Hi All
I am new here and am looking for information and advice regarding my ASHP. I have posted this in the ASHP area; but it is quite technical; so I thought it might be appropriate to post it here as well.
It is an Hitachi Mono 4 HP RASM4 [Yutaki RASM-4VNE] with a nominal output of 11kW. It was manufactured in 2019 and was installed in a new build house in 2022.The heat pump supplies a three storey 280m2 detached house with UFH on the ground floor and radiators on the upper floors. The outside design temperature is -3.4C [central UK].
Various heat loss calculations have been made which estimate the heat loss of the building to be 10.3kW. There does not appear to be an amount quoted for heating domestic hot water [300L DHW tank installed], or for defrost cycles included in that figure.
Qualitatively speaking, the heat pump appears to struggle at lower ambient temperatures, particularly if it is windy, leading to a fall in room temperatures. More specifically, we have issues with two rooms.
The main bathroom, which has a room volume of 25m3 is fitted with a vertical panel radiator rated at 5668BTU at deltaT 60C. However, the water going into the radiator is only about 38-40C; giving a deltaT of 20C. Weather compensation is set so that the heat pump should produce water at 48C, when air temp. is close to 0C. Applying the radiator manufacturer’s deltaT correction factor means a corrected output of approx. 1400BTU. The design temperature of the bathroom is 22C and to date, the developer’s plumber has not been able to achieve a temperature higher than 20C, by balancing the upstairs radiators. Usually, the bathroom temperature hovers around 17-19C dependent on outside air temperature. A lack of adequate insulation was identified in the bathroom ceiling. It was underdrawn with thermal plasterboard recently; but this has made no discernible difference. There was a discussion about putting a larger rad [higher BTU output] in the bathroom. However, the heat pump manufacturer/supplier advised against this as it would “result in the heat pump becoming over worked and running more defrost cycles, taking heat away from the primary circuit”.
The dining room has a volume of about 180m3 and has total glass surface area of 40m2 [bifold doors and roof skylight]. It has its own UFH manifold. Over recent weeks, with outside temperatures a couple of degrees above zero, the heat pump must run overnight to keep the room temperature acceptable and avoid it, otherwise, having to warm the room from about 16C in the early morning [i.e. reduce the warm up load]. I think the dining room is responsible for roughly a third of the total house heat loss.
The water pump runs at 100%, when I check it, and during cold periods we appear to be using about 80kW of electricity in 24 hours for the heat pump.
I have looked at various documents including MCS MIS 3005, MCS 021, MCS Best Practice Guide etc. They state the heat pump should be able to cover 100% of the output; and to achieve a satisfactory installation the heat pump specification should take into account the heat emitters and room design temperatures. I was told by one expert that the heat pump can only cover 96% of the heating requirements of the house at design condition.
I’m getting conflicting information from various sources: heat pump too small, heat pump borderline; or it should be possible to balance the rads to get 22C in the bathroom. The developer is sticking with the heat pump manufacturer/supplier who say it is the correct size. As an aside, two other properties on the same development have the same 11kW heat pump; but they are 220m2 and 250m2 respectively.
Any advice or opinion appreciated.
