Our Grant Aerona 3 air to water heat pump is rated at 9.1kWh output with a COP of 2.66 (water at 55C, air at 7C). It uses R410A refrigerant and is accompanied by a 200 litre Grant Monowave cylinder. The underfloor pipework and individual room thermostats are JG Speedfit. This review is as much about Grant’s service network as about the performance of the pump itself.
As a caveat, experience suggests that any problems with a heat pump or underfloor heating system could be as a result of:
- Poor design/specification of the overall system
- Incorrect installation of the specified system
- Heat pump is the wrong one for the job
- A fault with any part of the system
If performance is not up to expectations it can be very difficult to identify if it is one or a combination of the factors listed above.
Our pump was installed at the end of 2016 as part of a ground-up restoration and extension of a 1,500sqft Cotswold stone cottage. The property is electric only, massively insulated, double glazed, has LED lighting, and underfloor heating on the ground and first floors. The pump sits next to an outbuilding (also with underfloor heating) about 20 feet from the house, with an insulated underground conduit carrying the flow and return pipework.
The heat loss calculations for the MCS scheme were done by www.heat-engineer.com. They estimated an annual energy usage for space heating and hot water of 14,800kWh, with a SPF of 3.56. In reality, this estimate proved (initially at least) to be overly pessimistic. In the 12 months to March 2019, it was 11,811kWh (total electricity usage), but by December 2020 it had risen to 15,000kWh. I have no explanation for this.
The air source heat pump runs quietly enough and for four years it did so without incident. Our installer (no longer MCS registered) omitted to register the installation with Grant – something that I was not aware of until more than a year later. I only rectified this after three years when I commissioned the first service.
The control panel (unless I am missing something) does not tell you the power consumption and heat output of the pump. I’ve always taken regular (now daily) meter readings, but these are of our total electricity usage. It was these numbers that eventually led me to realise that our usage had been climbing steadily. That, and irritation of not always having hot water in the morning, prompted me to get a service.
Grant has a list of approved engineers on its website. One was booked for October 2020. They arrived, took the covers off, prodded around, and said it was OK. I was told the pressure gauge on the heating expansion vessel was broken as it read zero. I subsequently discovered this was wrong; it read zero because there was (and still is) no pressure.
Shortly after their visit the pump stopped working and we saw an error code on the control panel. The engineer returned, reset it and left. It stopped again and after a further visit he said that I (why not him?) would have to contact Grant. I spoke to Grant and discovered that the code meant the main PCB had blown. By now I had decided not to use this engineer again but told them to order and fit a new PCB. This took about two weeks to arrive and it fixed the pump. But there was still no pressure in the expansion vessel and the occasional lack of hot water.
Come January 2021, our daily power consumption peaked at 105kWh on January 8, and averaged 79kWh daily for the month. The house remained warm (set at 19C, 24/7) but the water would not heat. This was the coldest weather in four years and the air temperature was well below freezing for several days. At one point I switched the pump off for a day (the house kept warm) as I envisaged my bank account being drained by the unstoppable electricity usage. This episode has led me to question whether the pump can work efficiently at all below 0oC.
Following the cold spell another engineer was called, from the Grant list, to look at the system. He was more knowledgeable and asked some sensible questions, but spent most of his time getting to understand the installation. He marked up the various components, as a reminder, with an indelible pen. Several attempts were made to restore pressure to the expansion vessel, via the filling loop, but the pressure always fell back to, and is still at, zero.
Power usage is again at a sensible level (ie. less than in the same period last year) but this seems to be mostly due to mild weather in February and March. Hot water is still not guaranteed. It is clearly somewhat, but not exactly, related to air temperature.
I have looked at the Grant engineer list again and now realise that neither of the two used so far are MCS registered. Neither does new installations. I’m tempted to try a third engineer from the list who I see is MCS registered. I still want to fix the outstanding problems (unreliable hot water, no pressure in the expansion vessel and excessive power consumption in sub-zero temperatures).
Watch this space.