Oil boiler over sized ?

[RayPlumb]

I have not been doing oil boilers long (recently oftec registered)
I am quoting for a regular boiler change. Measured 4 bed house and including hot water is 12.5kw
Customer wants a floor Grant vortex. Although the smallest they do is 15-21kw will this be oversized.
If so any suggestions as not much choice in small oil boilers
Thank
Ray

 

[Best]

Hello Ray. Are you sure about your house calculations?
12.5 kW seems very little for a 4 bedroom house. Is the house terrace/semi or well insulated?
Remember the boiler can be set to various outputs if you look at MIs. Usually 3 options on output, with burner being factory part set for middle output. Smaller nozzle, oil pump pressure set correct and air adjusted to suit. Burner might also require addition alteration which manual will explain.
The 15 - 21kw white cased Vortex will do nicely if your calculations are correct.
But the 15 - 26 kW Vortex white cased boiler is in my opinion a much better boiler - so much more boiler shell for your money. I fit it, even on homes that only require the smaller boiler and set them accordingly.

The boiler house cased models (if available to you) are physically same size boilers though.
If the system has all controls (wall stats etc) then that will help.

 

[RayPlumb]

Thanks for the response Best. I checked house size idhee web site and energy saving trust which came in at 13.2kw? Detached house, 2 story , 6.2mx 9.8m , double glazed 80's built with approx 150mm loft insulation.
I understand in MIs that changing nozzel, pressure and air I can adjust it down to 15kw. Would that then be the set output and not modulate down and therefore cycle on off at 12.5kw. I'm I right that they oil boilers do not modulate down..
Thanks
Ray

 

[Dotty]

No, they don't.

Set it at 15kw, though this does seem a little low.

 

[ShaunCorbs]

Simple way and your sure then is work the room sizes out and think you will be more than 8kw without water be more like 22-25kw (with cylinder)

 

[RayPlumb]

Simple way and your sure then is work the room sizes out and think you will be more than 8kw without water be more like 22-25kw (with cylinder)

I am looking at 12kw (unfilled cavity walls) or 8kw (filled cavity walls, cannot be sure without drilling). Then added hot water allowance. What would you allow for hot water allowance cylinder is about 160lts. some of the boiler sizing sheets have 2000w some have 2000 or 3000w

 

[Best]

I allow 3.5 kW for hot cylinder at least.
12 kW seems extremely low for a 4 bed house with unfilled cavity walls!
Remember one very big radiator is 3kw.
I would do what Shaun has suggested and calculate each room allowing for any outside walls and particular room heat requirements.
I don't doubt that a detached house can just need the smaller boiler, but I would think it would always need close to 20 kW unless the house has modern insulation built in to every place.
I usually don't calculate a 4 bed house, - I just go with 26 kW boiler and set it to medium setting

 

[gpbeck]

If they're happy with current temperatures what's the current boiler doing? What's the total output of the existing rads?

 

[Best]

Another thing to consider is what if the customer builds an extension on to their house later?
If the customer is not worried about extra £££s, and not pricing against your quote, then go with the 15-26 kw Vortex.
I think your whole house calculations are wrong. Maybe try it again using different calculation formula?

 

[ShaunCorbs]

5kw I allow as if they up to 300lt it will still be ok and as best says allow for extensions

 

[RayPlumb]

Most mention whole house calculations am I doing it wrong??
My calculations on http://www.idhee.org.uk/calculator.html Calculated Boiler Size[/URL]
Internal total length 9.1m x with width 5.7m
The dwelling type is detached with double glazed windows with wood/plastic frames.
The external walls are cavity, no insulation.
The roof is pitched with more than 75mm of insulation.
The Dwelling is in the south east or Wales.
Allowed 3500w for hot water
Total 13.97 KW
or with Insulated walls and 2000watts for water
Totals 8.93kw

Also 2 Floors..

 

[RayPlumb]

I understand about upsizing for future extensions and bigger cylinders etc. I was told not to oversize boilers as they will be inefficient. As I am new to Oftec and will definitely have one of my first oil boiler installs assessed by oftec so want to get it right ...

 

[Millsy 82]

I wouldn't be too worried about 1.03kw myself and that is the smallest grant do so can't see the problem.

I do agree though for a 4 bed house it does seem quite low.

 

[ShaunCorbs]

I understand about upsizing for future extensions and bigger cylinders etc. I was told not to oversize boilers as they will be inefficient. As I am new to Oftec and will definitely have one of my first oil boiler installs assessed by oftec so want to get it right ...

In the old days yes but with new modulating boilers etc i wouldn't worrie to much

 

[RayPlumb]

In the old days yes but with new modulating boilers etc i wouldn't worrie to much

If I understand correctly .. oil boilers don't modulate..?

 

[RayPlumb]

If I installed grant 15/21 or 15/26 and adjusted it down to 15kw would that be ok.
Then if required I could adjust it up to 18, 21 or 26kw if required in the future....

 

[RayPlumb]

Most mention whole house calculations am I doing it wrong??
My calculations on Replacement Boiler Size Calculator Calculated Boiler Size[/URL]
Internal total length 9.1m x with width 5.7m
The dwelling type is detached with double glazed windows with wood/plastic frames.
The external walls are cavity, no insulation.
The roof is pitched with more than 75mm of insulation.
The Dwelling is in the south east or Wales.
Allowed 3500w for hot water
Total 13.97 KW
or with Insulated walls and 2000watts for water
Totals 8.93kw

Also 2 Floors..

Anyone with comments if I calculated above wrong... As a lot saying it is a bit low for a 4 bed house.

 

[Millsy 82]

If I understand correctly .. oil boilers don't modulate..?

I've never seen one

 

[Millsy 82]

If I installed grant 15/21 or 15/26 and adjusted it down to 15kw would that be ok.
Then if required I could adjust it up to 18, 21 or 26kw if required in the future....

Yep and yep.