Tag Archives: DIY fan coil

Easy brilliant DIY fan-coil heater

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DIY fan-coil heater

DIY fan-coil heater by Original Twist

Maybe 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

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 and bigger fan-coil units – i.e. hot water powered puffer heaters.

A kickplate heater can give an occasional guff of hot air and they cost under £200. 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 fan-coil units 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.

cupboards with fan-coil heater built in.

Lets assume that the low cupboard in this sketch is made of typical 720mm x 300mm kitchen carcass units, (just like your top cupboards). A pair of them (each 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 can also do air conditioning, hence the length of plastic guttering under the radiator to catch any condensation. However, a separate mini-split aircon unit (a very cheap heat pump) which delivers hot or cold air could be 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.

UV-C disinfection: For many asthma sufferers this will be the main feature. Fitting  UV-C tubes in the top sections will neutralise airborne bacteria and viruses, colds and Covid included. There are pros and cons about the ozone produced so I’d recommend thorough research before going ahead. UV light can damage your eyes so put kill switches on the doors and make sure there is no direct light escaping from the air inlet. Also, mount the fans underneath the shelves to keep the light off them. For the same reason it makes sense to put the transformer, relays, wiring etc screwed to the underside of one of the shelves.

A pollen filter, such as fitted to many cars, dropped over the low outlet will help too and can be Hoovered clean easily enough.

Controls:  The fans can be switched on automatically when hot water arrives at the radiator. A bi-metallic thermostat switch will do the job simply enough – RS Components do one for about £8. It clicks on at 40C and off again at 25C. The switch can be glued directly onto the radiator near the water inlet. See picture below.

You’ll also need a variable resistor for speed control of the fans. They are 12v so you’ll need a power supply, another cheap PC component.

If you are doing the ‘off-peak energy storage for cheaper heating’ thing the hotter water is available for a boost start which could almost instantly make your bedroom warm for getting up. Fan-coils are best for bedrooms where you need fast response times; under floor heating takes ages to come on and then stays on too long.

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. Turn up the fans to double this.

Noise? Just like putting your PC in a kichen cupboard so practically inaudible.

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, replacement  air then moves across the floor as a cold draught. A fan-coil, on the other hand, wafts 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 directed 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 like a radiator in its own right.

MVHR reheat  The MVHR system is essential in a modern house but as it is not 100% efficient it does cool the house slightly so using one of these fan-coil units as a re-heater is a neat and inexpensive route to perfection. 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 turn the unit on its side so the air is blown from a vertical slot, then go one step further and have a pair of them so you step between them into a hot air blaster, like a giant Dyson hand dryer. Electricity in the bathroom!  No worries there, keep it all 12v.

insulation matters

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 but the message is the same.

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.

Don’t miss ideas on off-peak energy storage; see the Large house heat pump system

If you’ve built this unit please let me know on the comments form below. We’d all love to see pictures.

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Radiators and heat pumps

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Radiators and heat pumps

Heat pump energy delivery can be calculated using just 3 parameters. The flow of water passing through the machine and the inlet and output temperatures – that’s all.  Heat pumps deliver energy at lower temperatures than gas or oil boilers so they need to flow more water to contain and transport that energy. That’s why the pipes coming out of a heat pump are fairly large.  When that high flow is confronted by the slightly smaller pipes of a system designed for radiators it can get bogged down, even more so when the house has zoned heating areas and many parts are shut down. For these reasons the heat pump must be able to modulate the output flow – not all can.

The same 3 parameter calculation – flow and temperature drop – works for the heat output of radiators and for that matter, to heated floors. In typical 15mm OD pipes, leading directly off a radiator for example, flow will be around 7 litres/minute  and, when supplied from a gas or oil boiler, at least 65C on the inlet, dropping to say 55C on the outlet (Dt of 10). The power extracted and delivered to the room in this case would be 4.9kW. So very hot radiators work well.

As flow is constrained by pipe size it is the design and size of the radiator that decides how much energy it can transmit and what the resulting Dt is. Heat transmission is restricted by any sludge on the inside and also the insulating boundary layer of air on the outside, just like the slower water flow near the banks of a river. The rate of loss of heat is proportional to the excess temperature of the surroundings, so clearly hotter radiators work best and also benefit from a stronger updraught which strips the heat off.

If we lower the temperature range to say 45 in, 35 out (typical heat pump) the calculation for power delivery would remain the same (Dt of 10 again) but the temperature drop is harder to achieve as the lower differential to the room cuts the updraught and also the heat transfer. The result can be a disastrous drop in performance – more than half. Your shiny new heat pump might have the nominal power output but the rads just can’t shift it to the rooms.

Assuming you are not able to dig up your floors for under-floor heating the solution is to blow air over the radiator surface to shift that boundary layer and replace it with cool air at room temperature, just like blowing on a hot cup of tea. That’s what fan-coil units do – they are fan assisted radiators and with their lower operating temperature they can make a heat pump system actually work. Alternatively bigger radiators would help, but as you probably already hate the smaller ones you have ,,,,,,,

Reverting back to our power calculation the other parameter is flow, so turning up the heating circulation pump speed is worth a try although pipe size imposes limits on this. Of course increasing pipe size is an often suggested solution and the right one if micro-bore pipes are in use. However my model suggests that the normal 15mm pipes will do as long as the delta T can be reached.

Quite often the main feed will be in 22mm pipe with 15mm take offs to the radiators. So just one pipe feeds all the radiators from your main heat source. This can easily flow about 14 litres/minute which with a 10 degrees Dt produces 9.8kW. The choice of power output of the heat pump is limited by the pipes it serves. Don’t buy a huge expensive heat pump because it won’t make any difference.

Of course if two or more pipes can be fed in parallel from the source then all these flow issues disappear.

Theoretically some heat pumps can deliver 60 degree temperatures but it is a struggle, particularly in very cold weather, and will result in a COP near to 2.5.  Running costs will be worse than gas. Fitting fan-coils will lift the COP to about 3 and give much better performance all round. The order of play should be to get the heat pump installed then start fitting fan-coil units and turning down the set point as you go. Check out here for an inexpensive DIY fan-coil unit  (pictured)

DIY fan-coil unit

At this point you might be dismayed to realise that your house needs more energy than your pipes and radiators can transmit. Don’t despair though. Have a think about installing a mini-split as well. These are independent, air to air heat pump units, quite cheap (under £600) and they can do air conditioning as well as make heat. They make excellent partners with PV panels which apart from running your heat pump in the winter will have masses of surplus power in the summer to run air conditioning.