how do wired thermostats and TRVs work together?

weil_fail · May 2020

for a hot water boiler system, if the radiators have TRVs, how does that work with a wired thermostat?

I feel like there is a catch-22 where if a room's TRV shuts off before the wired thermostat is satisfied, it would just keep calling for heat forever. on the other hand, if you didn't have a wired thermostat, how would the boiler know when to run?

are there certain brands of boilers designed for use with TRV and just use some other means, like return-water temperature and outside reset to determine when to run?

Jamie Hall · May 2020

Your thought about TRVs and wired thermostats isn't wrong. If there's a TRV in the space with a wired thermostat, and it shuts off before the thermostat does, the boiler will go on... and on... and on.

TRVs are used to reduce the heat in rooms which are too warm when the thermostat -- which controls the boiler -- is still calling for heat elsewhere. The thermostat is your primary control for temperature in the structure. Before you even think about adding TRVs, get it located properly and set to heat the structure as best you can for comfort. Then work with balancing the radiation -- if a space is to warm, reduce the heat to it slightly (on steam and some hot water systems this is easy -- partly close the valve or vent; on some hot water systems it's difficult). Then add a TRV if you really truly need independent cooling of a space.

Note that a TRV can't call for heat. It can only reduce the heat in a space which is too warm.

ChrisJ · May 2020

Some thermostats such as the Honeywell Prestige can have multiple sensors which all get averaged.

In my case I'm running single pipe steam but it still applies. 5 out of 10 radiators have TRVs and several are in rooms which have sensors for the thermostat. Under those conditions, if the average temperature between all sensors drops enough the boiler will fire. Rooms which want heat will get it and those that don't will not get heat.

It works incredibly well actually. But if you only have one sensor, the thermostat and you put a TRV in that room it would be bad.

Joe_Dunham · May 2020

Jamies right. TRV's defy the central control. The old public schools had pneumatic TRV,s in the classrooms controlled by a pneumatic wall thermostat. the idea was to give the teacher 100% control of the classroom. the boiler ran constantly. TRV's should be used to prevent over heated rooms and should not be in the same room as the thermostat. they're really meant for very large buildings

hot_rod · May 2020

Why do you need a thermostat?
A few ways TRVs can call the boiler on
Use a delta P circulator with trvs, some have “end switches” built in to close when the circ starts flowing

Phenix Controls have current sensing relays the recognize when the delta P circ wakes up

McDonnell Miller has a 1/2 gpm flow switch that works also, similar to how a tankless water heater fires up

weil_fail · May 2020

@hot_rod ,

by delta-P, I assume you mean a change in pressure through the system such that the pump can detect when all of the radiators have closed TRVs and shut itself off. is that correct?

lets say I have a 2-pipe hot water system, which puts all of the radiators in parallel. as each TRV shuts, the pressure will change. would the last TRV closing cause any problems since there would now be no flow at all? in such a system, how does the boiler know when it needs to run?

Jamie Hall · May 2020

If there is no thermostat or timer or what have you, @weil_fail , the TRVs -- all of them -- have to have end switches wired in parallel to turn on the boiler, so it will run when any TRV is open. AS @Joe_Dunham noted above, some schools or larger buildings back in the day just let the boiler run all the time -- more or less on the theory that someone, somewhere, was going to be needing heat. It wasn't the most efficient plan, but...

hot_rod · May 2020

weil_fail said:
@hot_rod ,

by delta-P, I assume you mean a change in pressure through the system such that the pump can detect when all of the radiators have closed TRVs and shut itself off. is that correct?

lets say I have a 2-pipe hot water system, which puts all of the radiators in parallel. as each TRV shuts, the pressure will change. would the last TRV closing cause any problems since there would now be no flow at all? in such a system, how does the boiler know when it needs to run?

Correct, the delta P circulator will start moving flow when any TRV opens. The Phenix relay senses the increase in current draw, down to 0.1 A. It makes a contact which connects to boiler TT. then the boiler runs of the operating limit, or ideally an outdoor reset function.

