Nudging this for visibility - *any* thoughts gratefully received...

For my part I was overwhelmed with the total number of questions.

So I’ll answer a simple one: if one ufh zone is colder then yes you get it warmer by increasing the flow. If you are already at the maximum adjust range (assuming you have turning flow adjustment which shows the flow) then you have to reduce the flow in all the others and increase the temperature.

Since ufh and radiators need sometimes very different temperatures why are you trying to combine them? The larger size of pump won’t give you the two temperatures you need. Do you know what temperature flows your two zones have?

Posted by: @judith

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Since ufh and radiators need sometimes very different temperatures why are you trying to combine them?

I did mention mixing valves and pondered the criteria when they are useful so I can understand better.

The larger size of pump won’t give you the two temperatures you need.

Do you know what temperature flows your two zones have?

Think you may be thinking too much.

So first 

Has your heat pump got its own pump, if so the plan is to use that and not change it. So to enable that to happen you could wing and hope for the best, or ideally make a best guess at system pressure drops and see where you sit on the pump curve. If it's an external pump, you need to calculate pressure drops and see if any of your existing pumps are ok. Suspected the secondary one will be ok as it's already pumping around the system.

UFH, does it have its own pump and mixer, or do you plan to delete?

Thermostat, it's there to act as an on off switch, so set to about 2 to 4 degs above target temp. Your controller own one is best. You then run WC. And balance system to get room temps where you want them.

PWM control will slow the pump during cycling of the compressor that's about it. Not the most important thing in the world.

3 port valve is required so hot water goes to cylinder to heat it up and not to the UFH etc. it's part of the operation logic.

Ask questions one at a time, from this point to get clarity of responses 

Thanks for the response, @johnmo.

Posted by: @johnmo

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Has your heat pump got its own pump

if so the plan is to use that and not change it. So to enable that to happen you could wing and hope for the best or ideally make a best guess at system pressure drops

If it's an external pump, you need to calculate pressure drops

see if any of your existing pumps are ok

UFH, does it have its own pump and mixer, or do you plan to delete?

Thermostat, it's there to act as an on off switch, so set to about 2 to 4 degs above target temp.

Posted by: @rhh2348

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How would I do either/both of those, please?

You need to get an idea of pipe size and length and number of bends etc. 

Look at current secondary pump current settings. Where it sits speed wise.

Posted by: @rhh2348

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Yes, has a pump; no mixer (that may not be by design).  My question was essentially, in what circumstances does one need a mixer?

You need to look how the UFH is plumbed into the rest of the system. The normal mixer arrangement provides some hydraulic separation, so multiple pumps don't start fighting each other. A closed tee for example would provide hydraulic separation also.

What you are proposing is easy enough to do, but you almost need to start from a blank sheet of paper.

Posted by: @rhh2348

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What space/room do I put that in?

A representative space, mine is in the hall, I know the living room gets lots of solar gain so it's out of that room. But think of the thermostat as a simple wireless switch, it should be demanding heat all the time, the WC curve looks after the room temp. Once you have WC sorted the thermostat serves no purpose, except a easy place to set holiday mode 

Posted by: @johnmo

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You need to get an idea of pipe size and length and number of bends etc.

Look at current secondary pump current settings. Where it sits speed wise.

You need to look how the UFH is plumbed into the rest of the system. The normal mixer arrangement provides some hydraulic separation, so multiple pumps don't start fighting each other. A closed tee for example would provide hydraulic separation also.

What you are proposing is easy enough to do, but you almost need to start from a blank sheet of paper.

A representative space, mine is in the hall, I know the living room gets lots of solar gain so it's out of that room. But think of the thermostat as a simple wireless switch, it should be demanding heat all the time, the WC curve looks after the room temp.

@rhh2348 Answers to your questions.

Seriously considering doing this. I think I understand the benefits (mainly, the ASHP works much better with open flow rather than the inefficiencies of a LLH and one room able to demand which causes temp overshoot/pump cycles) but could I ask for some pointers / tips from those who have done it please. What I know:

Use of one thermostat as a master On/Off essentially, set 0.5C above preferred temp - assume this goes in the most-used / most-central room / coldest (see thinking below) in the house (e.g. living room or hallway). Emitter 'temperatures' are controlled by flow temp and individual flow to emitter (by manifold valve or lockshield). NO, any use of an on-off thermostat WILL reduce your performance and significantly increase your running costs, it is akin you to install a switch on your dashboard, then using it, when you are in third gear with your foot flat on the accelerator to control your speed on the motorway at 70 MPH.

What do people do if they have one UFH room that is significantly-colder than all others? Does that just get more flow? Yes, but there must be reason it is colder, Identify that and correct it. What about if it then cools down quicker than the thermostat space elsewhere - or is that why the thermostat always goes in the room with fastest heat loss? Does one have to make a call to either a) run the system more/always just because of it, b) fix the heat loss, or c) accept it will always be cooler? The system should always run continually, and always reduce heat loss if you can, however, the colder room may be due to insufficient heat supply from the emitter. A correctly designed system will be a balanced system, maintaining a constant, even temperature in every room.

Extending the single-cold-room scenario above, how does one handle a colder upstairs generally (or, rather, all rooms lose heat quicker than downstairs) Corrct emitter design, such that if thermostat is downstairs and satisfied, can upstairs always be 'just' satisfied by lockshield taming if emitters on both floors are 'aligned' with each other and the thermostat is in the right space? Do not use a thermostat. The upstairs may start to lose heat quicker - so I assume either 1) I am right about the thermostat positioning in the room with fastest heat loss, or 2) that is why people run (close to) 24/7 - what are the scenarios when your system doesn't run, other than OAT > 14C (say) or for those who heat their screed overnight for it to emit during the day?

