Rad Fans, do they offer any energy savings

roseandrog48

Recently been reading ads re Rad Fans, they appear to use very little electricity ie 2 or 4 watts dependent on size, but do they really improve the effectiveness of the water filled rads?

tacpot12

They do have some effect, but radiator's output is limited by its size and the temperature of the hot water in it. Blowing air through the rad would basically cause the temperature of the water to cool down more quickly, and the air temperature to rise more quickly, but if the room is at the right temperature and the TRV (assuming you have these fitted) as turned the radiator down then the fan is just wasting electricity. (A thermostatically controlled fan would be better - I would not bother  running the fan unless the room, say, 5 degrees below the set point of the TRVs.) 

Also if the thermostat in the house has the boiler switched off, running the fans is again just wasting electricity. Radiators move a lot of air just using convection, they don't really need a fan to assist them. I would save my money.     

Effician

Don't have central heating or radiators but do have a wood stove & try to make the best use of fans as i can.

The rad fans serve a very similar function to stove top fans as in they get the hot air away from the heat source, i would class this as a first step as often they don't have enough draw or projection of air to prevent hot air heading to the ceiling within a few feet of the fan. You could check this by reading the air temp in the centre of the room 1' from the floor & 6" from the ceiling.

If you have the ceiling height & no room vented gas/open fires etc  a ceiling fan on winter setting makes for a far more evenly temperatured room.

Reed_Richards

A radiator fan will give you a bit of forced convention instead of just the natural convection (and a bit of radiation) from the radiator.  So I expect it would increase the heat output from the radiator, as would replacing the radiator with one having a larger surface area.  Thus the fan might be useful in a room with a small radiator that you can never quite get warm enough.  But the only circumstance in which it would save you money is if it stops you turning on an electric heater to supplement the radiator.

By "improving the effectiveness" of the radiator you are making that radiator more costly to run, unless it has a TRV in which case it shouldn't make much difference.  

matelodave

As said abve, they wont save you money but, as they move the warm air about a bit faster than natural convection, they may make the heating feel more effective but they can't increase the heat output of the radiator.

However that said, if you feel warmer because the warm air is stirred around a bit, you migh be able turn your room stat down a degree or so which could possibly save a few bob

Reed_Richards

matelodave said:

... they can't increase the heat output of the radiator.

I think they probably can if they are independently powered; it's forced convection vs. natural convection and forced wins.  There is a class of radiator that incorporates an electric fan which is used where there isn't room for an ordinary radiator.  "Hydronic plinth heaters" are examples of such.  The heat comes from your central heating but the fan increases the heat output. 

matelodave

A hydronic heater is really only a fan heater with an air-water heat exchanger instead of an element. If the incoming water temperature remains the same and the flow rate of the water remains the same then the same amount of heat is delivered to the heat exchanger.

Blowing air over it might increase the speed t which the heat is dissipated but it can't increase the amount of heat that is delivered although if you increase the temperature drop across the heat exchanger the return temp to the boiler will be reduced making the boiler work harder to maintain the flow temperature. If you increase the temp drop across the exchanger, say from 20 degree to 30degrees, the the delta T to the room temp will drop by 5 dgrees

As it happens I fitted a Myson hydronic fan convector to a previous dwelling. The benefit being that we could fit it into a space where a conventional radiator woudn't fit but the heat output was only a product of the delta T of the heat exchanger to the room temperature. The fan just moved it around. If we'd wanted more heat then we'd have had to fit a bigger unit. It did have the advantage that the fan was controlled by a thermostat so it

[Deleted User]

We have a plinth heater and it is pretty redundant without the fan. No real convection route.

We have used a fan on a rad during beast from the east when an obscured rad needed a bit of help. It did work but normally not required.

Edit. The fan we used is a few quid desk top at a few watts.
The Plinth martix is tiny compared to the rad it replaced and 18w motor (less I think) but recently had 7 rads replaced and they are better specced than the old ones fitted when the house was new and by eck do they lob out more heat.

Reed_Richards

Take a coil of copper pipe.  Pump water through it at a constant flow rate.  Now add a second coil and maintain the flow rate.  You have doubled the surface area so the coil "radiator" will give out twice the heat.  Same incoming water temperature, same flow rate (but the difference between incoming and outgoing water temperature will now be about twice what it was for the single coil).

Take a coil of copper pipe.  Pump water through it at a constant rate.  Now use a fan to blow air over that coil.  My contention is that it will lose more heat so the outgoing water temperature will drop.  If I am correct, forcing air over a radiator will boost its heat output in the same way that adding extra surface area boost the heat output.  Forced convection is why you feel cool if you stand in front of a fan.

QrizB

Reed_Richards said:

Take a coil of copper pipe.  Pump water through it at a constant flow rate.  Now add a second coil and maintain the flow rate.  You have doubled the surface area so the coil "radiator" will give out twice the heat.

This only holds if the temperature of the water doesn't drop significantly. For a room temp of around 20C, if the water entering the first coil is at 60C and the water exiting it is still at 58C (delta-T of 2C), then the second coil will produce almost as much heat as the first (another delta-T of 2C) and you've almost doubled the output. However if the water exiting the first coil is only at 30C (delta-T of 30C) then the second coil will produce much less heat then the first (since the maximum possible delta-T is 10C; to achieve a further delta-T of 30C would need the water to exit at 0C which is colder than the room).

In the extreme case, if the water exits the first coil at 20C you will get not benefit whatsoever from the second.

Reed_Richards said:

Take a coil of copper pipe.  Pump water through it at a constant rate.  Now use a fan to blow air over that coil.  My contention is that it will lose more heat so the outgoing water temperature will drop.

Again, this is only true if the water would otherwise exit the coil at a relatively high temperature.

You mentioned in a different thread that it's not unusual for a house to have undersized radiators compared to the power of the boiler. In this case fans could help make use of this "extra" heat and warm the room faster from cold, but once the room is warm the fans will not generally be beneficial.

Reed_Richards

I was trying to provide a simple thought experiment to prove that fans work; I should have said that I was assuming the water was hot by comparison with the room temperature and you are right to point out that my thought experiment unravels if that isn't true.

Perhaps It is easier to assert that if a fan can cool you then you must be heating your surroundings faster than your would otherwise do.  And it you accept this then it's not too much of a leap to accept that the same thing holds true for a radiator.