Effect of Hitachi Heat Pump over-heat

Published October 8, 2012 by Sean

I noted earlier (see this post) that the Hitachi Yutaki Air Source heat pump has a design flaw. By only sampling the control signal every 20 minutes, it is perfectly possible to start heating water with a setpoint of maybe 55°, run for several minutes, and then attempt to raise the entire space-heating circuit to 55° even though the controls would otherwise be asking for a flow temperature of only 30°C.

Now that I have accurate monitoring on my heat pump, I was able to demonstrate this effect. Below is todays log of heat pump flow/return, space heating/heat pump return, and room temperature. (Click each image to zoom)

Heat pump and room temperatures

Currently, the room stat is set for 21°C between 5am and 8am, and from 6pm. (19° from 3pm). Hot water is suppressed till 6:30 am, unless the stored water gets too cold or significant flow is detected (shower profile detection).

Space heating can draw on the lower part of the tank, the two sharper drops in the lower probe temperature are due to water being drawn off and colder water entering:

The days power consumption is here:

Heat pump only power profile

Two fairly major heating events account for a large proportion of the days energy use.

Zooming in to the temperature,

The two water heating cycles take about 8 and 15 minutes each, but once the hot water demand is satisfied, the system reverts to space heating mode. Normally in space heating mode, the heat pump only runs for a short time, at lowest output, before the temperature rises above the trip point (pump speed needs to increase, maybe a buffer tank would help). In the case where there is still heating demand once the DHW cycle has finished, the pump runs at near full power to overheat the radiator circuit. If I had a buffer tank, the pump would run even longer.

The problem is seen even more clearly in the power plot. The short spike of power is followed by a longer period of high power, causing significant wastage.

Comparing the different power states (and extrapolating for an evening heat with 10°C outside), idle, compressor pre heat and pump account for 30%, but in-efficient space hating also accounts for maybe 20%.

The impact is probably worst in the not-so-cold days where the excess heat is mostly wasted. It seems that the Hitachi Yutaki control algorithms are probably optimised around a fairly high load factor (my pump electric input in February was 538 kWh, which corresponds to 20% of full-load).

For that 538kW input, I measured 1125 kW heat output. COP = 2.1, excluding control circuits.

Filed under Heating

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