More electricity needed for DHW? How?

[Timbones (Tim)]

I’ve just discovered that my heat pump seems to draw 500 W more power during the heat pump cycle compared to space heating. I’m curious if anyone else sees the same, or can suggest why this is.

This is for a Mitsubishi Ecodan 11.2 kW with pre-plumbed cylinder that is heated via a plate heat exchanger. There are 3 pumps in the system: primary to the outside unit, central heating via LLH, and one to pump potable water from the cylinder through the heat exchanger.

I have an electric meter on the whole system, including the pumps and controller. I also know how hard the compressor is running, as a percentage of it maximum input power (3.73 kW). This means I can compare the two and calculate the consumption of the other components.

I already know that the standby power is around 20 W, and there are periods when the primary and central heating pumps are running but the compressor is not so I know they take about 50 W each. Plotting these on a chart stack up nicely, so I’m moderately confident that it’s pretty close, give or take 10%.

Then I look at a period of water heating, and find my numbers above don’t fit.
Chart below shows DHW up to 13:40, followed immediately by space heating:

Instead of 50 W for the pump, I need to factor in 500 W to get the total power match what the meter is reporting.

This can also be seen by simulating heat output using Carnot equation. I matched the heatpump factor (0.525) on the heating portion, then zoomed out to show both. Carnot is really expecting to see 2 kW more heat for DHW given the amount of electricity going in, but it’s just not there.

While I’ve long accepted that COP just isn’t so good during DHW cycles, I was surprised to find this out. Surely a pump that moves water from the cylinder to the heat exchanger and back doesn’t need to be 10x more powerful that the one for 20m of primary circuit? Am I missing something?

I looked at a few other similar systems and think I see the same effect there too, some more than others, so maybe I’ve not seeing the same thing.

[Timbones (Tim)]

The FTC manual suggests that pump 4 draws 72 W, and my system is set to a slightly higher speed for DHW cycles. I might try reducing the speed to see if that changes the consumption.

[Sarah_Hanson]

New contributor here. I have a recently installed Samsung AE080BXYDEG/EU with MIM-E03EN controller, so not directly comparable with your setup, but I’ve noticed the same thing (more power consumed on DHW than CH).
Power consumed is a function of ASHP flow (exit) temp. In the Samsung setup, flow temp during CH demand is constrained by “water law”, an energy-saving feature which decreases the target flow temperature linearly with increasing ambient temp. However, this feature is not available for DHW demand, and the ASHP winds up its exit temperature (thus draws more power) until the tank temp setpoint is reached. This is partly because the coil in my DHW tank is somewhat undersized (we didn’t upsize when the ASHP was installed).
I’ve partially got round this by 1) minimising the tank temp setpoint - the sensor is at the bottom of the tank and I get about 5degC layering, so the drawn water is plenty hot enough, and 2) heating the tank in 2x15 minute bursts (with 15 min rest between) rather than a single 30 minute programme (which I assume is smoothing out the thermal inertia of the system).
Hope this helps…

[MyForest (David Bowen)]

BTW @Timbones I see this other slightly unexpected thing.

The heatpump’s “what power level is appropriate for the next iteration” (blue) maxes out at 2.9kW but the actual consumed energy (red) goes quite a bit higher, peaking at 4.8kW.

Also, this run is vaguely interesting because it starts off getting more efficient as the pump gets into it’s stride but then the water gets hotter and it has to work more and the efficiency just keeps dropping.

This is my overnight run when the carbon and price are lower so I push it to a higher temp than I do during the day.