Hot water cylinder sizing

peter_333

I'm about to complete on a flat purchase, and one of the first things needed in the property is to replace the existing hot water cylinder. The property is Electric only (no gas) and we're replacing the old gravity fed system with a new unvented system (partly due to the age of the old system, but mainly to improve water pressure, which is horrendous right now). I was hoping for a little advice on appropriate cylinder size, as various sites I've visited seem to vary wildly in their recommendations.

It's a two bedroom flat, with only two adults living there (and the occasional family or friend coming to stay). There is a bath, which would be occasionally used, but obviously the dishwasher and washing machine use cold water only. Various sites seem to recommend anything from a 120L to a 200L cylinder!.

In addition, we will not be on an Economy 7 electric tariff (for various reasons) and as such the cylinder can be heating during the day. So could we get away with a slightly smaller tank because of this?

nofoollikeold

1. Before you even consider an unvented system, make sure the incoming mains water pressure and flow rate will support it. You need a MINIMUM 1.5 bar pressure (dynamic) and a flow rate of at least 20 litres per minute. Measured at your property, not just at ground floor level.
2. Unvented cylinders are extremely well insulated. The heat loss from a 200 litre heated cylinder should not exceed around 1.5 kW/Hrs per day, which at peak rate electricity amounts to around 35 to 40 pence a day. Careful management of heating times should enable this to be reduced.
3. I'd advise going for the biggest cylinder you can fit in. The difference in heat loss cost between a 70 litre and a 200 litre cylinder is only around 8 to 10 pence per day.
4. Key things to consider before going unvented are:
4a. Pressure and flow as set out above.
4b. The location of the cylinder, and above all the routing of the pressure release pipework to outside. This can be quite difficult with an inside airing cupboard with no external wall.
4c. You need to check the lease to ensure you are allowed to do it. Certainly some leases forbid the installation of combi boilers, generally because they have high instantaneous demands for water.

jk0

I always try to have a fall-back in my rentals, so have an electric shower, and gravity fed hot water system. This has saved my bacon a number of times.

ComicGeek

nofoollikeold wrote: »

The location of the cylinder, and above all the routing of the pressure release pipework to outside. This can be quite difficult with an inside airing cupboard with no external wall.

Doesn't need to be routed outside, can be taken into a soil stack with appropriate tundish and trap. But location still needs to be considered as you say.

peter_333

nofoollikeold wrote: »

1. Before you even consider an unvented system, make sure the incoming mains water pressure and flow rate will support it. You need a MINIMUM 1.5 bar pressure (dynamic) and a flow rate of at least 20 litres per minute. Measured at your property, not just at ground floor level.
2. Unvented cylinders are extremely well insulated. The heat loss from a 200 litre heated cylinder should not exceed around 1.5 kW/Hrs per day, which at peak rate electricity amounts to around 35 to 40 pence a day. Careful management of heating times should enable this to be reduced.
3. I'd advise going for the biggest cylinder you can fit in. The difference in heat loss cost between a 70 litre and a 200 litre cylinder is only around 8 to 10 pence per day.
4. Key things to consider before going unvented are:
4a. Pressure and flow as set out above.
4b. The location of the cylinder, and above all the routing of the pressure release pipework to outside. This can be quite difficult with an inside airing cupboard with no external wall.
4c. You need to check the lease to ensure you are allowed to do it. Certainly some leases forbid the installation of combi boilers, generally because they have high instantaneous demands for water.

Thanks nofoollikeold. The info about cost of heat loss is very helpful! If the cost of heat loss in the larger cylinder is as low as you say, then it seems a no-brainer to just get the bigger capacity.

nofoollikeold

Have a look at the technical specifications on the HeatraeSadia web site for unvented cylinders. Gives details on heat loss.