... the kitchen is already small but they then chose to put the radiator in the middle of one wall, meaning ...
As mentioned, you can get what are effectively fan heaters that fit in the space underneath the cabinets - electric, wet, or combination.
Which is as good a reason as any.
There's been an ongoing "conversation" with SWMBO. I like UFH, and when we looked at the house we've just moved into I commented that without UFH the kitchen floor (tiled concrete slab) is guaranteed to be cold - and also all those floors covered in laminate. "You and your underfloor heating" was the slightly sarcastic response. Not long after we moved in, I got a "not quit a complaint" comment about how cold the kitchen floor is
We got a large rug for the lounge, but that's not really the answer for much of the other space that's got laminate flooring.
I'm unclear as to why electric is so considered to be so poor performing?
Probably because they are frequently mis-specced. I suspect a lot of people just stick them in because it's easy, and then find that it doesn't work very well. If done right, electric can work (almost) as well as wet - the main limitation being the amount of power that it's practical to put into the floor.
One key difference is that wet systems are inherently self limiting - the floor (and any component of it) cannot get above the temperature of the water going in, but at initial startup you can effectively put massive amounts of heat in subject to the capacity of the boiler (typically 30kW or more). With electric, if a "powerful" system is installed, then it's possible for the floor or elements of it to get too hot - especially if controlled off a room stat with no sensing element buried in the floor itself. So that really means you have to limit the power input possible to avoid, or at least mitigate, that risk.
I've got an ideal place to break into the existing feed/return for the CH loop, couple of motorised valves, pump, manifold, controller... I reckon I could do it for £2500... worth investigating.
If you're putting a Radiator/UFH system onto a combi, can you just use the standard CH/HW S-Plan layout and use the HW section for the UFH?
Almost - I had to read that several times before I got what you meant, initially I thought you meant using the HW section of the combi
Fit a motorised valve in the feed to the existing heating run from the room stat(s) and wire it's microswitch to the boiler heating demand terminals. That's half of your S-Plan.
AIUI, the UFH controller is likely to have a dry contact for when it wants heat - you wire this in parallel with the motorised valve microswitch across the boiler demand terminals. If the UFH is not calling for heat, it'll shut it's valves and turn off it's pump - there should be no flow through it, and so you don't need a second motorised valve.
Done this way, the boiler will fire up in heating mode if either :
- The room stat(s) call for heat from the rads, the valve opens, and it's switch is triggered.
- The UFH controller calls for heat - it will run it's own pump, and control the valves for each circuit according to it's own stats.
A few other random thoughts :
The boiler may need a bypass valve. If the rads are off, and the UFH is on very low demand, then there may be insufficient flow for the boiler - it may have been setup with one rad that's never turned off (no TRV) as it's bypass. Some boilers have an internal bypass, many don't.
Depending on various factors, there may be times when the UFH is drawing a low of heat (flow) and it starves the rads of flow. The internal pump of the boiler will be pumping water round, but the UFH is drawing most/all of it and so little/none will flow through the rads. In extreme, it's possible to reverse the flow through the rads if the UFH draws more flow than the boiler pump is pumping. Some system design work is needed to work out if this is likely to be a problem.
Ideally, if the system is only supplying the UFH then you would want to turn down the boiler setpoint so it condenses better. This would need a boiler with a suitable interface (eg OpenTherm), and some integration with the other bits.
I have on and of been thinking about ways to combine UFH and rads - all under the control of electronic TRVs. The schemes I've come up with in my head have mostly been a case of a minimal UFH system per room - just the pump and blending valve with the minimum of plumbing needed - and tee the UFH feed and return into the rad pipe (either upstream or downstream of the rad).
The basic principle being that the TRV controls the water flow, the rad and UFH combine to heat the room, with the UFH being primarily for comfort. The UFH draws water from the hot feed before it gets to the rad, and returns it's return to a second tee between the first tee and the rad. So the UFH gets first bite at the hot water, then it goes through the rad. If the TRV has throttled the flow rate down, then the UFH may well reverse the flow in the section of pipe between the tees (but the rad will still be warm).
Another simpler scheme is to tee in after the rad, and simply pump that water round the UFH without a blending valve. In that case, the system would need to throttle back the flow rate if the rad return temp reaches the maximum permitted for the UFH system. There's less plumbing (no manifold, just a pump, some pipe, and two tees), and the floor will seldom be colder than the room - second worst case is the rad is throttled right back (warm room) and the water is "cold" (room temp) before it leaves the rad, worst case is rad shut down altogether so there's no warm water for the UFH (but that means the room is warm anyway).
Quickly adds up in complexity and cost - but avoids any problems with insufficient UFH output (especially when carpeted) or fighting between two sets of heating with different controls.
