DIY fan-coil heater
And actually the best fan coil heater regardless of price.
If you are wondering how a heat pump might (or might not) work at all with your old radiators and pipes this might give you the answer, and it won’t cost too much
You might be surprised to know that if your house build is anywhere near Passivhaus standards then minimal heat demand makes underfloor heating a waste of time and money. Well, that’s a big saving so bring it on but what to do instead?
Underfloor heating did away with horrible radiators, and we don’t want them back, so that just leaves warm air blowers such as kickplate heaters – i.e. water powered puffer heaters..
On the walls without plaster theme here I proposed a kickplate heater, in the lower cupboard part, to give an occasional guff of hot air and for slightly under £200 you can do just that. However these neat little units aren’t perfect. They have to shift a lot of air through a small aperture so they are intrinsically noisy, draughty and the heat exchanger, or a filter, can clog with dust and pet hair fairly quickly. Larger versions are available but they aren’t so neat and they are much more expensive.
So here’s the Original Twist fan coil heater which is cheaper, quieter and more reliable. Double click this picture for a better look. Can’t see the heater? Neat huh.
Lets assume that the low cupboard bit of your wall is made of typical 720mm x 300mm kitchen carcass units. A pair of them 600mm wide with a large slot cut out of the inner sides allows a 1000mm x 500mm x 70mm double radiator to fit inside (£48 Screwfix). Each of the top shelves is fitted just low enough to mount a 140mm fan from a computer (£10 – 20 Amazon etc) and these gently blow air past the radiator and out of the bottom. You can decide where the air inlet goes: probably top front but a wall panel channelling much warmer air from ceiling height is worth a thought. Note that this is much smaller and neater than the huge radiators you would be trading up to if you were trying to make a heat pump conversion work.
When paired with a suitable heat pump this unit also does air conditioning if that’s what you want hence the length of plastic guttering under the radiator to catch any condensation.
N.B. The Heat Pump Incentive does not support heat pumps with integrated aircon but you can still get it if you use a separate chiller unit – see the Originaltwist hybrid heat pump concept which does chilled water as a free extra. Before you get too enthused about that though, have a think about a separate mini-split aircon unit (a very cheap heat pump) which delivers hot or cold air with a much simpler installation. Cold air flows across the floor so spreads around the house surprisingly well. You don’t need multiple sources like you do for heating.
You might want to kill airborne pathogens. There is plenty of space in the box for a pair of UV lamps but if you do fit them then avoid or wrap PEX water pipes because UV will kill them too. Fitting UV tubes in the top sections with the fans underneath is the way to go.
Controls: The fans are switched on automatically when either hot or chilled water arrives at the radiator. An ESCO solar controller does all that for £45.50 or about half that with a frost stat and a 2 pole thermostat. If you are omitting the aircon side then any solar controller will do the job simply enough. You’ll also need a variable resistor for speed control. The fans are 12v so you’ll need a power supply and, like the fans, these are cheap PC components. Remember that the LED strip lights in your integrated beam floor could be 12V and might like to share that power supply.
Cleaning: Open the doors and Hoover it out – easy.
And that’s it really. A few simple components all easily obtained and making the perfect, reliable fan-coil unit. When turned down to essentially silent the fan pair will move about 100cfm and the heat output will be between 1 and 2kW depending on the water supply temperature. Turning up the fans will double this.
You might wonder why all this effort gives you anything better than the same radiator fitted as normal. The answer is that heat pumps don’t work efficiently at the 60c+ normally used with radiators. While under-floor heating is the best at very low temperatures a fan-coil unit works reasonably well at 40c. The only reason our unit has a radiator is that with all the fins in a double radiator you get a high surface area for not much money. The heat delivery couldn’t be more different. A radiator wafts hot air straight up to the ceiling where it tends to remain – with replacement air moving across the floor as a cold draught. A fan-coil on the other hand, blows warm air across the floor where it mixes with cooler air to give a faster and more comfortable heating experience.
Some extra mods
If you want to direct warm air somewhere in particular, patio doors for example, then a row of slats on the exit slot can send the air to the side. To go further with directing air flow you can also fit a duct or two to the bottom of the unit. This might be useful if you were trying to warm the air near those patio doors. In a new build you could fit a drain channel beside the doors (those ones with the clip-on metal tops) and blow hot air into the ends of the channel. That same channel could also be used as a handy route for the heating pipework and as an added benefit it makes the channel into a radiator in its own right.
If you have an MVHR system in mind you could run your return duct into the top of this unit with the benefit of hiding the outlet and getting a free blow from the MVHR fan.
What about the bathroom?
Now we’re talking. The perfect place to have hot air blowing especially when you are drying yourself. For a start I’d turn the unit on its side so the air is blown from a vertical slot, then I’d go one step further and have a pair of them in a corner so you step into a sort of Dyson hot air blaster, like a giant hand dryer. Smaller bathrooms could have an opposing pair. Electricity in the bathroom! No worries there, it’s all 12v.
Have a look at this chart on heating costs for various house types to appreciate why a Passivhaus with a small heat pump and some fan-coil units could be the way to go. You’ll see that the ASHP can meet the Passivhaus demand for almost no cost. Indeed if the ASHP is connected to PV panels then the running cost of the house will be close to zero. This chart is out of date now. I did a new one in October 2022 when all the new rates were fixed. here
The results are startlingly different so take a look and see if you agree with my comments.
I guess you might be in a state of shock now. Thousands saved on your heating strategy, the government might buy you a heat pump and air-conditioning can be a simple addition.
For more money saving shocks check out more building ideas on LIST OF POSTS.
If you’ve built this please let me know on the comments form below. We’d all love to see pictures.
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