Thermostatic radiator valve + in-floor hydronic heating
I watched the BS+Beer, November 2022 show where John Siegenthaler discussed hydronic heating systems. Toward the end, Mr. Siegenthaler mentions that he would discourage zoning with thermostats (and by inference zone solenoids or dedicated circulators). Instead he would suggest using thermostatic radiator valves (TRV’s) in each room. I think these are normally associated with hydronic radiant panels (or radiators).
My question is if anyone has installed (or has opinions on) a TRV on each in-floor zone and a variable speed pressure regulated circulator to drive the system. This seems like a much simpler zoning solution than dedicated thermostats and circulators. Thoughts?
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I would argue just the opposite. A thermostat and a zone valve is simpler and cheaper, assuming you have access to run the wiring. The thermostat has the benefit that you have a wired output that you can use to control the circulator so you don't need an expensive constant-pressure circulator.
TRV's are usually used with radiators as a way of simulating zoning after the fact, when actual zoning would require replumbing. They're a half measure.
I'm going to disagree with DC here. TRV is a good way to go. Since the TRVs are modulating, with the right reset curve you'll get some flow through all the zones which gets you the most efficiency especially with something like an air to water heat pump.
Lot of modcons and AWHP have a built in circ and if you set up your loop right you can plumb it direct to load. This means no extra pumps, relay modules, valves or thermostats, it just works.
Down the road, if you do want to get some electrical controls over it, you can get electric actuators for the TRVs. There is even ones that are 0..10V which would let you set it up with a modulating thermostat.
TRVs are your friend. They are simple and they just work. The price in NA is a bit high for them though, overseas is a much better option but you'll need adapters.
Maybe this works better than trying to get manifolds balanced?
Perhaps this is the constant circulation mode?
I dunno, as l0ng as someone understands the control strategy....
I appreciate both perspectives. I do like the modulation of a trv. The ideal for me would be a linear thermostat that would open a TRV proportionately to temperature in a room. (I doubt that thermostat or TRV exists.)
I started down this path because i was looking at hydronic towel warmers, and a lot of them seem to have TRV control. I would like to tie a hydronic zone in the bathroom floor to a towel warmer, so both would be activated at the same time. I know, a real "first world problem". Also, i had wanted to move away from thermostat zone control due the expense and complexity of 'smart' thermostats. But an apples to apples comparison of TRV to thermostat would have 'dumb' thermostats to control the zones. Going nostalgic, i'm now thinking about the round honeywell thermostats.
Likely, i will put the bath floor/towel warmer on a timer switch (along with erv boost), and control the hydronic zone with a traditional zone valve. Put the bedroom hydronic zones on a simple thermostat + zone valve. Probably one or two 'smart' thermostats in main living areas for remote control/monitoring, erv control, and humidity control.
As an aside on Akos' comment about AWHP's. At least my experience with those (monobloc systems), they require a buffer tank with the built in circ pump moving water strictly between the condenser and buffer tank to hold the buffer tank at a constant temp. The aren't really designed for on-demand use. You could set up TRV's with bypass and make use of the AWHP circ pump, but then you are constantly pumping conditioned water through the house. And i would guess the manufacturer would not warrantee that setup.
You better balance the flow in the circuits and set the reset curve properly so that (with open TRV) the room will get its temperature by itself or just overshoots a bit.
A TRV is basically to throttle the flow if internal gains make the room overshoot - like a party with 20 people or sun shining into the room. That way you have some flow in all zones nearly all the time and the pressure regulated pump can shine. A buffer tank is still worth some to reduce cycling in the shoulder season - no way to avoid that.
If you switch from the simple mechanical (expansion) type of TRV to electronic ones, then you can do a program over time and run different setbacks for each room. I would question this at least for a properly insulated house where you better use all emitters to be able to run the lowest possible reset curve.
If you want rooms to be warmer or colder on average (as a feature) then balance them for that.
I'm with Wastl.
As I have thought and lived with these systems, the more I tend towards simple/no control. I think the ultimate AWHP or Modcon system is a simple direct to emitters with no thermostat and just outdoor reset control.
I'm slowly eliminating controls on my setup as some are simply not doing anything. For example the bath floor heat ant towel warmer is always on. The rest of the zone vales are also mostly on, so even turning the zone pump off isn't needed. This let me drop the outdoor curve a couple of degrees, so it is definitely running a bit more efficient.
The one benefit of properly sized always on zones is you don't need a buffer as there is enough water volume.
It does feel wrong though as it looks too simple, but it does work and it works very well.
My understanding is that in Europe, where air-to-water comes to us from, this is a very common approach. No zone valves, no circulators, no buffers, no thermostats. The circulator in the heat pump provides constant circulation and the water temperature is determined by the outdoor temperature. To the extent the heat supplied doesn't match the heat load, the room temperature varies.
If you want less variation in indoor temperature, a fan coil unit with thermostatic control on the fan will provide that.
It's really hard to control the output of a radiator by modulating the flow.
Let's think about a common setup, a hot water radiator that takes in water at 170F and returns it at 150F. Let's say this radiator has a flow of 1 GPM, so it's producing 10,000 BTU/hr. Let's say you want to reduce the output to 5,000 BTU/hr. What does the flow need to be?
It has an average temperature of 160F, or 90F above room temperature. To have half the output the average temperature needs to be 45F above room temperature, or 115F. With the water entering at 170F, to get an average temperature of 115F the water as to be leaving at room temperature, 70F. With a drop of 100F, to get half the output, 5000 BTU/hr, the flow as to be 0.1 GPM, or 10% of full.
Let's say you only want to drop it to 90% output. You need the radiator temperature to be 81F above room, or 151, which means a leaving water temperature of 132F, or a delta of 38F. To get 9000 BTU/hr you need a flow of 0.47 GPM.
So in order to get modulation in the heat output you need much bigger changes in the flow. And it's hard to get those small flows dialed in. Using balancing valves is a fools game.
On the other hand, with on-off control, if you want 90% output, have it on for 90% of the time and off for 10% of the time. Likewise with 50% output. It's simple.
DC,
You have to think about this in the context of low temperature emitters. Once you start throttling the flow, the return water temp quickly drops to room temp. This means that output is proportional to temp delta and flow rate. Since SWT and room temp are pretty much fixed, output is proportional to flow rate.
I agree that flow rate modulation doesn't work as well with high temp emitters but even there, there is decent modulation near the bottom end of the flow range.
DC,
not to dispute your calculation here. If it comes to that mismatch of room load and emitter output, then something went wrong in the past.
Same as with the other high efficiency heating systems, you need to do your design properly.
I use a air to water system as Akos wrote - one circulator, no TRVs, with an "as good as possible" balance of water flow and it works good enough that I do not care the last temp. swings. Ok, this is not a responsive system and i had to accept that some rooms are a bit off because the radiators are not always "spot on" in size. Overall I rather accepted this and run the reset curve now quite cold. Its 35°F ambient here atm and I use 88-90°F as reset and am ok.