How do I supply radiators on demand while supplying floor heat

Do the radiators each have a TRV? Or is there just one for the whole loop?

In any event, what's usually done is to have each zone have a thermostat, which controls the circulator or zone valve for that zone (and may or may not control the boiler). Or, you can have TRVs if you need different temperatures within the zone -- but in that case you may find, as you note, that the boiler is merrily running along when all or most of the TRVs are closed, which doesn't help your efficiency at all.

So... the question is, why the TVVs?

If you are worried about short-cycling, install a buffer tank.

I would run an aggressive outdoor reset as the primary control so the TRVs only have to the fine tuning. Set the TRVs more open than you normally would. If the rads zones are too hot don't close the TRVs, instead your ODR more aggressive to reduce the SWT. If your outdoor reset is set aggressively your flow won't drop off much as it warms up. The TRVs will open and close to fine tune the space temp, but the SWT should get it pretty close.

Use a delta pressure (either proportional or constant) setting on the radiator and infloor pumps. You could use a constant or delta T pump as your primary pump.

Pump your radiators and floor separately since you want to be able to control the SWT separately to both of them.
Your max temperature is probably going to be determined by the radiator zones. Setup the boiler ODR to satisfy the radiators, and put a mixing valve on your infloor since you will likely want to limit the max supply temp to something less than what the rads want on a cold day. I'd suggest one with outdoor reset capability.

I would run a primary loop (or buffer tank) feeding your secondary loads, I would be tempted to put your loads in series so the the rad manifolds take off and return before the infloor takes off. This should maximize your delta T for efficiency sake. But if your rad return is too cold you'd either have to up your primary flow/temperature (upping temp will reduce rad flow as TRVs will close and hence increase swt to infloor) or put the loads in parallel so both take off before both return. I'd pipe it both ways with ball valves so you have the option. Use generously size pipe for your primary loop.

On your boiler you would need to set your start and stop offsets and timings to prevent short cycling. But with an aggressive ODR you should have pretty constant secondary flow, and hence some thermal mass to work with so a buffer tank may or may not be necessary. But I would recommend at least a small one, but pipe it in a 2 or 3 pipe configuration rather than 4 to keep your temp delta's as high as possible. If you over pump a buffer tank you end up pushing the hot water down and with warmer (less efficient) boiler entering temperatures.

You could probably setup the boiler for constant circulation (jumper the call for heat), and just use a warm weather shutdown.


Something like this... but you can probably just put the boiler in series on the primary loop and eliminate the boiler circulator. The temperature will cascade downwards from your high temp loads to your lower temp infloor. Assuming you can set the primary circulator to an appropriate flow rate for the boiler (delta T of 20-40F at max fire depending on the boiler). The buffer tank could be in series on the primary loop after all the loads take off (to the right of the in floor tees in my diagram.)
Rad and Infloor circulators would run constantly, DHW would run on priority. If necessary you could kill the Rad and in floor circs on DHW call if capacity is an issue.


Sorry for the hard to read labels, it's tough to write with a mouse.

Here's another idea...

I won't speak to the rest of your system -- but pumps don't like to run with no flow, since they depend on the water going through them for cooling (even with hot water!).