A couple of points:
700m of underfloor heating pipe for living space (@175mm spacing) is around 120m2 or 6kw of air source.
If you can go to ground source - it is nearly silent - you would need a 5kw heat pump and around 55m of “slinky pipe” externally buried at around 1.5m deep - depending on the ground type - it may need to be a little deeper, rarely more than 2m.
From a sizing perspective ( particularly air source) oversizing causes two problems:
The floor area will heat up quickly - using a lot of power, driving down the COP - once at set temperature, it may have difficultly holding it, without a lot of relatively short off / on cycles.
In really cold weather if the cycles (on / off) are too short, the pump probably won’t defrost properly - leading to progressively reduced performance - this is more likely in really cold weather when the defrost times are longer.
The key to a heat pump for long life and low operating costs is for it to be sized to operate for long periods at a low output. Every time the pump switches off, upon restart it will ramp up to full power. Sometimes an inverter controlled variable speed pump is a preferable solution, nut that is dependent upon the system design - summer hot water demand is an influencer in that respect.
In an earlier post, I cited Dream as being a decent option for a swimming pool - that is because they are fixed speed, virtually always on and you have an enormous heat dump to mask any design inadequacies. If they fail, it is not such a big deal if there is a lengthy wait for parts ( as once happened to me with a customer).
If in any doubt, get a local specialist to design the system for you. You will get a better result and probably save the cost of the design in reduced electricity consumption over the first winter.
Clearly you know heat pumps, many thanks for your comments.
It is at 150mm centres, interesting rule of thumb calculation you did for KW requirement.
I would like ground source but too much work, this has been a renovation that has long overextended so need to be finished. I wish we had a stream nearby, that would be perfect.
UFH does all the tiled areas, hall etc, there are radiators in the bedrooms so I'm looking to have something that gives a residual heat and can top up with radiators if required.
You say to run at low loads, I thought better to have it at max design load?. I would like this running at least 15 hours a day.
You say Dream are a fixed speed, I guess that also accounts for why they are cheaper so maybe it's better if I get a slightly bigger one? I'd value your opinion there as it looks like you have experience with these "Dream" Heat Pumps. They say max outlet temp is 60 degrees. Seems high to me, guessing that must be under absolute ideal conditions. I was going to be happy with 40 degrees as UFH is set at 35 degrees anyway.
I'm not concerned about summer, that requirement will be by solar collectors mainly with some PV panels back up into the Immersions heaters.
Ultimately the PV panels. plus a wind turbine into batteries will run the Heat Pump also so when that happens it will run 24/7 through winter only then shut down for the summer.
Once again many thanks for your comments, very much appreciated.