Simple control strategy…

[HughF]

I’m thinking about a simple control strategy for the ufh in my extension. It’s only a 15m3 area, 2 loops of 49m each in slab.

 

Rads in the rest of the house, single zone, trv heads in the upstairs rooms only, full weather comp. 
 

Could it be as simple as powering the ufh mixer from the ‘heating valve’ 230v output from the ashp?

 

I’ve brought both the hw/3port live and heating/cooling valve live up into the airing cupboard in a length of armoured.

 

I have a wireless room stat that will be hooked to the volt-free call-for-heat.

 

Do I need to consider a slab sensor or other, or can I just run this simple, low and slow and drive it off the heating zone valve feed? It’s a relay output, on a 6A breaker in the ashp.

[ProDave]

Your problem is going to be the fact the UFH will take a LOT longer to heat up than the radiators.  So if you just connect it and treat it as another radiator, don't expect the extension to be warm until some time later than the rest of the rooms, and then probably stay warm after the other rooms have cooled down.

 

I would have a 2 port valve feeding the radiators and another 2 port valve feeding the UFH and then a time clock so the UFH could come on earlier.  But you can't really do that just with the ASHP's controller.  That is why I don't bother with using the ASHP's controller as a timer and just treat it as somewhere to set and look at parameters, and my timing is done with a 3 channel central heating time clock.  Two heating zones with separate times and DHW on the third.

[HughF]

Hmm, lots to consider. I’ll have a good think.

[JohnMo]

If I recall you have a thick slab, so heating times could be reasonably long, i.e. most of the day at a low temp. Or you could run a slightly hotter and batch charge. If you had E7 or time of use tariff just charge during that period. All you need a 0.1 hysterisis thermostat.

 

My UFH is in 100mm concrete, and everything takes 6 to 12 hours (heating and cooling down again).

 

Problems you may have is short cycling if you try to run it independently of the rest of the heating system. Think you are the same as me, no buffer.

 

The very simplistic approach with least risk of short cycling is to run it all together UFH and rads as a fully balanced system.

 

If your wireless thermostat has a hysterisis of 0.2 or above I would get something better other you will get big overshoot or undershoot of UFH room temp.

 

Or could you run the radiators on WC, with night setback, so the bigger part of the system is on most/all the time at lowest flow temp, and just have the UFH doing its own thing on a 0.1 deg hysterisis thermostat driving mixer pump.

[HughF]

Arghhh….. decisions decisions…. I’m now thinking of putting a spare radiator that I have left over from the order (it wouldn’t work in the kitchen) on the wall in the extension and running just on rads. 
 

The time lag between the two systems being troublesome, along with the wiring and controls issue. Rads are easy, pump in the ashp just drive’s everything based on WC and a programmable stat. My wife will understand rads, stat comes on, rads get warm.

[HughF]

On 29/09/2023 at 13:04, JohnMo said:

If I recall you have a thick slab, so heating times could be reasonably long, i.e. most of the day at a low temp. Or you could run a slightly hotter and batch charge. If you had E7 or time of use tariff just charge during that period. All you need a 0.1 hysterisis thermostat.

 

My UFH is in 100mm concrete, and everything takes 6 to 12 hours (heating and cooling down again).

 

Problems you may have is short cycling if you try to run it independently of the rest of the heating system. Think you are the same as me, no buffer.

 

The very simplistic approach with least risk of short cycling is to run it all together UFH and rads as a fully balanced system.

 

If your wireless thermostat has a hysterisis of 0.2 or above I would get something better other you will get big overshoot or undershoot of UFH room temp.

 

Or could you run the radiators on WC, with night setback, so the bigger part of the system is on most/all the time at lowest flow temp, and just have the UFH doing its own thing on a 0.1 deg hysterisis thermostat driving mixer pump.

Ok, this seems a reasonable strategy as I’ve already plumbed up the system and didn’t think to add any zone valves in the already cramped airing cupboard.

 

I’m thinking ufh stat with a slab sensor, driving the mixer pump and a relay to short out the call for heat. Then the rads will come on with the ufh, but not the other way round (because of the lack of a zone valve on the heating circuit)… set the blending valve to a sensible low figure and just let it slowly charge up.

 

I’m planning on going to octopus on their HP tariff.