Flow temperature, radiators and the Heat Emitter Guide

Ask why one heat pump installation costs half as much to run as another in a similar house, and the answer is almost always the same: one produces water at 40 degrees and the other at 55.

A heat pump's efficiency falls as the water temperature it must deliver rises. The relationship is steep. Every degree taken off the flow temperature is worth roughly two to three per cent on efficiency, so the fifteen degrees between those two systems is worth something in the region of a third of the heating bill.

The flow temperature is set by the radiators, not by the heat pump.

How much heat a radiator gives out

A radiator moves heat into a room by two routes: convection, as air rises across its fins, and radiation from its surface. Both depend on how much hotter the radiator is than the room.

The figure that captures this is the mean water to air temperature difference: the average of the flow and return temperatures, minus the room temperature. A radiator running at 75 flow and 65 return in a 20 degree room has a mean water temperature of 70, and a difference of 50. That is the condition British catalogues quote outputs at, written as ΔT50.

Output does not rise in proportion to that difference. It rises with the difference raised to a power of about 1.3, a relationship set out in BS EN 442. The practical effect is that halving the temperature difference costs rather more than half the output.

Run the same radiator in a heat pump system at 45 flow and 40 return in a 21 degree room. The mean water temperature is 42.5, and the difference is 21.5. The radiator now gives out (21.5 ÷ 50)^1.3, which is about 33 per cent, of its catalogue figure. A radiator rated 1,500 W delivers around 500 W.

A radiator sized for boiler temperatures gives about a third of its catalogue output at heat pump temperatures, so the limitation sits in the emitter and not in the machine.

The oversize factor

MCS turns this into a single number in the Heat Emitter Guide, published as section 4 of MGD 007. Take the radiator's rated output at ΔT50 and divide it by the room's design heat loss, and that ratio is the oversize factor.

An oversize factor of 1.0 means the radiator delivers exactly the room's heat loss when run at boiler temperatures, and nothing like it when run cooler. A factor of 2.0 means it delivers twice what the room needs at boiler temperatures, so it can deliver what the room needs at much lower ones. Higher is better, and the arithmetic to convert a factor into a temperature is simply the output law worked backwards.

The stars are the Heat Emitter Guide's shorthand: six is the best, and each star is worth about five degrees of flow temperature. Underfloor heating, with the whole floor as its emitter, sits at the top of the table almost by definition. Existing radiators in an uninsulated room sit at the bottom.

A worked example

MGD 007 works through a room adapted from CIBSE's design guide, and it is worth following because it shows the two levers and how much each is worth.

The room loses 1,671 W. Its radiator is a 1600 × 700 double panel, rated 1,938 W at ΔT50. The oversize factor is 1.2, and the flow temperature needed is above 65 degrees. No stars: this is not a room an ordinary heat pump can heat.

Improve the fabric. Fill the cavity, fit A-rated double glazing, insulate under the floor, draught-proof properly. The room's heat loss falls to 976 W. Same radiator, and the oversize factor is now 2.0. Flow temperature 55 degrees, two stars.

Or change the radiator. Leave the fabric alone and fit a double convector of the same face size, 1600 × 700 × 135 mm deep, rated 3,269 W. The oversize factor is again 2.0, and again the answer is 55 degrees and two stars, so either route reaches the same flow temperature.

Do both. Heat loss 976 W, radiator 3,269 W, oversize factor 3.4. Flow temperature 45 degrees, four stars.

Two points come out of this. The two levers are worth about the same, so a house with limited money to spend can pick whichever is easier. And they compound: doing both is worth far more than twice doing one.

Getting more output without more wall

The example above is the useful one, because the replacement radiator occupies exactly the same wall as the original. It is deeper, not longer. A single panel becomes a double; a double panel becomes a double panel with two sets of convector fins. Output can be nearly doubled inside the same footprint, which is what makes retrofitting a heat pump into an ordinary house possible at all.

Depth is not free. A K3 radiator projects around 155 mm from the wall against 100 for a K2, which matters behind a door or beside a stair. And there is a limit to what convection can do: the innermost fins of a very deep radiator sit in air that has already been warmed by the fins in front of them, so the third panel adds less than the second did.

Other things that help, none of them dramatic on their own: fitting radiators where the air can circulate instead of behind furniture, taking off the boxing that somebody put round them in 1988, and reflective foil behind radiators on external walls.

Underfloor heating

Underfloor heating is a heat pump's natural partner because its emitter area is the entire floor. A screed floor at 30 to 35 degrees delivers what a radiator needs 55 degrees to deliver, and does it evenly.

Retrofitting it is the difficulty. Taking up a ground floor to lay pipe in screed is major work. Low-profile systems that sit on top of an existing floor exist and are much less disruptive, but they raise floor levels by 15 to 25 mm, which brings doors and thresholds into the job.

Many retrofits end up mixed: underfloor downstairs where a floor is being replaced anyway, radiators upstairs. That works, provided both circuits are designed for the same flow temperature. A system running underfloor at 35 and radiators at 50 has to produce 50 and throttle the underfloor down, which throws away the advantage.

The pipework, which is usually forgotten

A heat pump system is designed around a smaller temperature drop between flow and return than a boiler system was, typically 5 degrees against 10 or 20. Delivering the same heat with a smaller drop means moving more water, sometimes twice as much.

Existing pipework may not carry it. Microbore, at 8 or 10 mm, is the usual constraint, and 15 mm feeding a large radiator can be marginal. The symptom is a radiator that never gets properly warm at its far end however the valves are set.

A good survey looks at pipe sizes as well as radiator sizes. The Heat Emitter Guide is honest about its own limits here: it compares a radiator to a room and says nothing about whether the pipes can supply it.

What this means for a quotation

The design flow temperature is the most consequential number in a heat pump quotation, and it should be stated plainly. A quotation that does not mention it has left the most important decision unrecorded.

Ask which room sets it, and what it would cost to change that room's radiator. The answer is often that one or two radiators are holding the whole house five degrees hotter than it needs to be, and that changing them costs a few hundred pounds against a running cost saving that recovers it in a couple of winters.