In our case, we have three thermostats that can call for heat for radiators. The previous owners, however, zoned ALL our rads into one enormous circuit, so if one thermostat calls for heat, and there are TRVs that are cold and will allow hot water into the rad. 

Your pipework zoning will be important to answer this question. 

Do you have a buffer tank/store? If yes, your heat pump will be heating that and circulating heat. So if one thermostat has called for heat it'll take the same amount of energy to heat one rad or three (or more) rads if you know what I mean?

Thanks @editor.  I suppose I was hoping it would be a simple answer, which would apply to any set up, but I suppose I was being optimistic 🙂

In our property, we have 8 rooms, each room has a thermostat, which controls the underfloor heating in that room.

I'm trying to work out if only one room is calling for heat, are we using the same amount of energy as if all 8 rooms are calling.

Our coldest room (kitchen) is calling for heat pretty much 24/7 because it's the biggest room with the biggest heat loss, but the other rooms are generally not calling for heat.  I'm wondering whether I should increase the desired temperature of the other rooms so that they are all on, all of the time.  Thinking being that if the water is being heated to 40 degrees anyway, might as well use it throughout the house rather than just the one room. 

We do have a buffer tank, so I think from your last response this means that we might as well be circulating the hot water in all the zones, as it wouldn't use any more energy.

@novemberromeo 

It's not just that the water is being heated.  As it's pumped round the house, it loses heat and comes back cooler than it leaves.  I assume your ufh opens up valves and allows flow only when it's calling for heat?  If it's being pumped round less of the house it will lose less heat and come back less cool, say 38C instead of 36C.  The ASHP has less to do to heat it back to 40C.  The buffer tank complicates things because (I think) it will always have to be filled up with hot water and kept hot but I think this still applies.  Sorry if I'm stating the obvious ...

Very decent point @kev-m

I think i'll try it for a week and see what effect it has on the energy usage...

Posted by: @novemberromeo

Thanks @editor.  I suppose I was hoping it would be a simple answer, which would apply to any set up, but I suppose I was being optimistic 🙂

 

In our property, we have 8 rooms, each room has a thermostat, which controls the underfloor heating in that room.

I'm trying to work out if only one room is calling for heat, are we using the same amount of energy as if all 8 rooms are calling.

Our coldest room (kitchen) is calling for heat pretty much 24/7 because it's the biggest room with the biggest heat loss, but the other rooms are generally not calling for heat.  I'm wondering whether I should increase the desired temperature of the other rooms so that they are all on, all of the time.  Thinking being that if the water is being heated to 40 degrees anyway, might as well use it throughout the house rather than just the one room. 

We do have a buffer tank, so I think from your last response this means that we might as well be circulating the hot water in all the zones, as it wouldn't use any more energy.

 

Hi,

Sorry to burst your bubble, but you will definitely use more energy if you increase the thermostat settings in the other rooms

@novemberromeo, at this stage it's all about experimentation and understanding how things work most efficiently. Keep as many notes as possible of what you've done and capture as much data as possible. Over time, this will allow you to set up things in a such a way that will make the house warm and not cost a fortune to heat. It does, however, take time, patience, and trial and error.

Well I tried it...

Average energy required before change (each zone calling for heat as needed) : 35 kWh

Average energy required after change (each zone calling for heat 24/7): 44 kWh  (25% increase

So I've now reverted back to letting each room call for heat when it needs it...

@novemberromeo, that’s really interesting. Please continue to keep an eye on this and share data going forward.

Posted by: @novemberromeo

Well I tried it...

Average energy required before change (each zone calling for heat as needed) : 35 kWh

Average energy required after change (each zone calling for heat 24/7): 44 kWh  (25% increase

 

So I've now reverted back to letting each room call for heat when it needs it...

For larger properties this is my thought as well. When you have large areas/zones that don't need heat but its being sent there it can really take away from rooms that don't need it. For example we have a large open plan south facing lounge with lots of glazing that heats up quickly through solar gain during the morning and afternoon that quite easily brings it up to temp. There is no point in the heat pump having to heat the water to send it through all that extra pipe work to then work harder to reheat it again when it comes back. We also have a log burner in there which we use most evenings and that again allows the heat pump to focus on other areas.