Are you having the floors up anyway? We considered adding underfloor during a kitchen refurb but chickened out because we weren't sure what state the floor was in... and once the old units were out, that seems to have been the correct choice! We knew the kitchen had been expanded by taking the old utility and part of the garage. What we didn't know was this was done in two stages, leaving the floor as at least three slabs, one of which has been poured on top of the old utility floor, tiles still in place, now as a sandwich.

I would get someone who'd added an underfloor zone to a Mitsubishi system before, if possible, but not necessarily the original installers as long as they left you all the design and commissioning paperwork.

This is only my opinion and experience  so beware.

We did exactly what you are proposing refitted kitchen turned into entire downstairs floor removal and UFH throughout only the retrofit type didn’t dig up floors.  This was installed with an 8.5kw ecodan. Still have rads upstairs.

Works perfectly fine but there are things I know now that I didnt at the time for the ufh.  If you want to achieve low flow temps then pipe centres should be as close as possible if you look at heat geeks they tend to be 100mm centres where as most boiler fired installs would be wider.  Mine are 150mm so somewhere in between.

what flow temps are you running your rads at?  Ufh doesn’t want me to more than 45C on tiles and less on wood and that will be the max the rads will see so is that enough to heat upstairs?  You could cool down the ufh water but that defeats the point.

You don’t need zones or thermostats or wiring centres or actuators for the ufh.  Just one big loop. It just needs to pump to turn on when your main thermostat comes on.  I had all this and don’t use any of it.

Even though ours doesn’t have a big slab to heat up It works really well at storing heat from overnight without making upstairs hot due to lower flow temps and in the recent weather it won’t come on again till the following night so can be run for 9 months a year on off peak rate.  Oh and you get cool floors in the summer if it’s really hot

This comment is from practical experience, rather than from being being a trained professional:

The relative length of each UFH pipe loop does matter.

I'm using a very common 16mm OD UHF pipe. It has 2mm wall section, and 12mm bore.
It's a PEX style, with aluminium sandwiched between two layers of polyethylene.
As such it will stay where I put it, but I took care to ensure it didn't kink when turning it around corners.

The longest run is almost 100m, and the shortest is about 30m.

At present the two UFH manifolds draw their water from a thermal store via a dedicated (modulating) pump.

If the shortest loop is open by the valve on the manifold, then it accounts for almost half of the available water-flow!

Water is only going to be running at an acceptable rate through the longer loops when the shorter ones are closed off.

So in your case, with no thermostats and control valves, you'll need to ensure those loops are very close in terms of length.

Posted by: @gary

↑

This is only my opinion and experience  so beware.

We did exactly what you are proposing refitted kitchen turned into entire downstairs floor removal and UFH throughout only the retrofit type didn’t dig up floors.  This was installed with an 8.5kw ecodan. Still have rads upstairs.

Works perfectly fine but there are things I know now that I didnt at the time for the ufh.  If you want to achieve low flow temps then pipe centres should be as close as possible if you look at heat geeks they tend to be 100mm centres where as most boiler fired installs would be wider.  Mine are 150mm so somewhere in between.

what flow temps are you running your rads at?  Ufh doesn’t want me to more than 45C on tiles and less on wood and that will be the max the rads will see so is that enough to heat upstairs?  You could cool down the ufh water but that defeats the point.

You don’t need zones or thermostats or wiring centres or actuators for the ufh.  Just one big loop. It just needs to pump to turn on when your main thermostat comes on.  I had all this and don’t use any of it.

Even though ours doesn’t have a big slab to heat up It works really well at storing heat from overnight without making upstairs hot due to lower flow temps and in the recent weather it won’t come on again till the following night so can be run for 9 months a year on off peak rate.  Oh and you get cool floors in the summer if it’s really hot

@gary Thanks a lot, that's incredibly helpful! Will check out the Heat Geek stuff for sure, but it's great to know your actual experience in a real install.

45C is more than enough for the rads upstairs for the majority of the winter, but when the temps get down to -5C they'd need to be at 50C. However having said that, we plan on replacing the rads upstairs to be higher output K3 rads before the winter.

Your welcome.

To clarify what I meant by 1 big loop was that all the loops for the different areas should be open at the same time not that it should be one set of ufh pipes.

Video on UFH for low temp systems

We’ve just had ufh in our (not yet finished) extension but the rest of the house has radiators (and will stay that way). We really had to insist that we had 100mm spacing on our pipe work because the plumber insisted 150-200mm was plenty. The ufh designer that the plumber used had installation guidelines that said 150mm was plenty for low temp systems. So we insisted and paid more because it was more pipe work. Obviously the plumber ignored the installation instructions “there’s no need for that” so only we the customer read them.

Despite this it works fine on our gas heating system running at 60C. The thermostatic mixer holds it down to 30C quite nicely, and the secondary pump runs when there is a call for heat. What I was surprised to find as we’re moving onto flooring is that vinyl tiles must never see more than 29C. Yet they say like so many floor coverings suitable for ufh but the devil is in the details. Never assume with anything. So even without the ashp yet we’re glad we’ve got the extra pipes in or we might had had to change our other choices.

Whether we can run the ufh open with the radiators I’m not sure, it will all need some very careful balancing. I’ve balanced the ufh flows (plumber just set full) with the flow controllers as per the installation instructions, but we might need to keep the mixer or something with the ashp. Or it might even be the evil LLH for the ufh?

@judith that's really great advice -- we'll be sure to check the installation guidelines for the UFH and the specs of the flooring to make sure it's suitable. I think 29C would be too low for us...!