How do I supply radiators on demand while supplying floor heat

DaveH

Here is my work in progress. Since the picture I have added 3 circulator pumps for each manifold. Each lower manifold supplies the radiant floor heat.

The upper manifold supplies all the wall hung radiators to the 2nd floor bedrooms and bathrooms.

Being the indirect is up and going for DHW and on it's own heating loop, I would like the radiant and radiators all to run at 130 degrees. My question is, with the radiators having a TRV, how do I supply the radiators so the boiler isn't constantly firing to heat and circulate the water; possibly making the efficient system inefficient.

Jamie Hall

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?

DaveH

Yes. Each radiator has it's own TRV, so to speak. They are not the Caleffi style but an unmarked flow control on where the supply line comes in to allow each person in the room to open or close based on the temp they desire.
The boiler can modulate 10:1, but even at 10k btu's, one would prefer it to not constantly run.

psb75

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

Jamie Hall

You are going to have very widely varying demand. As @psb75 says, a buffer tank is going to be the way to go.

SuperJ

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...

DaveH

The drawings on my picture make more sense to me! That picture was an earlier picture before my pumps were installed and before my original post. I have 3 pumps. One to the rads and one to each lower manifold, since they are separated living areas (mother in law area).

I have the outdoor reset hooked up and DHW gets priority. The floor heat seems to work well, but have only had it going for a day or so, since it's still mild outside.

It does make sense to pipe through the rads then to the in floor, but then the rads would only be available on a call for heat??

Or, can I keep the pump running, powered and pumping and open the valves on the rads when desired? Again, they are not TRV valves, and have no thermostatic control just flow control. If they are all closed then the boiler wouldn't fire for that portion of it. But, can those pumps run with water in them but not be pumping, or is that bad on them?

Jamie Hall

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!).

SuperJ

You can deadhead some small modern wet rotor pumps that have a delta p control. Some wilo pumps actually will shut off when they detect no flow with no external controls.

But a zone control box with a pump output would be a fine addition.