Is it possible to use two rads off one TRV?

[RFw3Duwrd7Yu]

I'm replacing radiators and plumbing throughout the house in anticipation of installation of an air source heat pump next week. There are two rooms which have double radiators, and especially because I'm using (expensive) smart WiFi enabled TRV valves, I'm wondering if it's possible to connect the two radiators to make them essentially function as a single unit?

As I see it there are essentially three configurations:

(1) Run totally separate 15mm feeds from my 22mm feed/return loops to each of the two radiators with separate TRVs on each (not preferred for reasons above)

(2) Run a single 15mm feed from the 22mm heating feed/return loops and tee off the second radiator off this 15mm feed or...

(3) Somehow connect the two radiators directly to one another. But I fear that this would functionally mean that the second radiator would be running off the return feed from the first. Is there a way to run rads in parallel that will avoid this problem?

 

[oldbuffer]

1. If the radiators are next to each other, you can connect the two at top and bottom (2 x 1/2" x 22mm compression male irons, bit of 22 mm copper pipe for each of top and bottom) and treat as a single radiator.
2. If they are not next to each other your option 2 is the way to go, but you'd need to be careful that 15 mm would feed enough heat for two radiators. There are too many variables to be specific about the heat carrying capacity, but if the total output of the two radiators is more than 5 to 8 kW you might have to go for your option 1.
1. You might find that, allowing for odds and ends and the work required, your option 1 is the best bet.

 

[RFw3Duwrd7Yu]

Thanks @oldbuffer - the rads are right next to one another (well, about 18" gap, mounted on opposite sides of a corner in the room), so your suggestion that I connect them up seems right. I should probably mention that the rads are K2 900x1200 & K2 900x900, so a total of just around 5kw (assuming the system is running delta T45 since it's an ASHP) - think that's too much to run as a single unit? I guess, worst case scenario I could go with your version of (3) and switch to (1) or (2) later is rads are underperforming.

 

[RFw3Duwrd7Yu]

Also curious to know why you've specified 1/2" BSP male to 22mm compression irons - isn't 1/2" equivalent to 15mm?

 

[oldbuffer]

1. Radiator fittings are nearly always 1/2" BSP female thread, which is about 19.1mm internal diameter. Hence the 1/2" BSP threaded ends of the irons.
2. The top and bottom "channels" of most radiators are more than 15 mm, so I use 22mm pipe to connect them, hence the 22mm compression ends of the irons.
3. "Irons" are of course generally made of brass!
4. You will need to bend the 22 mm or use an end feed solder elbow to connect at right angles. Bend looks better but in my view requires a pipe bender.
5. I'd suggest:
5a. Using Flomasta pipe seal on the threads.
5b. Screwing the irons into the radiators and letting the sealant set (30 mins or so).
5c. Measuring / bending / trimming the pipework to fit.