Here’s my data on this. As stated before, I let a bolus of hot water enter my radiators, completely fill them and then closed the TRV to observe the cooling using an infrared thermometer. The expected curve is simiular to an exponential decay, but not quite. It would be exponential if the heating power of the radiator was just proportional to dT (radiator - room temp), however there’s an exponent on this difference, i.e. Power ~ (T_rad - T_room)^k. k is typically 1.3 for natural convection in Type 33/22 radiators. This means an exponential decay will not nicely fit over the whole temperature range but will fit locally in a short-enough segment.
In any case, I left the fans running to cool the radiator down to 35°C and then turned them off. Looking at the time ranges immediately before and after the switch-off, I can fit exponential decays to these sections and compare the decay rates. This gives me the relative heating power with/without the fans. The example plot is a type 33 radiator, approximately 1 m long with 5 fans. I’m getting a decay constant ratio of 1.85, meaning I output around 85% more heat at 35°C flow temperature. Experiments on other radiators yielded similar results - between 75 and 110 % increased power.
Nice experiment. Another method would be to use the feature in My Heatpump app to estimate emitter size with and without the fans. I reckon my fans increase output by about 1kW.
@ectoplasmosis asked if I could detail my radiator fan setup…
I have 3 different fan solutions:
3x sets of SpeedComfort fans under a very long K2 radiator. These are a little pricey but have a very neat mechanism to fit under any radiator. They claim to double the output, though independent testing showed only 20-25% increase. The threshold of the temperature sensor is set too high, so I use a smart plug instead. Ever so slightly audible when room is silent.
2x 140mm fans underneath a K3, controlled by the temperature relay I linked to earlier. Couldn’t figure out a good way to fit them, so they’re just resting over a couple plastic baskets on top of some planks. Quieter than the SpeedComforts.
A dual PC fan unit with USB connector, plugged into smartplug. Has rubber feet, sits on top of box under radiator. Very cheap, not particularly quiet.
I’ve the same on on rad. My wife has banned me from turning it on ever again !!! However, mounting it on some 3rd party hangar might be a good solution, hmmmmmm…
The fans are just inherently loud,. I’m not sure how much the decoupling will help. They use super crappy low throughput fans - I was surprised how loud they are compared to the flow they can create.
Haha I totally know that feeling. But it’s all a learning process. My father in law isn’t bothered by the noise and will take my SpeedComfort off my hands while I build my own solution.
I like this idea, nice and simple. However, have you thought about how to trigger the smart plugs at each fan NOT using an App? I think it would be good if the circulation pump live could trigger all the smart plugs to switch on. From a brief look around they all seem to expect to be app driven
My second prototype is working nicely. Full control over two channels, temperature sensing and Home Assistant integration via ESPhome. Im quite happy with it! I thought on how to make this available - I’ll put all design docs on GitHub, and if there’s enough interest I could order a batch of populated PCBs through JLCPCB. I’ll do a small design uldate before though to make it more friendly to automated assembly.
I’m also interestet to build these radiator fans, are you ready with your project/testing and do you have you a part list what is needed to build this up incl. with a little giude how to set it up ?
as usual with these projects, there’s always something to improve but I have a fully working setup. I’ll post it even if it’s not fully to my satisfaction yet.
