Can someone explain heat loss figures please

Johnandabby

It's a shame that the new unvented cylinder wasn't installed in the same place as the previous cylinder - as other posters have noted, the problem is probably that the loft is much colder than the habited areas, and therefore increasing the rate of heat loss from the cylinder. I assume the insulation in the loft is between/above the floor joists rather than between the rafters (ie insulation installed flat between the loft space and the rest of the house, rather than following the pitched roof itself) - this together with the ventilation of the roof space provides a perfect place to lose heat from the cylinder. This should have been thought about by the installers, as it will make a big difference, but most don't really seem to care.

Short term you will probably need to reheat the cylinder a couple of times each day - long term, insulating the roof rafters or relocating the cylinder to the first floor airing cupboard might be worth considering. Don't just remove the loft insulation, you will spend more money on the extra space heating than the saving on the water heating. Check the cylinder temperature - the higher the cylinder temperature the higher the heat loss as well.

brig001

This is just for the tank itself:

1.8kWh/day = 75W constantly

This will probably be assuming an ambient temperature of 20 and a water temperature of 60 deg C, so 40 deg C difference.

Now with the tank in the loft, you have closer to 60 deg C difference, so your losses will now be 112.5W constantly.

If we assume the loss is constant as the tank cools (it isn't, but will be close enough), 112.5W over 24 hours is 9720kJ per day. Water has a specific heat capacity of 4.181 kJ/(kg degC), you have 210kg, so your temperature loss will be 11 deg C per day (ie. 24 hours).

49 deg C should be warm enough for a shower.

Are you heating all of the tank?
Where is the thermostat?
What temperature are you heating it to?
Where is your boiler? If it is next to the cylinder, it could be thermo-syphoning back round the boiler cooling it that way.

HTH,
Brian.