There are lots of loopcad layouts here on BH with room-by-room loops. Figure heat loss per room etc...

Now supposing I have PH-level insulation and a thickish slab - can I just slather my slab below the screed with a regular loop design, ignoring the individual rooms? My thinking is that concrete/screed is a good thermal conductor and, for that reason, I won't really be able to control the temperature in individual rooms anyhoo. The screed will pretty much all be at the same temperature, and the heat output to the individual rooms will be governed by that temperature, whatever I try to do?

p.s. my place has 3 rectangular slabs roughly the same size.

Edited 2 hours ago2 hr by Alan Ambrose

I believe for 16mm pipe, loops should not exceed 100m length.

About 16 sqm ? - have I got that right for 150mm spacing ?

I think you'd struggle to pump through a much longer small diameter pipe and even if you could, most of the heat would be dumped into your screed in the earlier part of the run so heating would be very uneven.

I agree you can ignore zoning exactly to individual rooms and not much point trying to control their temperature separately, but you'll still need a manifold and ideally similar length loops to even out the heat distribution and keep flow through the pipes easier.

Many of us with passive-type slabs treat the whole thing as one zone.

That said, there could be a benefit to at least trying to keep each loop to one or a couple of rooms with similar heating needs. That way, you can tweak per-room heating by adjusting flow rates at the manifold. You'd do that entirely passively - tweak the flow rates when you first move in to get the room temps you want, then probably never look at the manifold again.

As it happens, running all loops completely open worked fine for us. The one carpeted room downstairs is a little cool in winter, but it isn't used that much, and when it is, it's generally the kids gaming or watching TV, so it heats up fast enough. I haven't touched our manifold in over a decade.

As @torre says, there's probably an advantage in keeping all loops about the same length. I think each loop in our system is nominally 100 m long (one might be a little shorter).

We're not PH level, but very well insulated with UFH in a 70mm slab over Hanson Jetfloor. We have a total of 12 circuits grouped in 7 thermostatically controlled zones. This enables us to set and maintain comfort levels to suit the use of each zone, and for relevant times of day.

Surely, a single loop/zone risks losing such control, even if you could deliver even heat distribution across the entire circuit/slab?