Any point in over-speccing BTU vs. what is required? Read more: http://www.diynot.co

[Russel]

[FONT=&amp]Hello guys, my first post, so go easy on me haha [/FONT]

KEY INFO:

ABOUT ME: DIYer who doesn't know much about plumbing
BOILER: Potterton Combi 28 HE, installed 2012
PROPERTY: Ground floor mid-terrace flat approx. 80 sq m / 860 sq ft
TOTAL NUMBER OF NEW RADS: 9

[FONT=&amp]I'm self refurbing a ground floor flat in a mid-terrace property (built 1880). Renovation is extensive, stripped back to bricks / joists. [/FONT][FONT=&amp]After destroying it, I've began to build it back up.

I've taken accurate measurements of each room, inputted these into 3 different online BTU calculators and am repeatedly coming to a figure of 30,000 BTU, or so for the whole flat.[/FONT]

[FONT=&amp]The boiler is a Potterton Combi, rated at 28kW DHW / 26kW CH Pnc (condensing)...whatever that means. By my maths, 28kW = 95,000 BTU[/FONT]

[FONT=&amp]I have 2 questions:[/FONT]

[FONT=&amp](1) Is there any harm / negative effect in over speccing and get radiators equivalent to, say, 40,000 BTU in total, if not a little more (boiler has a max of 95kBTU, as mentioned)?[/FONT]

[FONT=&amp](2) I don't know whether my system is delta 50 or delta 60 as all the pipework has been ripped out, so I don't know what the inlet / outlet water temperatures would have been...what should I assume, as this has an effect on BTU values?

Thanks very much!
[/FONT][FONT=&amp]
[/FONT]

 

[chris watkins]

Re: Any point in over-speccing BTU vs. what is required? Read more: http://www.diyno

Hi Russel

You need the services of a design engineer if you are not going to employ a plumber/heating engineer who can work all that out for you.

It is far two much to go into on a forum, BTW. Btu's went out before I started over 30 years ago

or you could buy yourself a very good book & read up on it all.
CIBSE - Domestic Heating - Design Guide 2015 (Domestic Building Services Panel)

Not withstanding the above.
Your new installation falls under Building Regulations Part L1b & must fully compliant with the current approved document. BC should also be notified as you are effectually replacing your heating system, just because the boiler is remaining does not stop this.


At some point you will need the services of a gas safe engineer anyway who will need to recommission the old boiler to suit the new system, why don't you make contact with someone to maybe sort out the bits you can & can't take on yourself and those that you will need to pay them to do?

 

[Russel]

Re: Any point in over-speccing BTU vs. what is required? Read more: http://www.diyno

Funnily enough, I have a copy of that guide, albeit dated 2006 I had a leaf through it a while ago.

I think it is a good idea to get an engineer in, but, in the meantime, given the collective wisdom / experience on this forum, I could perhaps get some approximate answers, so I can look at sizes, positioning etc. Then get a specialist to confirm things.

One thing that concerns me, is what I read in that guide:

"Historically there have been problems with oversizing of heating systems which can lead
to inefficient operation, particularly at part load operation, to control problems and to a reduction in plant operating life."

Eek!

EDIT: Realistically, when you chaps are quoting a client for a new system, say boiler and rads, more often than not, will you guys more often than not use an online / Mears calculator or will you guy take the time to plug all the possible variants (and there are loads!) into some sort of spreadsheet / do a full and very detailed piece of analysis?

 

[Riley]

Re: Any point in over-speccing BTU vs. what is required? Read more: http://www.diyno

Work it out properly not finger in the wind. You'll have a far more effective efficient system

 

[rpm]

Re: Any point in over-speccing BTU vs. what is required? Read more: http://www.diyno

[FONT=&]Hello guys, my first post, so go easy on me haha [/FONT]

KEY INFO:

ABOUT ME: DIYer who doesn't know much about plumbing

By using different forums at the same time this could get messy with conflicting help/advice.

 

[chris watkins]

Re: Any point in over-speccing BTU vs. what is required? Read more: http://www.diyno

Work it out properly not finger in the wind. You'll have a far more effective efficient system

Every time if you are a Pro.
2006 will give you a start with rad sizing to suit condensing boiler but you will need to understand the latest requirements for the controls to meet the current Building Regs.

Best of luck Russel

 

[thereisawizza]

Re: Any point in over-speccing BTU vs. what is required? Read more: http://www.diyno

Love the "key info" at the start, it's like the stats on American Wrestling at the bottom of the screen

 

[Howsie]

Re: Any point in over-speccing BTU vs. what is required? Read more: http://www.diyno

Read the book Chris mentioned, it's all in there and it has work sheets. Then buy the new book because that one is well behind the times.

Good luck.

 

[joni os]

Re: Any point in over-speccing BTU vs. what is required? Read more: http://www.diyno

Design on delta 50 to increase boiler efficiency. Be generous with calculation of heat loss through air changes. Do not assume zero heat loss through party walls, as these properties may not heat to your standard. Incorporate insulation into your refurb. not just between joists but internally, with celotex or similar to front and rear walls, otherwise, however efficient your heating, a 1880 solid wall will feel cold.

 

[Howsie]

Re: Any point in over-speccing BTU vs. what is required? Read more: http://www.diyno

You'll have to get a plumber in for the gas pipe sizing and routing anyway, so why not get them to do this for you? At least you've some comeback then.

 

[Best]

Re: Any point in over-speccing BTU vs. what is required? Read more: http://www.diyno

No harm in slightly oversizing radiators in my opinion.
They should be compatible with a condensing boilers ideal lower potential water flow temperature to rads.
But you would still need to have the rads calculated accurately. What I mean by that is if you oversized output for each area at say, 10%, that would be fine for any room stat, but if some rads were 20% oversized you would have some rooms reaching temperature quicker and any room stat would close off either too early or too late.
I prefer to oversize slightly for very severe weather times, or when calculations just don't seem enough. Nobody ever complains that their heating system can cope in -10 winter or is very fast to heat the rooms.