Heat pump or immersion hot water?

[Windbag]

So up until recent energy changes, I used the ASHP for heating and a solar iboost for hot water (large tank two elephants). All ok.

Now it seems I can buy at 36p and sell my export at 49p. Oooooo. That changes things. We now move from working with the weather enjoying the wind and the sun, into nocturnal creatures only doing things in the dark

The immersion is now just doing an hour first thing in the morning, the top up based on export is of cause 'off'

My question. Would it really be cheaper to heat water with the ASHP? Considering the pipe runs/ heat loses etc

[Andy]

The answer is I guess it depends. I have a GSHP and this is a typical recharge of the cylinder. There have been a few showers so the bottom is cold and the top is not. 41/19 ºC. The average COP was 2.9 to take the top to 55ºC and bottom to 51ºC. 1.3kWh was put in for 3.88kWh out.

However if the whole tank was at 19ºC then the average COP is much better as it starts at >5 COP as opposed to 3.1 as in this case.

The pipe losses wouldn't be huge if properly insulated. What's the distance? The following link shows 8w per m for insulated 22mm pipe at 55ºdelta. https://www.engineeringtoolbox.com/copp ... -d_52.html. However it will be less for a heat pump as it doesn't send the water into a tank at a high temperature. It slowly ramps up the temperature relying on the oversize coils in the cylinder.

I don't get the payment for exporting so I stick the sun in the cylinder. I always use the GSHP to heat the cylinder in the winter as it is significantly cheaper. I tended to program the recharge for cheap periods so the house loop regain was also covered by the cheaper rate.

As the summer months approach you will be getting similar if not better numbers than I am.

[Ken]

This will be a close run thing and very much will depend on your usage and pipe runs etc. Remember there are always start and stop losses and then there are dead leg losses ie before heating the tank one has to first heat the pipes etc and this heat is completely lost after the heat cycle.

Often the coil will be towards the bottom of the tank but the immersion is higher up thus not heating so much water.

My gut feel is for low usage of HW and in non heating season the immersion heater is cheaper and for high usage and heating season the HP is cheaper.

In the non heating season you could measure the leccy input into both systems to compare, perhaps a month under each system. Love to know the results of that.

[Gareth J]

I don't know quite how you work out/consider depreciation of a heat pump but, if there's an expected lifespan/service schedule based on hours run, that might be important. An immersion heater is extraordinarily reliable/low maintenance.

[Yuff]

Our heat pump doesn’t heat the water to a high enough temperature so I heat it for 30 mins just before the off peak tariff ends, and then the iboost heats the water up,to a nice temperature during the day.
During the winter I used the immersion but I don’t think I need it now when there is a decent amount of solar.
All new to me though as I have only had this up and running since November 22

[nowty]

I'm on deemed export for FIT so the export thing is not even worth considering for me.

I use my GSHP to heat the water up to just under 50 degrees, then use the immersion to get it higher. The COP on my heatpump really suffers above 40 degrees so its not much better than the immersion at the high end.

In Summer when there is an abundance of solar, I just let the solar diverter heat the water on the immersion and give the heatpump a rest.

[Windbag]

Why do you need your DHW hotter than 50c or are you storing and blending?

[nowty]

Why do you need your DHW hotter than 50c or are you storing and blending?

To reduce risk of legionella.

[Windbag]

My gut feel is for low usage of HW and in non heating season the immersion heater is cheaper and for high usage and heating season the HP is cheaper.

That's a good point, thinking the heatpump is on anyway so the lagtime losses are lower. But the outside air temp is also lower?

Thoughts?

[Windbag]

Why do you need your DHW hotter than 50c or are you storing and blending?

To reduce risk of legionella.

OMG. How much money do you spend on something that doesn't exist in the domestic setting! Legionella only exists in standing warm water. Domestically is always getting a dose of chlorine... Every time you turn on the tap.