Can someone explain heat loss figures please

cornerstone13

Hoping someone knowledgeable can help please (or move this thread if necessary)

We have recently had an unvented water cylinder installed in the loft. The blurb advertises a "reheat total time" of 22 minutes which is being achieved. However, we are finding by the end of the day the water is only lukewarm, even though we haven't used very much. Our previous cylinder, also heated through the boiler but sited in the first floor airing cupboard, kept the water at "shower temperature" all day.

The blurb quotes "Standing Loss Cylinder kWh/day" of 1.8 and we'd like to understand what this actually means and if there is a way we can check. The cylinder has a capacity of 210 litres - how could we work out how long it should stay hot/warm for ?

The loft is fully insulated and ventilated; the new cylinder is insulated by the manufacturer and was installed by fully certified plumbers as part of a building project.

Any help gratefully received

DVardysShadow

Differences

• Located in loft rather than airing cupboard
• May be less well insulated.
• Heating coil may be higher up the cylinder
• Initial water temperature may be lower

All these will tend to reduce the performance of your cylinder.

Canucklehead

Hi: I assume your plumber has insulated all exposed pipework in the loft space with suitable insulation.

Secondary return fitted?

Leak on DHW pipework e.g. under ground floor.

HTH

Canucklehead

cornerstone13

Thanks for these.

Yes the exposed pipework in the loft has been insulated and there is a big reduction in the surrounding temperature as the cylinder is now in the loft rather than the airing cupboard, which could easily make a difference - that's why we were wondering how we could check the "standing loss" rate as presumably surrounding temperature would be in the formula; we'd like to know what that formula is.

To our (simple) minds, the initial water temperature should be the same - it wasn't changed during the build project; similarly, we don't think there is a leak on the DHW pipework as the ground floor pipework wasn't touched and we can't see any evidence of leaks anywhere.

We can't answer about the position of the heating coil in the cylinder - we know where the current one is but not the old one ! We'll ask the plumbers about the secondary return - what is that for ? (ignorance is bliss !)

Many thanks

DVardysShadow

Heat loss calculations can be complicated and I really am not going to start on this forum.

In general, the heat loss is proportional to the temperature difference. So if your water is 60degC and you airing cupboard is 25degC, the temperature difference is 35degC. If your loft is 5degC, the difference is 55degC, so you will be losing heat 1.57 times as fast. Assuming the same cylinder. If the new cylinder is not as well insulated, the heat loss will be faster. If it contains less water, the temperature drop will be faster even though the heat loss is the same.

ormus

any equipment tests are conducted in a controlled environment, using a standard. as a starting point.
ie 20c degress is a pretty universal standard for room temps.
also you would need to know the complete system being used. pipework & insulation etc...
so the 22 minutes reheat total time, could be a bit meaningless to you or me, in our houses.

MX5huggy

The calculation method is covered by BS 1566, They heat the cylinder to 65 in 20 degree room then for 3 days measure how much energy is used by a 3kW immersion to maintain the 65 degrees.

closed

is there a thermostat clamped to the outside of the cylinder? Are you heating the water at the same time the heating is on?

Canucklehead

Hi.

Was the unvented cylinder a replacement for a vented one?

Is there any water being lost out of/through the tundish?

GSR.

cornerstone13

Thanks for everyone's help with this - at least we now realise we're obviously not going to be able to check the heat loss ourselves !

Yes, the unvented cylinder is a replacement for a vented one and there is not water being lost via the tundish. By GSR, do you mean Gas Safe Registered ? The plumbers are definitely registered for everything they need to be (I used to work in insurance, so check this type of thing quite carefully).

Not sure if there is a thermostat clamped to the outside of the cylinder (will have a look), but we set the hot water to come on before the heating in the morning, so they're not on at the same time.

We think, from the questions and points you've raised, that it is very likely to be the move up to the loft that has changed the efficiency, so we shall look at ways to make the surrounding temperature of the cylinder warmer.

Thanks again

Canucklehead

Hi.

I would be worth asking your installer to check for leaks.

It's possible that , even though you have not had work done on the ground floor, you have a leak.

Pipe and fittings that were used to low gravity pressure might suffer from the higher mains pressure. I have had fittings hold up with flux alone on gravity . When converted to mains it leaked. It had never been soldered.

GSR (Gas Safe Reg)

PS
Unvented cylinders usually come with stats fitted.(not clamped on but probe type)