How do people handle different emitter 'profiles' - e.g. concrete screed UFH downstairs vs rads upstairs - flow rates can be controlled via pump / flow restrictor / valve/lockshield but is the minimum flow temp the lowest both can reasonably operate with, even if that means one (say UFH) is running a higher one than it needs - or can whichever needs a lower one mostly be tamed via valve/lockshield? What happens if minimum flow to UFH loops result in excessive heat emitted; I note mixing valves for UFH manifolds but I assume these are to reduce flow temps significantly - like, by 10+C? Correct system design, you are heating a house, not a room. If you want to switch off a room, the adjacent room's emitters MUST be designed to cater to the extra heat loss. There is no insulation between rooms in the walls, therefore the heated room wall, next to the unheated room becomes a very big radiator. The calculations of the heated room MUST assume that the adjacent room is external and the external wall is uninsulated. In reality, this will triple the size requirement of the radiator, making it 6 times the size of a fossil fuel high-temperature radiator. This is not viable or practical, as I started with, heat the whole house, not a room, the heat is stopped by the insulation in the external walls, ceilings and floors.
How common is it / what are the criteria/thresholds that flow temps and rates (and anything else?) are too different such that it actually makes sense to run two circuits off separate thermostats, albeit all emitters on the respective circuit (stay) open together?   NO, unless you want a very high bill.

In terms of pumps, how would one shortlist/chose - I assume it's based on their capabilities around: flow rates offered, head, operating noise, energy consumption, PWM control - are these ball-park defined to make shortlisting easier?  One that will do the job, they are tools, if you use the wrong tool, it will not work. They all the same these days.

How does one calculate pump head/pressure required - I read some Jon Cantor materials but it didn't quite go in. But I have worked out my runs. Are there recognised 'reduction' figures for 1/3/5/10 metre straight / 90deg / couplings / Ts for copper + push/fit that you essentially add up at a specified flow rate? Almost impossible. Correct flow rate can only be determined on commissioning, anybody who says otherwise has never commissioned a system correctly. Experience will give a guide, but will not be accurate. Even if a system is designed to the fitting and meter of pipe, friction loss figures for pipes and fittings are estimates and averages, they vary from component to component, manufacturer to manufacturer. No calculation can be accurate if your inpuuts are estimates and averages.

Is it usual to replace existing pumps with one pump for everything (15mm UFH and 22mm rads pipe runs c.750m in total length) and leave the 3port in, or can/is it better to remove 3port but have one pump for DHW cylinder and one for heating - as generally only one is in operation at once. I always try to use 1 pump, but more may be needed if 1 is not big enough. I have a system where I have had 4 pumps installed one after the other to achieve the desired flow rate, I prefer a good quality full bore 3 port diverter. The Honeywell-type valves are NOT suitable for any system above 7 Kw. With heat pumps, flow is king, if you do not have the flow, you cannot move the heat, you have a cold house. Quite simple actually.
If the space heating had to remain as two zones and the flow rates/temps were broadly the same, could the same pump be used for 1 or 2 or 1+2, subject to something like a 2port valve controlling flow to each circuit, or are two pumps always required for two circuits - I assume this is the case as the flow rate would need to change significantly if both circuits were open at once. Zoning is a last resort, but only if it is designed correctly. If the heating system is not specifically designed to do it, DO NOT do it. It will significantly increase your running costs even if you are attempting to heat a smaller area.

I hope this answers your questions, feel free to respond.

Regards

Brendon

@heacol, many many thanks for your reply.  I've broken my comments into individual posts so it's easier to manage further quoting:

Use of one thermostat as a master On/Off essentially, set 0.5C above preferred temp - assume this goes in the most-used / most-central room / coldest (see thinking below) in the house (e.g. living room or hallway). Emitter 'temperatures' are controlled by flow temp and individual flow to emitter (by manifold valve or lockshield).

NO, any use of an on-off thermostat WILL reduce your performance and significantly increase your running costs, it is akin you to install a switch on your dashboard, then using it, when you are in third gear with your foot flat on the accelerator to control your speed on the motorway at 70 MPH.

Extending the single-cold-room scenario above, how does one handle a colder upstairs generally (or, rather, all rooms lose heat quicker than downstairs)

Corrct emitter design

How do people handle different emitter 'profiles' - e.g. concrete screed UFH downstairs vs rads upstairs - flow rates can be controlled via pump / flow restrictor / valve/lockshield but is the minimum flow temp the lowest both can reasonably operate with, even if that means one (say UFH) is running a higher one than it needs - or can whichever needs a lower one mostly be tamed via valve/lockshield? What happens if minimum flow to UFH loops result in excessive heat emitted; I note mixing valves for UFH manifolds but I assume these are to reduce flow temps significantly - like, by 10+C?

Correct system design

you are heating a house, not a room. If you want to switch off a room, the adjacent room's emitters MUST be designed to cater to the extra heat loss. There is no insulation between rooms in the walls, therefore the heated room wall, next to the unheated room becomes a very big radiator. The calculations of the heated room MUST assume that the adjacent room is external and the external wall is uninsulated. In reality, this will triple the size requirement of the radiator, making it 6 times the size of a fossil fuel high-temperature radiator. This is not viable or practical, as I started with, heat the whole house, not a room, the heat is stopped by the insulation in the external walls, ceilings and floors.