Heating hot water cylinder -gas or economy7?

Scot_39

Is that 3kW electric or 3kW into tank.

What sort of cop ratio does the hw cycle get in summer / winter when temp diffs higher ?

matelodave

its normally about 1kw electric and then what ever the tank accepts - I dont have the facility to monitor COP or independently monitor the heatpump power consumption but I can estimate what its using by when its on and by reading the smart meter half hourly data.

It does use a bit more in the winter but thats because its colder outside and the incoming water to the tank is colder. This time of year it only runs for about 45 mins a day, whereas in the winter it might take another half an hour to reheat the water.

edit 17th May

Interestingly I forgot to turn the hot water on yesterday morning so it was pretty cool this morning, down to 34degrees. It took about 70mins to reheat the tank to 45 degrees and consumed just over 2kwh of leccy (outside temp was 14 degrees and heatpump flow gets up to around 52 degrees when heating the hot water. According to the Daikin spec sheet , 15 degrees outside to 55 flow equates to a COP of 3.07.

The heating coil is 25%-75% of a 200 litre tank, so I guess we are heating around 150 litres by 10 degrees which theoretically needs 1.92kwh -  which actually means that we might have been better off using the immersion heater  and not using the heat pump. A 3kw immersion would have done it in around 40 minutes

Bear in mind that these are all slightly finger in the air calculations as I don't have the means to do proper measurements but it does show that there are probably a lot of inefficiencies in heating the hot water tank. Not least being the amount of heat that gets transferred from the inner coil to the tank water, especially in our case, where the Dt is only around 10-12 degrees. Our tank is only around 600mm away from the hydrobox so there's not a lot of efficiency loss in the plumbing.

A more modern high efficiency heatpump cylinder might be better, although that said, our tank was supplied by Daikin so should be designed for the purpose.

I guess similar factors would come into play, even when comparing with a 90% efficient gas boiler, It doesn't really matter how efficient it is if it cant transfer all the heat into the hot water because the coils may not have the capacity to extract all the heat thats being generated