned my concerns elsewhere in response to your posts about safely and efficiently integrating a mfBB system with a conventional type boiler: I repeat them here.

fishfoodie

Probably tangental to iamlegend's question, but do Boiler manufactures supply power/efficiency curves for their different models. This would be the best way I would assume of guaging whether a particular model would be appropiate or not, as it would show its operating efficiency at different power outputs.

Would I have to send an email to the mfgs to get this kind of info ? ,as scanning the various websites, they are singularly lacking in technical details on their products.

Brian Dublin

iamlegend2008 wrote: »

Hi

Im currently researching which boiler model to use for an 1800 sqft dormer. The oil boiler will be used as backup since the house will hopefully be airtight and has 50mm insulated slabs on external walls etc.

Using the online guides I have calulated my heat and water requirements to be only 16KW (including 2KW water)

The nearest firebird condensing boiler is the C26 which is 15-26KW (50,000-90000 BTU) preset to 21KWH
The nearest grant condensing boiler is the HE50/70 which is 15-21KW (50000-70000 BTU)

I believe that either of these boilers would suit my needs but wouild like to know which is better suited to mu heat needs and why ?

Any help would be appreciated.

Cheers

Rule of thumb for sizing your heating output requirements.
Standard B.T.U. calculation
Length X Width X Height in feet
Allowances are as follows
Lounges and dining rooms add 5%
Bedrooms add 4%
Common rooms hallways and such 3%
North facing rooms 15%
Hot water allow 20%
General loss 10%
Double Glazing deduct 10%
Insulated walls deduct 10% depending on standard if in doubt don’t allow for.

Your total B.T.U. IS hourly rated.

To calculate B.T.U as Watts B.T.U X 0.293
To calculate B.T.U. AS kw B.T.U X 0.000293

plus get yourself some TRV and size your boiler larger than required.
as for the better boiler the Fire Bird if only for its B.T.U 50/90 TO THE Grant 50\70
its always a good idea to have some extra required B.T.U As you never know when you may add on a room down the line. you are always better off with a boiler that can cope with the extra B.T.U . when required