KW allowance for 210 litre cylinder

[kozak1968]

Does anyone know if there is an accurate formula for working out the requirement of heating hot water for a 210 litre unvented cylinder. Using the whole house method works o.k for standard vented cylinders, but i just want to know if more allowance would be required for unvented cylinders due to the coil rating. It seems a grey area to me.

 

[Chalked]

Look at the manufacturers instructions. It will say how much the coil will take.
They can be between 10 -22kw depending ont he coil in that size of cylinder.

 

[kozak1968]

Does anyone actually allow an extra 10 - 22kw to a system? Seems a bit much for a domestic property with ground floor underfloor heating and 8 rads on first floor of a new build in my opinion.

 

[ShaunCorbs]

normally allow for 10 tbh

 

[mfgs]

To allow an extra 20kw would be ridiculous. I allow an extra 7-10kw and explain to the customer about timing the hot water so its not on with the heating during peak demand if they need a quick recovery time.

 

[joni os]

Boiler sizing does not conform to rigid formula. Experience and insight, taking into account, demand and expectation of client is necessary. The old method of looking at heat loss of property and adding a few Kw for DHW will prove inadequate for well insulated homes if the fast recycle times of cylinders is to be achieved. Quoted recycle times, raising 70% of water through 45 C can also be misleading. If thought necessary to install 210 litre DHW cylinder I would size the boiler to heat all 210 litres in 30 minutes ie 22Kw and assume that during those 30 minutes the ambient temperature of the rooms would not be grossly effected. In less well insulated property or commercial setting with high DHW demand further consideration is necessary.

 

[ShaunCorbs]

Boiler sizing does not conform to rigid formula. Experience and insight, taking into account, demand and expectation of client is necessary. The old method of looking at heat loss of property and adding a few Kw for DHW will prove inadequate for well insulated homes if the fast recycle times of cylinders is to be achieved. Quoted recycle times, raising 70% of water through 45 C can also be misleading. If thought necessary to install 210 litre DHW cylinder I would size the boiler to heat all 210 litres in 30 minutes ie 22Kw and assume that during those 30 minutes the ambient temperature of the rooms would not be grossly effected. In less well insulated property or commercial setting with high DHW demand further consideration is necessary.

if that was the case you would end up with a 28-30kw boiler for a flat

 

[joni os]

Only if the flat had a 300 litre DHW cylinder. In which case anything less would not meet the recycle times the cylinder was designed to achieve.

 

[kozak1968]

I think experience counts for more than rigid calculations.

 

[Chalked]

A lot depends on the type of job and customer expectation. A
So diversity can be woked in too. Most of the time a boiler is not heating the cylinder from cold. The house is normally never lower than 12 degrees, so it only has to raise the room temperature a few degrees. So in essence a smaller harder working boiler may be specified.
But on large domestic properties, with high heat loss, large cylinders and large rooms, imho need sizing to heat the rooms and hot water within an hour. These type of people who expect things to just work, don't want to back from a skiing holiday and wait six hours for hot water and rooms to be warm.
Expectations need to be talked through with customers. If they are happy that in some instances water may take an extra hour and the house might feel a little colder, if the heating has been turned down for a few days. Most will say undersize and make the instsll cheaper. Some will say, I expect it to be quick, then a boiler sided to the whole house and the output of the coil are needed.

 

[Chalked]

if that was the case you would end up with a 28-30kw boiler for a flat

Not exactly true Shaun. A flat might have a small cylinder with an 8kw coil and maybe 9kw of heating. So would still only spec an 18kw boiler.

Saying that, I would have fitted a combi!:6:

 

[ShaunCorbs]

Not exactly true Shaun. A flat might have a small cylinder with an 8kw coil and maybe 9kw of heating. So would still only spec an 18kw boiler.

Saying that, I would have fitted a combi!:6:

210l is about 16kw but understand