We have a large open-vented heat store in our loft with 3 spare ports (top, middle, bottom) heated by solar thermal and stove. It seems to work fine. We'd like to combine it with ground floor UFH and first and second floor radiators (see diagram). I'm wondering about the best way to connect and control these different temperature outputs. Would anybody be able to give me any pointers? I'd be very grateful.

(I also buried some temperature probes in each UFH zone in the screed. I haven't wired these up to see if they work, but I thought it might give us an option.)

Thanks Tim. The heat store is quite big, around 1,500 litres, so I'd hope it would be okay to heat the house between fires. The SAP calculation suggests that we'd need 2kw to keep the house at 21º when the temperature is -3º, but I suspect this is a bit ambitious. If nothing else, the store would be needed to distribute the heat to the radiators and UFH around the house.

We're also using the store for the DWH. It has 2 internal finned coils (not shown for simplicity).

Posted By: ComeOnPilgrimWe have a large open-vented heat store in our loft with 3 spare ports (top, middle, bottom) heated by solar thermal and stove. It seems to work fine. We'd like to combine it with ground floor UFH and first and second floor radiators

I would have 3 zones, UFH, first and second floor radiators all taken off the middle port and returned to the bottom port of the heat store. the zones would be each driven by its own pump with a non-return valve to prevent back circuits and each would have a recirculating temperature control valve so that the temp. of the CH water to each zone can be individually controlled and the heat drawn from the store is minimised. After that each zone can (should) have room stats or TRVs to control the temp. of the zone.

If you take the CH from the top of the store you will compromise the amount of DHW available.

Don't forger that whilst the calculation by Tim above is valid, by the time you have reduced the store temp by say 25 deg and if it starts at 80 (top to bottom) then you will be down to 55 deg which would be OK for the UFH but a bit low for the rads unless they were designed for this supply temp. and since you are taking the CH from the middle port you will get somewhat less that half a days heating. Or realistically in winter you will get a warm house in the mornings but will probably have to light the stove mid morning or so and make sure you have a full store late evening to carry over to the next morning.

My heat store is remote from the house (attached boiler house) and I put a couple of adjustable thermostats on the side of the store operating warning lights in the house so that I can monitor the temp. of the store to see when the boiler needs firing without the need to go to the store to check the temps - very useful). If your store is in the loft how will you check the temp. of the store?

Posted By: TimSmallWhat are the design temperatures for the rads and the UFH?

The UFH calculations (specified by the pipe supplier) are: Design Flow Temperature: 45º, 3.5kw.
I've only bought a few radiators so far, and the specifications are not all consistent, but here is what I've got:
Bathroom radiator: 666w @ 60º
Bedroom radiator: 600w @ 70º
Bathroom radiator: 300w @ 60º

Posted By: Peter_in_HungaryI would have 3 zones, UFH, first and second floor radiators all taken off the middle port and returned to the bottom port of the heat store. the zones would be each driven by its own pump with a non-return valve to prevent back circuits and each would have a recirculating temperature control valve so that the temp. of the CH water to each zone can be individually controlled and the heat drawn from the store is minimised. After that each zone can (should) have room stats or TRVs to control the temp. of the zone.

Hi Peter, would you have the pumps at the heat store, or at the manifold on the relevant floor for each zone or both? The UFH has a pump at the manifold, but presumably this is to push the water round the UFH coils, not to draw it from the heat store in the first place?

For the rads, you'll need a heat loss calc for the room, and then ideally size them for the same temp as the UFH.

Posted By: Peter_in_HungaryFor convenience I have them at the heat store. It also means that there is minimum hot pipe from the store before mixing down to circuit temperature. The pump(s) push the water around the circuit but also pulls the water through the mixer valve which takes water from the return and the store to maintain the set temperature

Thanks Peter. So the three ports could be combined through a mixer valve, which then splits into three, each with a pump and valves, which respectively supply the the floors? Do you think it would be possible to achieve the same with a single pump?

Thanks Peter! Very insightful. In the meantime I've found a link to a site showing a system that uses a single pump and motorised valves: https://www.underfloorheatingsystems.co.uk/2013/08/14/is-it-possible-to-combine-traditional-radiators-with-underfloor-heating/

Thanks very much Peter! I've drawn up a revised schematic, with motorised valves for the time being, though it would look very similar with pumps and one-way valves instead. See attached. I forgot that I had put the radiator circuits in a manifold arrangement like the UFH.

Is there an off-the-shelf system that could manage this, or is it something that I need to make myself?

Without the zone valves all of the heating will be on together, with the zone valves room stats will be able to control the different areas. An alternative would be thermostat valves on the rads. Zone valves and room stats make the system automatic, thermostat rad valves need manual intervention (unless you get the expensive electrically operated ones. If your house is anything like mine I feel that zone valves are a bit of a waste because most of the doors are open most of the time so heat (air) flows fairly freely around typically from downstairs to upstairs.

This is what is looks like now. I'm pretty sure that I can take out the zone valves if the radiators are arranged with a manifold, like the way UFH circuits are arranged.

I @djh, yes, there are service valves, drains, header tank etc, but adding these in makes it look way too busy.

Well spotted on the UFH cold mixer! That's a mistake made while adding in the actuators for the UFH loops. I'll correct it.

Actually, looking at it, I don't think I need the mixer valves for the radiators either as there is a mixer at the tank. I only need a mixer for the UFH as this is cooler.

I don't see any non-return valves anywhere! Are they left out for clarity, built in to the actuators or not there for some other reason?
I would put a NRV on each of the flow legs.

Back flow can be a problem when multiple circuits are driven from one source. It all depends upon the pressures set up within the system. Water will take the path of least resistance so results can be unpredictable especially when actuators are involved that continually alter the dynamics of the circuit. I put them in as a matter of course because non-return valves are not too expensive and it is easier to put them in at installation time than to retrofit later.