Air-Source Heat Pumps (ASHP) and radiator replacement: How to work out the optimum radiator size for maximum efficiency?

@Tim_OVO I think you’ve got the jist of it though hot water to the central heating circuit is not supplied from storage in our cases (that would be a better solution as described by transparent but requires space for a thermal store), the heat comes straight from the pump via some valves. 
 

I had four rads changed as part of my work, though not the ones suggested by Northern Gas, others were changed based on my calculations because the NG ones made no sense (the utility room would have had the biggest rad in the house if they had their way!! 😂). There are radiator size calculators online but they tend to be quite basic. You need to put in dimensions of the room, the wall, ceiling and floor materials/u-values (how well insulated they are), in which part of the house the room is and which way the face. Both NG and Reina group did these calcs and I’ve now seen the paperwork, they were actually very similar so something was lost between the designer of my system and the person who bought all the kit. The rads changed were all changed from single emitter to doubles. 

The radiators are then sized to produce an output for the room. For Gas systems they typically work at 60-70C+ so they can use a smaller output rad to achieve the same level of heat input to the room. To work out radiator sizing you need to understand the deltaT which is the difference in temperature between the required room temp and the system output temp. So 20C in room, 70C from the boiler is a deltaT of 50C. Heat pumps work at lower temperatures so a lower deltaT is used. From my paperwork it looks like a design flow temperature of 50C has been used, a deltaT of 30C. 

Its a fairly involved set of calculations to do for each room which is why the installer would carry out a survey. On traditional systems plumbers have got used to what size works and experienced plumbers would be able to estimate what’s needed or just oversize the rad and let the thermostatic valves do the work because boilers are designed to stop/start regularly, whereas heat pumps work best when they can run continuously in tick over so sizing the rads correctly for a heat pump will bring more benefits. 
Ive bounced around a bit here but hopefully that makes sense. 

Some really great input in terms of what is factored in when calculating required radiator sizing. 

From room size, to levels of insulation and even room facing (I’m guessing southerly facing rooms will generally require less heating/smaller radiators as brilliantly demonstrated by @Transparent’s heating capturing southerly facing glass roof.

@hydrosam makes some really Important points on the difference between these calculations for heat pumps as opposed to gas boilers. Seems like the importance of getting them right increases with a heat pump, to allow the correct temp to be supplied without needing to power on and off like a gas boiler might do.

 I’m guessing underfloor heating would be even more effective in allowing for a more constant power demand. Anyone got underfloor heating already installed or considered getting this fitted post-install?

I would love to get under floor heating and am looking at retrofit panels which can be laid on top of an existing concrete floor. Much disturbance though so will give radiators a good try first.

@Jess_OVOwrote:

Seems like the importance of getting them right increases with a heat pump

Ah… I wonder if that’s why the radiator manufacturers are wary of publishing online calculators for houses with Heat pumps?

There are plenty of radiator-sizing calculators available for houses with a standard boiler pushing out 70°C, such as this one from Stelrad. But it’s noticeable that they don’t allow you to change the water temperature, only the characteristics of the room.

That suggests to me there less margins for error with the lower temperatures supplied from a Heat pump.

Choosing to install underfloor heating removes these constraints of course. The radiating surface is massive in comparison to a wall-hung radiator.

Some great insight into radiator changes needed before a heat pump installation. Check out our summary of the advice our members gave below.

A good tip on managing heating with a heat pump:

Some call outs here to underfloor heating as a good option with heat pumps:

And a top tip online calculator which can help work things out:

For those of you who want to try the heat calculations for yourself, the MCS Heat Loss Calculator is available on the mcscertified website, along with a user guide. There are also a couple of webinars which explain how to use the various sections to produce radiator sizing for a heat pump.

In the Design Details tab you can specify the construction of the floor, windows, door, external and internal walls, ceiling and roof for each floor of the property. You then input details for each room specifying which floor it is located on, its dimensions and area of external wall and windows etc. along with the size of internal walls and the adjoining room temperatures.

The Heat Pump Rad sizing tab then lists each room in turn and gives the calculated heat loss in Watts. You can enter the size of existing radiators and it will give the output of the radiator at a delta T of 50 degrees C (for gas boilers) and also for the chosen delta T for your heat pump. It uses correction factors supplied by Stelrad:

So if you wanted a room temperature of 21 degrees C and the mean water temp was 51 degrees C, the radiators would have to be twice as powerful as those for a gas boiler.

 

Non-heat pump owner here, so this is more like a comment/question than an answer. Worse than that, it’s a comment from an amateur/armchair heating engineer rather than a practising one.

(And a request to be educated by those who know better!)

I looked at the heat being put out by our radiators last winter, and decided that they only ever come close to being used at their rated output when a room has been allowed to get cold and needs unexpectedly to be brought to a comfortable temperature, and sooner rather than later.

