21 June 2021

Grant Aerona 3

Grant Aerona 3

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.

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Mars 21 April 2021 at 20:55

Very helpful review Mike. Thank you. Two quick questions:

1. Which part of the country are you based in?

2. Do you only have underfloor heating (no rads)?

Mike Patrick 22 April 2021 at 10:01

Witney, Oxfordshire

Yes, no radiators, underfloor only

Neill 26 April 2021 at 11:36

This sounds like a bit of a sorry tale that should not be beyond some professionals to sort out. Disclaimer, I’m no expert heating engineer! It seems quite common that initial heat loss calculations are rather vague and under estimated.
The heat loss estimate is 14,800 kWh per year. Really roughly then assuming 6 month heating “season” we can see that this equates to 83kWh per day each day for 6 months. That seems like a lot but does include hot water.
However, if we assume the 9kW unit might realistically over a long period of varying outside temperature and humidity supply 8kW then it would need to run for 10 hours a day in the cold season. If we guess an average CoP of 2.7 say then it will consume 30kWh per day of paid for electricity.
In reality there will be defrost cycles, these will be perhaps more dependent on outside humidity than temperature and particularly troublesome around 2-4 degrees. The higher the flow temp demanded by the system the more often defrost will happen and the less efficient everything will be. The lower the temperature demanded the better. The demand for higher temp water may relate to the variable hot water heating performance. And, as below, any air in the system will make for greater inconsistency.
You say you have underfloor heating, in which case the system should be set for quite a low flow temp (maybe less than 40 degrees even), the 55 degrees mentioned must surely be for hot water only.
The upper and lower floors will have different heat demands and should therefore be on separate manifolds with their own circulation pumps and both of these are best separated hydraulically from the heat pump via a buffer tank, low loss header or the physical separation of a heat exchanger. This then allows for the two heating circuits to run at the best flow rates for their needs whilst still maintaining the required minimum flow through the heat pump circuit. You don’t want one common flow rate through the whole system.
All circulating liquid systems need some internal static pressure to generally overcome localised hot spots and to avoid air being retained/drawn in. The pressure gauge on the expansion vessel should be reading the overall system pressure, i.e. it is not just reading the pressure in the expansion vessel. If the gauge is working and the system filling loop is opened to generate some pressure (e.g. 1.5 bar say) and this then drops slowly (or quickly) to zero then there MUST be a leak or alternatively there must be a lot of air that is quickly vented out of the pipes.
Most heat pumps will have a minimum pressure below which they will throw fault codes.
A key point is that the system was working and much more efficiently than it is now. Air is a poor medium through which to transfer heat. So, if there is a leak and the original water has been subsequently “replaced” by air this could explain the gradual drop in efficiency and loss of pressure. It may be best to separately close down every circuit and flush/bleed each one individually.
It could be that a competent plumber might be better suited to finding the root cause of the pressure loss/leak issue but even so you’d expect Grant to have some interest…
Good luck

Mike Patrick 26 April 2021 at 11:54


Thanks for those helpful comments.
The 55deg is the figure stamped on the heat pump itself, although the thermostat on the water cylinder is currently set to that too.
We have separate manifolds for upstairs and downstairs. There’s a buffer tank on the return for the heat pump and separate expansion tanks for the heating (with the zero reading gauge) and the hot water. I do plan to have an engineer visit but assessing their competence in advance is an issue.


Grant UK 26 April 2021 at 15:04

Thank you for your review Mike – it is always good to get feedback from customers and, as a Company, we will always take on board comments and make changes or improvements when appropriate.

If I can respond to some of the points in order, hopefully it might explain a few of the issues you may have been experiencing:

Firstly on the usage, this wouldn’t be unusual given the circumstances the country has found itself in over the last year. We have seen an increase in household electricity bills due to homes being occupied more as a result of the three lockdowns as well as the very rare weather we have experienced, with long cold spells without the temperature rising above zero in most areas. This along with lights being on for longer periods of time, rooms being heated to higher temperatures and for longer especially as bedrooms are being used as office spaces – these factors combined with the cold snap have seen homeowners see an increase in their utilities. We always recommend that the usage of a heat pump is gauged over a 12 month period as over 60% of your bill will be based on November, December, January and February. This should average out over a year’s usage with minimal usage during the late Spring and into the Summer.

Regarding your comments about the Hot Water, it sounds as if the timings just need to be altered slightly so the unit is coming on slightly earlier to heat the cylinder, it is more likely sporadic based on the hot water usage from the day before. If it has to heat a whole tank up from cold it will take longer than half a tank. This will most likely be a very easy fix.

If the heat pump is installed by one of our accredited installers they would normally register the unit via their Portal, this is simple to use and then gives us an indication of when and where the units have been fitted. We have searched our system and cannot find your appliance registered on our system currently. If you are happy to send me the installer’s details (via private message or email) one of our Renewable Business Development Managers will be happy to contact the installing engineer. If we deem extra training is required then this will be offered. Also, if you would like to speak to one our Renewable Business Development Managers directly, please let us know and they will be happy to assist you and discuss any issues you have.

Posted on behalf of Kevin Ellis, Grant UK’s Renewables Sales Manager


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