The Fimer Windy 3HP (“hp” = heat pump) is a through-wall reversible air-conditioner which can function as a heat pump in winter and an air conditioner in summer (the two are essentially the same process run in reverse, just imaging you’re trying to cool the Earth’s atmosphere in winter and venting the waste heat into your house).
It’s a single unit that mounts on an internal wall getting it’s supply and vent through a single hole in the wall. Because of this there are no bulky units to fit outside your house, and the whole thing can be installed as a DIY job. It’s just about the easiest and cheapest way to get an air source heat pump into your home.
I recently fitted one to replace a segment of my gas boiler’s central heating that had never worked since we moved in, and as an experiment in heat pump heating this winter.
Big box goes on wall
The unit itself is a fat box about the size and shape of a large suitcase. It mounts onto a plate screwed to the wall and comes with a standard 3-pin plug, although I’ll be wiring mine into a fused spur once I’ve finished monitoring it via a plug-in meter. You’ll need to get out the core drill and punch a 150mm hole through your wall. From there two concentric plastic sleeves carry air in or out and the box simply hangs on the plate on the inner wall. It weighs 23kg, so it can be fitted single-handed if you’re careful. I’d get a friend in if you’re hefting your chunky heat pump above about knee height.
Internal air is drawn in the top through removable washable filters and vents horizontally out the front. This can create a bit of a breeze at full blast, so have a think about what you’re pointing it at. That’s not necessarily a bad thing though, my wee girl loves standing in front of it and having the warm air blow in her hair!
There are more powerful 4Hp and 5HP units available. They’re the same physical size, but are rated higher.
It heats pretty effectively, drawing up to about 700W maximum, and heats our small 3.5x4m room quickly. Stated COP is 3.14 in heating mode (at 7ºC outside, 21ºC inside), and max thermal output around 2.2kW. Since the air is blown out it doesn’t feel dangerously hot like a 2.2kW convection heater would, and the mixing of the air heats the room evenly and quickly. Compared to the floor-standing 3kW heater we were using air source heat pump is a massive upgrade, safer around the kids, and much cheaper to run.
Under control of its thermostat the unit seems quite accurate, coming up to the temperature set point without overshooting, and keeping the temperature nicely within about 0.5ºC.
The vent is pretty low-profile and unobtrusive (dodgy amateur concrete job notwithstanding…)
The vent on the outside wall is quite small and tidy, although not everyone will be a fan of the plasticky grille.
The wall unit contains no buttons, which is good for child-friendliness if mounted low down, but does leave you somewhat reliant on the tacky wee IR remote. Apparently an RF version exists too.
Power consumption on standby is good, at about 3W. This is low enough that measures like timer switches become pretty marginal, so it can just be left on standby if switching it off is inconvenient.
The unit itself is quite modestly styled and should suit a range of decor. It’s paintable if you want it to blend in further.
Not so good
Due to having the whole gubbins (compressor, fans, etc) inside one box it can be noisy. It’s about comparable to a powerful fan heater, and mine does have a slight whistle which is a little annoying, but not teeth-grindingly so.
The main problem is the controls, specifically the ability to automate. There is no proper facility for timed on/off cycles. The only facility it has is a delayed on or off function. In “Timer On” mode you set the machine up how you want and switch it off, leaving the remote pointed at it. At the right time the remote sends the “on” signal and off it goes. Likewise for “Timer Off”, set it running, leave the remote in view and it switches it off at the appointed time. Unfortunately you can’t set both on and off times, it’s one or the other. So every night I have to set mine to come on the next morning, then it has to be switched off manually. A bit poor really, Fimer really need to sort this out.
The remote must be left pointing at the unit for timer mode to work, I get around this limitation by putting some sticky velcro on the remote and leaving it stuck inside an alcove opposite. Again, you’ll need to think about where you’re positioning it to get around this (or try to find an RF version).
Power consumption when not heating or cooling is not great. When switched on the fans run constantly even if there’s no demand for heat (or cooling), drawing about 30W continuous. Nice for a bit of air movement, but it would be better if the fans stopped after a reasonable time of inactivity.
Due to the entire condenser being mounted indoors it will collect condensate in its internal tank even in heating mode, you’ll either need to empty this occasionally or fit the included auto drain kit. I recommend the latter, it includes about a meter of rubber tubing, so have a think about where you’ll drain condensate to. Even if you don’t fit the full drain kit, fitting the tap from it makes draining the tank a lot easier, or else you’ll be needing an allen key to remove a fiddly drain plug every few days.
Dehumidify mode seems fairly ineffective, I didn’t see anywhere near the stated 1l h-1 figure and it draws about 500W continuously. I’d recommend a proper demidifier with a decent sized tank if you need to control damp.
The box sticks out a fair way from the wall (about 300mm), so it won’t suit hallways.
The COP the manufacturer claims is 3.14 in heating mode. This means the best case scenario is that it will use 1kWh of electricity to create 3.14kWh of heat. Realistically you should probably expect more like 2.5 IMO, but maybe I’m being cynical. Whether this is greener than a gas boiler for example will depend on the CO2 intensity of your electricity supply.
My old gas boiler is 78% efficient, meaning that to create 100kWh of heat it will use 128kWh of gas, releasing 23.5kg of CO2.
The Windy 3HP will use something like 40kWh of electricity to do the same (assuming COP=2.5). At bog-standard grid carbon intensity that would release about 21kg CO2e, although taken as an annual aggregate your supplier may be better or worse than that (UK fuel mix: energy companies emissions compared). So marginally cleaner in carbon terms, but using electricity does allow you to get the supply from a clean source. For example, on a couple of mornings I’ve had enough sunshine to run the heat pump off my modest little 2kWp PV system at least some of the time, even in November. During spring and early autumn I’d expect to be doing this quite a lot. I hope the polar bears appreciate it.
Obviously if you’ve got a very clean gas boiler and a dirty electricity supply (Scottish Power is the worst by the way) then it might not make sense, but if you’re heating with oil or resistive heaters or have clean electricity available then you’ll be streaks ahead.
The refrigerant is a non-ozone depleting one (R410) as required by law. It’s still a pretty horrendous greenhouse gas so make sure it’s disposed of properly at end of life, or you’ll probably negate all the carbon savings it’s ever made.
Not a machine without flaws, but at about £400 delivered it’s got to be the most affordable air source heat pump available. It’s easy and cheap to fit, effective and will make your home a happier place. If you’re looking for an all-singing all-dancing air source heat pump to heat your whole home, this isn’t it. But if you’re using electric heating or want to take some load off your fossil-fuel spewing gas boiler then it’ll do the business.