Radiators and heat pumps

Radiators and heat pumps

Heat pump energy delivery can be calculated using just 3 parameters. The flow of water passing through 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 applies to radiators and for that matter, to heated floors. In typical 15mm OD pipes, leading directly off a radiator, flow will be around 7 litres/minute  and, from a gas 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.

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, like the slower 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 stripping heat off for the benefit of the room.

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.

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.

Ultimately just one pipe feeds all the radiators from your source and it might well be typical 22mm copper tube with an I.D. of 20mm. 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.

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.  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 and the RHI grant established 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.

DIY fan-coil unit


At this point you might be dismayed to realise that your house needs more energy than your pipes can transmit. Don’t despair though. Have a think about installing a mini-split as well. These are independent units, quite cheap 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.

DIY fan-coil heater

DIY fan-coil heater

DIY fan-coil heater

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.

cupboards with fan-coil heater

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 (£48.14 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 channeling much warmer air from ceiling height is worth a thought.

When paired with a suitable heat pump this unit also does air conditioning hence the length of plastic guttering under the radiator to catch any condensation. N.B. The Renewable Heat 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.

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. 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.

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.

You might wonder why all this effort gives you anything better than the 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 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 in the room and gives a faster and more comfortable heating experience.

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 if not better. A ground source heat pump performs a bit better – similar to gas.




I guess you might be in a state of shock now. Thousands saved on your heating strategy, the government is going to 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.