Posted by: @enc

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Will UFH be cheaper to run than the rads we currently have if all designed and insulated correctly

Generally no, because it increases the downward heat losses - you dT between floor and ground is higher.

Posted by: @enc

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Does the UFH have to be zoned separately to the rest of the house?

Not necessary to zone, but to make it run in unison with the upstairs radiators, would need to have a variable mixer valve to allow flow temp modulation and be run on a separate WC controller, unless you HP has this functionality built-in.

Do you have enough floor depth to add plenty of insulation? Ideals are 150mm of PIR or more, otherwise downwards heat loss with be quite large and way more expensive to run than radiators.

Have you thought about fan coils, these could be a straight replacement for radiators - generally much smaller than the equivalent radiator, variable output, so could easily be added to existing system and run as a single zone with upstairs.

I have all of my upstairs radiators and downstairs UFH on one zone , and all of the flow pumped from the ASHP (no separate circulation pump for UFH). I was a bit worried about this at first but it works very well once balanced between upstairs and downstairs (that took about a week to do, just contestant tweaking). I’m achieving very good cops. 
there is a huge comfort factor in UFH which is just so superior. Plus the flow temp can be lower.. In my house octopus initially wanted to change 4-6 rads upstairs but I got them comfortable on my old ones . Turns out they are plenty big even at 32-38c. The UFH pretty much heats the whole house , upstairs rads are off most of the time, most days 5c + outside.

The manifold is fed by a warm water pipe and has a return just like a radiator would have (I think). Perhaps bigger diameter than standard probably 22mm, you may want to check if that works without additional pipe work, I’m not sure about that.

good luck!

Posted by: @enc

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We are getting our downstairs renovated and thinking of taking the opportunity to upgrade to underfloor heating.

What type of floor structure will this be?

I have successfully undertaken "pipe in concrete pad"

and pipe with heat-spreader plates between the joists of a suspended floor

These have different characteristics.

A concrete pad takes hours to come up to temperature, but retains heat for longer.
It's a good strategy for storing heat derived from solar during the day.

The suspended floor approach can raise room temperature within 20mins,
but I store the heat elsewhere (in a thermal-store cylinder).

Either approach can work well with tiles or an engineered-wood floor...
... but not carpets of course, because they prevent radiation from the surface.

We have ufh in a small downstairs extension and it is far superior to the radiators elsewhere. It’s a small area so a simple thermostat mixer dropped to ~25C from the radiator main circuit running at ~35C  It’s not perfect in terms of efficiency because there’s no WC in this small area.

However if you have UFH on the whole of your ground floor and heat rises so you might nearly heat the whole house with it. Do the rooms have doors which you close? If you don’t allow the heat to rise by internal air flow then it obviously it won’t heat upstairs.

When was your house built? It will determine how much insulation you have under your downstairs floor from the building regs at the time, and possibly whether you have air leakage that takes the heat away before it gets to your rooms.

If you can do ufh I certainly would it’s lovely to walk on walk floors and it’s much more space efficient.