Provided that situation can be avoided I estimate that, even on the day when we burned the most gas last winter, the radiators themselves only needed to be at around 40 - 45 degC, to maintain a comfortable temperature (around 20 degC). (That wasn’t on quite the coldest day, but there was a strong wind and, in our house, that makes an important difference.)

That’s a deltaT around 25 (rather than the 50 used in standard tables), and radiator output is reduced to about 40% of nominal (output is roughly proportional to the 1.3 power of deltaT, and (25/50)^1.3 ~ 0.4).

This is not inconsistent with the fact that our rather old boiler probably puts out 70% of the 36 kW rate at which it burns gas (when it’s running) as useful heat, i.e. 25 kW: the sum of the nominal outputs of all the radiators on the heating circuit is around 22 kW. This is not very different from 25 kW, and I am encouraged that the system has been reasonably well designed.

On average, on that day when we burned the most gas (300 kWh in 24 hours is an average rate of 12.5 kW), and assuming a 70% efficiency, our demand for useful heat would have been 70% of 12.5 kW, i.e. just under 9 kW. I believe (based on pipe sizes and how long it takes heat to travel from the boiler to the furthest point in the circuit) that, when the CH pump is running, water flows in the heating circuit at a rate of 0.3 kg/s. If the useful heat is 9 kW, that would correspond to a DeltaT at the boiler (this is the amount by which the return flow is warmed up by passing once through the heat exchanger in the boiler, and completely unrelated to the other deltaT, introduced above) which is close to 7.2 degC.

This is a lot less than the “conventional” difference between flow and return temperatures, that one can read about, for a gas boiler, which is 20 degC. I am sure that what happens in practice is the boiler only burns gas for about 30% of the time, but the pump continues to circulate water around the heating circuit all the time. This is effectively what is called pulse width modulation. It does modulate its output after all, albeit in an unintelligent way.

My impression is that heat pump owners must modify their expectations. No longer can they “turn the heating up” and expect an immediate warming. Getting more heat energy in the same time is an increase in demand for heating power and, with a heat pump, that limit is much closer to normal operation than with a gas boiler. Instead, one has to travel back in time and turn the heating on earlier, so that the heating power can remain the same but the heat energy is increased by having that power in effect for longer.

Control systems can do this kind of thing, but instead of travelling back in time they use forecast weather conditions to change the way they operate. I have read that at least some heat pumps can modulate their output, but I suppose that modulation isn’t as easy as in a gas boiler.

I would love to know whether heat pump owners are happy: does the control system do what they need?

Simon

PS Interested though I am in the possibility of replacing that ancient gas boiler with a heat pump, the scale and effectiveness of possible improvements in insulation, reduction in air changes per hour, etc is far from clear. So, in the meantime, later this month the horrendously inefficient gas boiler will be replaced by a more efficient one. It’s a cop out, I know. But in a few years time? Maybe by then I’ll know enough and have done enough to understand whether a heat pump can do the job.

For those of you who want to try the heat calculations for yourself, the MCS Heat Loss Calculator is available on the mcscertified website, along with a user guide. There are also a couple of webinars which explain how to use the various sections to produce radiator sizing for a heat pump.

In the Design Details tab you can specify the construction of the floor, windows, door, external and internal walls, ceiling and roof for each floor of the property. You then input details for each room specifying which floor it is located on, its dimensions and area of external wall and windows etc. along with the size of internal walls and the adjoining room temperatures.

The Heat Pump Rad sizing tab then lists each room in turn and gives the calculated heat loss in Watts. You can enter the size of existing radiators and it will give the output of the radiator at a delta T of 50 degrees C (for gas boilers) and also for the chosen delta T for your heat pump. It uses correction factors supplied by Stelrad:

So if you wanted a room temperature of 21 degrees C and the mean water temp was 51 degrees C, the radiators would have to be twice as powerful as those for a gas boiler.

This is interesting:

So if you are looking at getting the most out of your ASHP by running it at lower flow temps than originally envisaged, you can cross ref your room by room heat loss calcs to see if you radiators are up to the job of delivering the desired room temp.

This IS interesting, another self serving resource. Albeit potentially a bit tricky to get one’s head around the numbers. But maybe that’s me, I’m better at letters than numbers. 

These methods we’ve got on this thread, at the very least will be helpful for someone to cross reference with what the installer has recommended, in case there’s any doubts! 

Have any trialists ran these calculations at home to see if what they’ve got fitted are within the right range? 

@Tim_OVO i did this weekend, and this confirmed that because we run ourhome at 19 degrees and not 21 I can turn down the heat pump temperature to 45oC or less and should still be comfortable on the coldest of days. The lower the pump temp the better the COP. I will certainly be giving it a try.