New hot water cylinder

[Agile]

The basic standard for cylinders now requires a certain minimum tramsfer rate.

Your boiler is probably correctly matched for the house at about 10-12 kW and that would be about right for a standard cylinder.

The transfer rate is only when nthe cylinder water is cold and is less when its already warm.

In any case 12 kW will only heat about 5 li/min so you cannot rely on that to reheat the cylinder.

I can see no reason to pay more for a high output cylinder in your situation. Merely sizing it correctly will provide you with what you need.

You can see the power input of your boiler by measuring the gas comsumption at the meter and see in the FAQ how to do it. Then multiply by about 0.7 to get the actual heat output.

Tony

 

[Schmill]

Thanks to all, another quick thought that occured to me...

I'm aware that 1L of water = 1Kg, if I am changing the HW cylinder from the existing 114L to one of say 250L, that is quite a lot of weight being placed on effectively a 500mm diameter circular base. Do I need to reinforce, or brace the flooring / joists in some way to cope with the new heavier tank?

Also, I read in one other post, just as a 'side mention' that the stopcocks for the HW supply COLDWATER from the loft should be ABOVE the top level of the tank.
Is this the case? I can only think that it is so you don't have to drain the cylinder under certain conditions, (not that I can think what they would be)..
Does this mean I need to move the 2 stopcocks aswell assuming I fit a taller tank?

Thanks

 

[mysteryman]

Weight considerations are one of several good reasons for using a smaller high recovery cylinder.

The important thing is the ability to reheat x litres of water per y number of minutes, rather than to store 4 times x .

 

[Agile]

In a smaller house with a smaller boiler there is little point in fittin g a seriously over powered boiler just to reheat a small cylinder quicker.

Far better to have a larger cylinder and a correctly sized boiler to ensure the maximum efficiency.

Tony

 

[mysteryman]

True, but I was not advocating an oversized boiler.

 

[Agile]

The OP has not mentioned the power of his boiler but in an average house then expectation is that its probably only about 14-15kW.

In that case there is no advantage in fitting a high recovery rate cylinder because the standard cylinder coils are rated at a minimum of 15 kW.

Tony

 

[Schmill]

Ok, my boiler is a British Gas RD2 450.

From the information I have been able to dig out from it, the heat output is:

Boiler Output to Water kW (Btu/h) 14.7 (50 000)

So yeah, presuming that means it can transfer 14.7 kW into the water, and the coils in a standard HW cylinder are spec-ed to be a minimum of 15kW transfer, then it would seem to me, (with my limited understanding), that without upgrading the boiler there is no point in going for a higher transfer rated cylinder, (as it would just have the capability to transfer more heat over, but the boiler wouldn't be able to provide it anyway)?

As much as we are planning on changing the boiler at some point, with any luck that won't be for a few years, so that's not an option.
Also presuming that at some points the CH will be on as well as the HW cylinder, the full 14.7kW wouldn't even all be going to the HW anyway?

If the above is all correct, then I guess the question comes back round to whether or not I need to take any particular floor precautions, extra noggins, doing 'something' to the joists, or just placing an extra piece of flat board across the existing floor in the airing cupboard?

Thanks again

 

[Agile]

Its normal to time the HW to heat up before the CH in the morning so the entire boiler output is available.

During the day the room stat will turn off the boiler when its satisfied so then the whole output is for the HW.

The calculated maximum heat is only required when its at -1° outside which is only for a few days each year.

Tony

 

[Schmill]

So the verdict is...

Big tank & perhaps strengthen the flooring somehow?