Yes but will need clever design and it may not necessarily work in very cold weather also will be needed on all radiators

I have the same problem...

https://mcscertified.com/wp-content/uploads/2019/08/MCS-021-.pdf

The MCS heat emitter guide gives these factors for running radiators cooler, sorry for the hasty screenshot attached. The left column is the flow temperature, the right column is the oversize factor compared to the "catalogue" rating, which is based on running at 70degC.

So to run at 60degC the rad should be 1.8x as big as its "catalogue" rating.

to run at 40degC the rad should be 4.3x as big as "catalogue".

So for Peter or I to change our rads from 60 to 40degC, they need to be made (4.3/1.8) = 2.4x bigger, or run them for 2.4x as many hours each day. (Which is much the same as what Ed said). This is difficult, especially as the HP also needs time for defrosting and maybe to heat DHW.

To run at 50degC the increase required is only 2.4/1.8 = 1.3x bigger than at present, or 1.3x more hours per day, which may be easier.

There might only be a few weeks each year that full output is required, and the rest of the time they can run cooler. For those weeks supplementary heating might work (got a log burner Peter? Or fan heater if you have amps available)

Or maybe a booster fan on the radiators for only those weeks!

Edit: I used this to estimate the "catalogue" rating of my existing rads http://mcssco.wpengine.com/wp-content/uploads/2019/08/Supplementary-tables-of-heat-emitter-outputs-1.pdf

It's possible to get fans that fit on the tops of the rads and circulate air out at 90 degrees to the radiator face.

Doesn't work like that unfortunately...

Well actually you do require more water flow rate if the deltaT is less, but you also require more surface area of the radiators, and you require more air flowrate through them too.... hence the fan, or the bigger radiators with more fins.

Posted By: owlmanPeter, Have a look at these; I imagine the concept would be better than a fan at the bottom of the rad. There are several other variations on the same theme.

https://www.radfan.co/" rel="nofollow" >https://www.radfan.co/

I sent Radfan an email asking them if a radfan could increase the radiator output to compensate for a lower flow temperature. Their answer was "Don't know" They also said that thy were a small company and could not afford to do the test but given the number of ASHP being installed if a Radfan negated the need to upgrade this would be useful for them to know so could I let them know how it works out. I.e. Could I do the research for them !!

Their website says theyve done testing previously. Given the potential market to avoid rad replacement, its a bit short sighted of them not to be testing at HP temps.

Fans will increase heat output but they will not necessarily increase comfort. With low-temperature emitters you want most of the heat to be radiant. The increased air movement caused by the fan may be experienced as a 'draught'.
Obviously it depends on the building, the occupants and the climate. It might work. If it was me, I would probably try a fan in one room first.

GarethC quoth: "I prefer look of these to the radfans: https://speedcomfort.co.uk/ "

I was curious how they are fitted to the radiators, but on https://speedcomfort.co.uk/optimal-savings-in-3-steps/ I found:

"Once the SpeedComforts are in place, it is important to reduce the central heating system water temperature. Boilers for most installations are normally left at the factory settings (usually 80-85 degrees). The SpeedComforts ensure that sufficient heat enters the room at a lower central heating water temperature.

"The extent of how low the water temperature in the central heating system can be reduced depends partly on the insulation qualities of the house and the quality of the central heating system. Initially, the flow temperature can be lowered to 60 degrees. If this works well, gradually lower the water temperature further."

There's also a graph of power increase by water temperature in the FAQ.

So they explicitly discuss the use case under consideration. They have already done some testing and maybe have got reports from practical use in the field? It's worth asking them first.

Oh, and they set revor a challenge in the where is it made FAQ!

