I have a 5y old myson one, I think it may have a 45C cutoff, that could perhaps have changed with newer units. they have a summer/winter mode switch. In summer mode, the fan always is on if there's power to the unit... the thermistor state is irrelevant. So you'd put it in that mode and then switch the power some other way. linked to your heat pump's on/off state (output relay if it has one), linked to a room stat, linked to a DIY'd flow temp sensor to relay device of your own... various options exist.  They are pretty low-tech so if you wanted to keep the summer mode option separate, then finding the thermistor-to-relay wiring and re-routing it wouldn't be hard. I don't have a heat pump yet but I'll be doing one of the above when I do.

or use the thermix one without the thermistor and link it to power switching as above.

or you might be able to ask them to make the thermistor with user settable temp?

what I do wish is that these companies made significantly wider ones (1000mm plus) so as to get the higher output across the room with lower temps, without having to go full on UFH. Generally in a a large kitchen where you'd need that higher output you can find somewhere for a wider unit (I can). I'd really like to have a couple of 1000 or 1200 wide units rated at 4kw (at CH temps) thus 2kw (ish) at lower temps. I've been contemplating buying several metres of finned copper tube and mounting it behind multiple kickspace grills, with an array of PC fans behind it and a fan controller, but not got as far as actually committing to do.

Ian

You may find something suitable on this website.

https://heatingforce.co.uk/blog/hydronic-plinth-heater/

Posted by: @derek-m

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You may find something suitable on this website.

Indeed I think I found out about hydronic plinth heaters from a website like this!

Posted by: @iancalderbank

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So you'd put it in that mode and then switch the power some other way. linked to your heat pump's on/off state (output relay if it has one)

I was hoping to avoid something "held together by tape" at this stage.

I have noticed that Cool Energy have water fan coils which may be suitable. The vertical spacing requirement (1.9m above the floor) is a bit of a problem in a room only 2.2m tall! Also I am confused about how the central heating system/heat pump controller can signal for it to turn on, since their CE-iVT9 heat pump manual is light on details. Plus the price point is rather high, at £450 for the smallest unit. The larger CE-HW800 might get @iancalderbank close to what you want for your kitchen (up to 3.8kW given a flow temperature of 50), although it is £525 and requires wall space.

I discovered the following website which looks promising for boosting radiator output.

http://openecohomes.org/diy-fanned-radiator/

along with this youtube video.

As a counterpoint, this youtuber created his own fan unit (controlled with an arduino nano, of course) but reported little difference in output; I think pushing air through the radiator would have been more effective, and the DIY-er in the first website takes care to ensure an even distribution of air through the middle of the radiator.

thats exactly what I've done already. This pushes air through the center of a slightly undersized radiator and makes a noticeable difference to its output. Its not as tidy as the one in that blog but same principles

its 9x artic F8 PWM PC fans. they are slightly too deep for a P+ but work fine. They are fixed together with cable ties, magnets bolted through the holes, the magnets hold it to the bottom of the radiator. I have bunch of F9 fans that I'm planning to fit to a K2, should fit perfectly.

Its controlled by an ESP32 with a relay board for 12v fan power on/off and a 5v PWM output to slow them down. There's a temperature probe taped to the back of the radiator, the controller only switches on when the radiator has warm enough water. and pushes the fans a bit harder when the water gets hotter, to get the heat out more quickly. with PWM fans the noise level is not noticeable.

my thought for a long and wide kitchen design was based on this - I know how to do the fans bit, and the W2A heat exchanger bit should be do-able with finned tube. But may well stick to commercial plinth units.

@chickenbig The Reverso (italian supplier) fan coils that Cool Energy stock are nice units. They are available in wall, ceiling or floor mount.

The CE-iVT heat pump just has a 'call for heat' input (which is jumpered together from the factory) so you could use a thermostat to switch a zone valve, the aux contacts in the zone valve would jumper out the call for heat input and fire the heat pump.