The Delta P circ should go to rest at around 7W, when all TRVs close so it can be powered 24/7. As soon as any valve starts to open, even slightly the circ will start reving up.

I believe the delta P circulators were developed around the TRV control logic. Millions of TRVs used in Europe and around the globe.

The McDonell Miller FS1 flow switch can be used, it detects down to 1/2 gpm. I think the relay is a bit more sensitive to micro flows, however.

Here is an example in a home run piping system, parallel would work also.

weil_fail · May 2020

how does it know when the valve is open? does it constantly or periodically try to pump in order to test the pressure?

do all radiators require h-valves for bypass in this scenario, or just at least one? (none?)

I know you can buy highly integrated condensing boilers with pumps, etc. built-in, do any brands sold in the US incorporate delta-P pumps and sensors?

hot_rod · May 2020

The circ is always spinning and detects a change in pressure dynamics. There are two types of H valves a one pipe and a two pipe. The one pipe has an adjustable bypass so a bit of flow bypasses to the next radiator.
Sketch your piping, but if there is a flow path when a TRV opens, flow will be established.
The delta P will work with any zone valved system. The beauty of TRVs is they are proportional, non electric, modulating valves. No transformer or valve power draw to contend with. A common delta p will be around 7- 40 W. So you are moving heat energy very efficiently, always at the correct flow rate.

Big Ed_4 · May 2020

The Delta P circulator does not need a pressure by pass .. It controls water flow automatically

With TRV's on the radiators , I would recommend an outdoor reset with a room sensor control . An option and wired to the OD reset .. Instal the room sensor in the coldest living area with heat .. Program the OD reset to turn on the heat if outdoor temperature falls below set point. .. 65* is the norm .. Use the room sensor to fine tune for your comfort and control ... And set the TRV's to your needs ..

weil_fail · May 2020

what are the best brands for designing a system like this for residential?

I really like the compact wall-mount condensing systems, but it seems like none of them do delta-p by default, so would I just use any brand and attach a flow sensor somehow? should I be looking for one without an integrated pump, so I can add a purpose-built delta-p circulator instead of a flow sensor?

Jamie Hall · May 2020

I'm curious. Several people have suggested using a delta P pump and all TRVs, but there seems to be a gap here -- and maybe someone ( @hot_rod ? Please?) can fill it in. It would seem from his comment that the pump is always spinning in such systems -- and that makes a certain amount of sense, as otherwise if a TRV somewhere were to open (and thus call for heat, in the absence of any other control) there still would be no flow.

Is this the way it operates? The pump is always running? Just seems to me that this might not be the most efficient approach for conventional radiators or baseboards, although if there is also a mixing valve to modulate water temperature as there would be in a radiant floor, perhaps... but then, what happens if someone wants more heat and cranks up their TRV?

Perhaps I'm missing something here in the control strategy?

SuperTech · May 2020

The Alpha pump can be set to maintain a constant pressure in the system, so the flow will meet the requirements of the system regardless of how many TRVs are open. The auto adapt mode is supposed to "learn" how much flow the system requires after it sees the changes in delta P as the system operates.

I'm not familiar with the Taco VR1816, that delta P circulator may have the same capabilities.

Jamie Hall · May 2020

Oh I got that far. But what does it do when there is no flow at all? Keep running? If there is no flow, there will be no pressure differential regardless of how much -- or little -- it pumps, so what does it target?

hot_rod · May 2020

Here is a good read on how the various delta P functions work

https://www.hpacmag.com/features/high-efficiency-circulators-brushless-dc-motors/

Jamie Hall · May 2020

As usual -- a good read. And it sort of answers m y question -- basically, these things in this type of application are intended to be powered -- and running -- all the time.

Jamie Hall · May 2020

As usual, a good read. Thank you, @hot_rod ! So basically these things in this type of application are in fact running all the time, regardless of flow...

And I shouldn't complain, eh, since the filter pumps on the fish pond run 24/7/365 too (and they are brushless ECM motors, too, by the way, very similar, except no control electronics).

Big Ed_4 · May 2020

And just think how the ODR flattens the curve

how do wired thermostats and TRVs work together?

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