Posted By: WillInAberdeen
So for Peter or I to change our rads from 60 to 40degC, they need to be made (4.3/1.8) = 2.4x bigger

so we need 240% more output. (Doh! Edit: Should have said we need 140% more output)

The speedcomfort FAQ refers to testing that shows 18% more output (at 44degC) which unfortunately is not in the necessary ballpark. So seems like retrofit radiator fans are helpful, but sadly not transformational for enabling ASHPs everywhere, which is presumably why their manufacturers are not shouting about them as a solution to that problem.



Dedicated fan convector rads are available to swap out with standard rads, eg Myson ivector and others, with substantially more air flow and fan power than the retrofit radiator fans can manage.

However they seem to be 7-10x more expensive per kW than basic double-fin radiators, and would need power and make noise.

So if wall space allows it, the cheapest way to add emitter capacity seems to be to fit more and bigger of the basic double-fin radiators, rather than fan-convectors. I'm tending that way for most of our house, swapping old single panel radiators for basic double-fin ones, or additional radiators in certain rooms. They are remarkably cheap, a 2kW radiator is <£100 plus fitting.

Might also retrofit some luxurious UFH using an overlay board system, but that is 20x more expensive per kW than the basic radiators.

We do have a stairwell which doesn't have wall space for more or bigger basic double-fin radiators. Might consider a fan-convector there, or possibly a vertical designer radiator, which is about 5x more expensive per kW than the same thing mounted horizontally

I'm also going to accept a higher ASHP flow temperature than originally planned, the trade-off between better SCoP versus massive radiators seems to be about 50⁰C CH for us on the coldest winter day, giving a SCoP about 3.2-3.5

Posted By: WillInAberdeenhelpful, but sadly not transformational

Yes, I'm sure there will be niche applications where an extra 18% output will make all the difference, compared to fitting a slightly longer or taller radiator. But can't immediately think of any!

Posted By: GarethCHowever, in the situation of using a heat pump, I'd imagine we would increase the flow rate of water. In fact, you'd have to, wouldn't you, for any emitter to be able to increase its output due to forced convection, because the rate at which the water can transfer heat is a function of temperature and flow rate?

It depends on what the temperature drop from flow to return is. If the heat output and the flow rate is the same at the lower temperature with the fan as it was at the higher temperature without the fan then the drop in temperature will be the same. Of course, that'll be proportionally a larger fraction of the flow temperature but, still, it doesn't necessarily follow that you'll need a higher flow rate.

I'd love it to work! Been thinking about it for several months.

1: CH flowrate:

The CH circ pump moves heat from the boiler/ASHP to the radiators, proportional to: the water flowrate, and the temperature *difference* between flow and return. If we keep both those constant, the same amount of heat is moved, irrespective whether flow is at 70 or 40 degC.

However if we double the flow rate, we can halve the temperature difference , eg have the return come back at 65 instead of 60degC, or at 35 instead of 30degC. That helps by increasing the average skin temp of the radiator by ~2.5degC, which might help a little, particularly at lower temperature, UFH works like this.

But 2x flowrate needs bigger pipes, or else 4x pump pressure and so 8x pump power and pipe noise. Also the boiler or ASHP will see a 5degC higher return temperature so poorer efficiency.

So overall it isn't strictly necessary to increase the CH flow rate, but that does have some pros and cons to be traded off.

3. Combinations:


>>> "40% increase in size, plus, say 30% increase in output per unit area due to fan assist, giving 82% increase in output vs. the same sized rad"

Not sure I followed, but if you increased the size by 40%, I don't see the 82% being "Vs the same sized rad"?

Did you mean "Vs a rad of the same length and height"? The fan could certainly help, but for context the capacity of a double, finned, rad is about 100% more than a single plain rad. And buying a 20% taller rad costs literally a few quid more.

So there might be occasions where 20% extra capacity from a clip-on fan makes all the difference between "comfy" and "shivering" and that would be great. I haven't come across any, mostly it seems that I need >100% extra capacity, so then I'm finding I have to bite the bullet and replace the rads, which is not hugely expensive as I described above.

If anyone gets any more info, or does a DIY trial, I'd love to see it!