radiator sizing

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A friend wants me to plumb a granny flat which basically is about the size of a garage 3 metres wide x 6 metres long split into 3 roms; bathroom = 1m90mm x 3m, bedroom =3m x3m, and living room with open kitchen = 3m x 3m930 mm. The height from the solid concrete floor to the ceiling is 2m 400mm. There are 2 window openings with dimensions 2m260mm wide x 1m300 from base to lintel and this is split between the living room/kitchen and bedroom,with the other window approx 900mm x 890mm in the bathroom.

There is a door to outside which i have no dimensions for in the living room. A double block cavity internal wall spans the 3 rooms which he said had timber behind it and double block cavity external walls make up the rest.Timber stud split up the rooms which he said hed plasterboard,and the roof is pitched.

The old oil burning boiler has been removed from which is now the bathroom with the what i assume to be the 22mm flow and return copper pipes just standing there on their own rising up out of the floor with the pump positioned on the bottom pipe which has to be the return. I dont think he drained the system down because he said when he removed the 22mm top or flow pipe water was gushing out hence the ballfix on the end.

The bungalow has 10 other rads 6 singles and 3 doubles ranging from 28 inches to 44 inches,all at 20 inches ht.

My question is this, is there a practical rule of thumb based on experience for sizing the rads and the boiler required. What i mean is would an experienced plumber be able to look at above scenario and go, bang,bang,bang, heres the sizes of rads and boiler you need,or would he also have to go through the rigorous calculations of heat loss w = room volume[m3] x temp diff[ 0C] x number of air changes per hour x a constant fig of 0.33 not to mention heat loss through building fabric, plus the rest.

I understand he would have to do this on a large property but would he just use a simpler method here. your advice is much appreciated.
 
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The first thing to do is a Boiler sizing calculation.

Use the Sedbuk Calculator

Then deduct 2kw from the result to get the heating requirement.

That ensures you do not install too big a boiler. If the boiler you are fitting works with a 20°C differential you should increase the result by 20% as the rads will give out about 20% less heat.

Without going to great lengths doing a room by room heat loss calculation, just assume that the heat loss in all rooms is proportional to the room volume. So we get

Room 1 Rad = Total heating kw x (Room 1 Volume/Total House Volume)

If the walls have cavities, have they been filled. Also the loft. Granny may be eligible for grants for this work and the heating. Take a look at Grants and Offers
 
Dumb question maybe, but what do you need new rads for if the boiler has been removed?
 
The first thing to do is a Boiler sizing calculation.

Then deduct 2kw from the result to get the heating requirement.

That ensures you do not install too big a boiler. If the boiler you are fitting works with a 20°C differential you should increase the result by 20% as the rads will give out about 20% less heat.

Without going to great lengths doing a room by room heat loss calculation, just assume that the heat loss in all rooms is proportional to the room volume. So we get

If the replacement boiler is going to be heating water then there is no need to deduct the 2 kW allowance.

If the radiators are going to give out less heat then there is no point in upsizing the boiler because that will have no effect apart from reducing efficiency.

If the new area is about the size of a garage then the heat requirement is likely to be about 3-4 kW assuming the loft is well insulated!

Tony
 
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Theres only going to be 3 extra rads on top of what his original boiler supplied. The reason it was removed was because i dont think granny would fancy sharing the bathroom with it. The decision to relocate a new boiler to an area outside with its own we boiler house is what he has in mind. The plastic duct is already in place for the flow and return pipes approx 1o yards outside of flat. Would it be better to run these in plastic or is it personal preference. Obviously if copper do they need to be buried within the duct to a depth of 750 mm under ground or just plenty of armaflex suffice.

Using the calculations for rad sizing ive worked out that granny would need a rad with an output of 1779.1 watts for the living room/ kitchen, 980 watts for the bedroom,and 961.43watts for the bathroom. The one thing ive found out is that however your limited by space eg the bathroom is reasonably small and a towel rail with that wattage is pretty dam big and could be an eyesore not to mention the space limitations because he wants a toilet,ped basin,and wet room type shower in there as well. I guess for all the time spent in the bathroom a smaller towel rad would suffice but then my time working it out has been wasted.

I have put one extra rad onto my own central heating before with no bother,and all copper with fancy crank passover and end feed soldered joints to boot[ i was pleased with mysef] maybe im just being overcareful and these 3 extra rads shouldnt pose any problem to a new boiler with similar rating as the old one.

Is it possible to get a manifold for 22mm flow and return branching off to 2 rads one on each side. The reason i ask is that there isnt much room between where the pipes come out at the bottom of the wall and the other side of the tracked channell to allow me to get 4 soldered 22 mm tees in,and the pipe wont give to allow it to spring back into the tees. thankyou again for any advice.
 
All those fancy calculations come to a total of 3720.43 kW which is the same as the 3-4 kW which I told you without even stepping foot in your country!

Tony
 
10 rads plus 3 extra,if the original house was typical ni property it would of been a 50/70 warmflow converted from solid fuel originally.rule of thumb would be upgrade to a 70/90 and insulate everything.hotpress will have either a plain cly 36x18 with 28mm flow and ret,ov will come off the flow[top fipe]cold feed into the bottom [ret] and only him up above will know what horrors await you in the roof space c/f teed into the vent prob and a few manual air vents,this is all assumption of course based upon experience of ni heating sys enjoy :LOL: .cabin pack would be a nice solution avoid the condensors if you can bassets are still knocking out standard warmflows/turcos if it helps.
 
Just want to say thks for your help,and this has been my first rad sizing calculations,which ive actually enjoyed even though my maths at school was woeful. Just one more question regarding the positioning of the heating pump. I understand its best working under positive pressure to prevent pumping over and taking in air. As the boiiler will be installed approx 10 yards outside beside the garage, could the new pump be located here or nearer the hot water cylinder. Is it best to position it on the flow or return. thks for everything from the 40 year old apprentice.
 
The first thing to do is a Boiler sizing calculation.

Then deduct 2kw from the result to get the heating requirement.

That ensures you do not install too big a boiler. If the boiler you are fitting works with a 20°C differential you should increase the result by 20% as the rads will give out about 20% less heat.

Without going to great lengths doing a room by room heat loss calculation, just assume that the heat loss in all rooms is proportional to the room volume.
If the replacement boiler is going to be heating water then there is no need to deduct the 2 kW allowance.

If the radiators are going to give out less heat then there is no point in upsizing the boiler because that will have no effect apart from reducing efficiency.
Thanks for pointing out, indirectly, the ambiguities in my post.

When I referred to deducting 2kw, that was not to give the boiler size but the heating requirement. The boiler size should be as calculated - unless a combi is used.

The 20% upsizing was intended to refer to upsizing the radiators not the boiler. So if the calculator says 18kW, deducting 2kw gives 16kw for heating. increase this by 20% gives 19.2kW. So you need radiators which total a nominal 19.2kw. You buy an 18kw boiler.
 

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