Which size combi boiler?

[Mrs Tara Plumbi]

Mr TP told me
there are different water regulations in Scotland.
And he told me if you telephone the local water board for that area there will be able to tell you the temperature of the incomeing water in winter, he thought it might be about 5C.
If you are putting in a whole new system you need to calculating heating/rad sizes for a much colder winter temperature than down here, but you know that.

Don't know if this is helpful to you.

 

[GerrythePlumber]

Rad sizes are calculated by room size and u values not by how hard a winter you expect. Boiler size is calculated by the size of your home and where it's located. The incoming main is not normally a different temperature, no matter where you are in the country. Aberdeen will have a minimum depth for the water main to be laid, the same as everwhere else. Unless you live in the permafrost of the Arctic tundra, your installation should be no different to one in Cornwall. Insulate, insulate, insulate.

 

[doitmyself]

Rad sizes are calculated by room size and u values not by how hard a winter you expect.

The difference between the outside temperature and required inside temperature is an essential part of the calculation. If you need a 2.2 kW rad to heat the room from -1C to 21C, you will need a 3.2kW rad to heat the same room from -11C to 21C.

The standard winter temperature has always been -1C, but -3C is often used now and the on-line Boiler Size Calculator uses different temperatures for the various part of the country, based on historic data. So Scotland uses a much lower temperature than SW England.

Long-pass

I see you are thinking of having a combi boiler because it will release space in the kitchen. As you have a cellar, you could install a system boiler and an unvented hot water cylinder in the basement.

In that case the boiler can be sized using the Calculator, but it would probably be sensible to install a boiler which is 10-20% larger than the calculated size as this will allow for the much colder winders we seem to be getting and your hot water cylinder will get heated quicker.

 

[GQuigley67]

be prepared for cold winters in eberdeen anyway, i suggest you size your heating based on bad winters imho.

 

[AWheating]

Generally scotland has an average temp a few degrees lower than the rest of GB, but can have spells of very cold weather. I think a base outside temp of -5*c would be a good idea, this is the value you would normally use for exposed locations.

 

[GerrythePlumber]

You live in a home with granite walls! Therefore, solid construction, no cavity, no unsulation. A heating calculator will take this into account, an if used correctly, will give you the correct size of radiators. No-one can predict winter temperatures. No one knows if there will be good skiing next year. Michael Fish doesn't know his storm from his elbow. Stick to the tried and tested methods as worked out by boffins and you can'go wrong.:30::30:

 

[GQuigley67]

I think they are considering lowering the average winter temperature used for doing heat loss calculations now because of the previous bad winters we have had

 

[long-pass]

I did my radiator calculations based on it being -3 outside, but had trouble finding the u value for granite walls, i don't know how thick they are either. i took a wild guess and gave them a u value of 2.5, if this is a silly guess i would much appreciate a more accurate u value if anyone has one, or if my -3 is way off the mark either. cheers

 

[AWheating]

[DLMURL="http://www.tywiafonyroesoedd.org.uk/English/buildings/Documents/6%20Caroline%20Rye%20-%20The%20U-values%20of%20traditionally%20built%20walls%20theory%20vs%20reality.pdf"]http://www.tywiafonyroesoedd.org.uk/English/buildings/Documents/6 Caroline Rye - The U-values of traditionally built walls theory vs reality.pdf[/DLMURL]


some examples in above

 

[long-pass]

Thats quality AWheating. am pretty sure that will come in handy for current and future calcs. cheers

 

[GQuigley67]

I have a piece of A4 with some information that you can use to calculate U values when I get the chance I'll scan it and post it on here. It has most if not all the materials you'd find in a building.

 

[long-pass]

that would be great 6Quigley67. It would save plenty of head scratching and fruitless hours trawling the net

 

[GQuigley67]

it is a table of the thermal conductivity of various materials, density is also given for comparison purposes. All you need is the thickness of material for your calculations

Here is Granite's figures

Bulk Density (kg/m3) = 2662
Thermal Conductivity (W/m Deg C) = 2.93

I'll post the A4 sheet when I get the chance to scan it and try and make up a tutorial for anyone who doesn't know the calculations if they are interested. I have lots of notes on it

 

[Mrs Tara Plumbi]

Hi.
I think you started by telling us you were changing the boiler ad you have one bathroom and there are 3 of you in the family.
You have got lots of great advice from people here.
Some people are always prefer system boiler... if possible.
I think there are good things to say about combis too, so it is not necessarily second best just to save on space.
Never ending instant hot water on demand is wonderful!
Only down side is that when you are enjoying a 3 hour shower the boiler will be heating your hot water instead of your rads, I have never found this to be a big problem.

Now you tell us you are going to replacce the rads do make sure you work it out and not just replace what is there which might be a tad cold for your southern bones.
When I went to the Hebridees to stay in Mrs McClouds' B&B it was freezing 11C in June.
She was throwing open all the doors and windows because to them that was a glorious summers day!

 

[long-pass]

Hi Mrs Tara Plumbing, in my original post i did say i was replacing the central heating system and installing a combi.I have been thorough (i hope!) with all the heat calcs. My better half is from aberdeen and every time i complain about the cold in london she laughs and tells me to do starjumps (they actually do warm you up). I am very grateful for all your and everyone who has contributed advice and helpful posts. thank you and keep them coming. cheers