Sizing New Oil Boiler

CBYR1983

I have a very old oil boiler that I thinking of replacing. It runs on gas oil, large asbestos flue to roofline, probably only about 60% efficient, if the scale descended far enough it might be a Z energy rating.

So looking at replacing it, will employ someone professional but want to make sure they don't put in too small a boiler to save themselves money.

I read from the firebird site that the boiler is sized based on BTU/watts.

I presume this is related to radiator output, though hopefully you will correct me if wrong. Funny thing is that I got new radiators installed recently so I know the BTUs of the rads - took them from the Quinn website.

I'm puzzled though - my rads are double panel convectors (two panels, two sets of convectors)- the BTU output is considerably higher than a Double Panel Plus rad (two panels, but only one set of convectors), which it appears uses the same amount of water.

If I use the BTUs for the former it would suggest I need a larger boiler - but does that make sense? Surely it's more based on water content.

Thanks

Micky Dolenz

it is based on size of heated area, so for example you could have ten rads in a room, but the room will only need so much heat to heat it.

There are loads of BTU/KW Calculators online that will ask for size of room/ house/type of insulation etc and it will give you size of boiler.

JohnnieK

Were the rads sized correctly in the first place or were they a straight swap to replace the old ones with extensions on the valves. If it is the later the chances are they are over sized. In general it is the total heat requirment (the out put of all the rads) plus around 4 to 6 kW for the cylinder.

wing52

If the new doublle panel/double convector rads are too big,

You might get awy with running the new boiler at a lower temperature.

An 'a' rated boiler (condensing) gives its highest efficiency at @50

degrees centigrade. Hope that helps.

CBYR1983

Yes the rads are probably oversized, but it still doesn't make sense to me that the boiler should be bigger when the rads use the same amount of water as another rad with lower output.

Are we saying that because the rads have fins they cool the water more quickly?

Surely it has to be taken into account that they're high output rads. It would mean that if I would be sizing the boiler for the same BTUs regardless of the amount of water in the system.

Like if I replace an old rad (no fins) for a new one with fins, the BTU output increases, but surely your boiler is still right-sized?

JohnnieK

CBYR1983 wrote: »

Yes the rads are probably oversized, but it still doesn't make sense to me that the boiler should be bigger when the rads use the same amount of water as another rad with lower output.

Are we saying that because the rads have fins they cool the water more quickly?

Surely it has to be taken into account that they're high output rads. It would mean that if I would be sizing the boiler for the same BTUs regardless of the amount of water in the system.

Like if I replace an old rad (no fins) for a new one with fins, the BTU output increases, but surely your boiler is still right-sized?

As mentioned before if sized correctly the BTU's (kW) should not increase. If an old radiator 1.5m long by 500mm high with no fins is replaced with a correctly sized radiator with fins it will be some what smaller than the older radiator and pipework would have to be altered. The old radiators would hold more water than new radiators as well.

The fins increas the surface area of the radiator giving off more heat quicker and would cool the water more.

Rad manufactures give charts of out puts of there rads. One rad might give off 500 watts at 50oC and the same rad would give 1kW at 75oC. If you have a room heat loss of 1kw you need a rad of the same output regardless of temp. You would have to decide at design stage what temp your system will be at either 50oC or 75oC and size your rads accordingly. If your total room heatlosses are 7kW's add 4kW for hot water thats 11kW some engineers add a further 10% for recovery that would be 12.1kW so your boiler size would be 12.1kW.

kscobie

you can get a rough idea from a boiler sizing page on www.sedbuk.com, not 100% accurate, but not too far out, hope that helps, all boiler ratings there too

Carlow52

CBYR1983 wrote: »

Yes the rads are probably oversized, but it still doesn't make sense to me that the boiler should be bigger when the rads use the same amount of water as another rad with lower output.

Are we saying that because the rads have fins they cool the water more quickly?

Surely it has to be taken into account that they're high output rads. It would mean that if I would be sizing the boiler for the same BTUs regardless of the amount of water in the system.

Like if I replace an old rad (no fins) for a new one with fins, the BTU output increases, but surely your boiler is still right-sized?

the BTU output of the newer rad is higher so it will require more heat delivered to get it to work to its designed output, however if the heat is not delivered to it then it wont:)

I would size the boiler based on room requirements plus losses plus HW.

Also if u are zoned u can use a smaller boiler and get it to work harder.

Carlow52

wing52 wrote: »

If the new doublle panel/double convector rads are too big,

You might get awy with running the new boiler at a lower temperature.

An 'a' rated boiler (condensing) gives its highest efficiency at @50

degrees centigrade. Hope that helps.

I believe that the 50 refers to the return temp and not the out temp

JohnnieK

No higher than 55oC on return, but it actually only equates to around 3% of a saving when condensing properly. So dose that justify the headache of trying to tinker with the system to get it right? Great on a newly designed system but can be very difficult on an old one.