Controlling zone valves for a boiler/chiller system?

Michael08

I am leasing a small space in a commercial building. The building has a boiler/chiller system that pumps heated or chilled water through two separate heat exchanger coils in our fan coil unit (FCU). Presently, the system uses an old Honeywell pneumatic thermostat that has a single pneumatic tube leading to the hot-circuit and cold-circuit pneumatic valves (there is a "Y" fitting that splits the air from the thermostat to the two valves). One valve is NO and the other is NC, so when the thermostat sends compressed air to the valves, the NO valve closes and the NC valve opens. Both valves are on the supply-side, before the FCU. One of the old valves is badly corroded and no longer functions. The valve is obsolete and cannot be rebuilt, so it must be replaced. The thermostat also appears to be defective.

Rather than continue with the old pneumatic system, I'd like to upgrade to an electronic programmable thermostat (with remote monitoring & control) like the Ecobee thermostat I have in my home. My plan is to replace both pneumatic valves with electric zone valves like Taco's "Zone Sentry" valves. I do not need circulation pumps as the heated and chilled water are continuously pumped throughout the building from large industrial pumps in the building's mechanical room. Although the entire building contains many zones, in my small leased space, there is only a single zone controlled by a single thermostat.

I've seen a lot of schematics and info on setting up a multiple zone heating system with a zone controller, but I haven't seen anything where a single thermostat is controlling both a hot circuit and a cold circuit. I suspect it will be fairly straightforward, and I possibly don't even need a zone controller (although I probably need relays to handle the power between the thermostat and the electric control valves, and a zone controller may be the best option for doing that).

This is how I need the system configured: a single programmable electronic thermostat that can call for heating or cooling. When the thermostat calls for heat, the zone control valve on the heated water pipe to the FCU will open (chilled water valve will be NC). When the thermostat calls for cooling, the zone control valve on the chilled water pipe to the FCU will open (heated water valve will be NC). Added bonus (desired, but not required) would be if both heating and cooling could be called at the same time to provide dehumidification when the weather is too cold for cooling alone. I would prefer to use another Ecobee thermostat because I'm already familiar with it and I can control it from the same app I already have on my phone for my home thermostat.

Could someone please provide some guidance on how to configure this system? If a schematic could be provided, that would be awesome! I also welcome any other advice. Thanks.

EBEBRATT-Ed

If it was me, I would just fix the pneumatics.

Are the valves two-way valves or 3 way valves (how many pipes connected to each valve 2 or 3)

The first thing I would do is get the Cv rating (flow rating) of the old valves, this is important, so you don't screw up the water flow to your air handler or effect other equipment.

Then I would buy new valves with an= (as close as possible) Cv rating and the same piping arrangement two pipe or 3 pipe. If 3 pipe be aware that some 3-way valves are diverting valves, and some are mixing valve and that makes a big difference, so you need to find out what you have.

After that it becomes a wiring and control issue. Do the thermostats control the valves directly without any boiler or chiller control?

hot_rod

if you want the Ecobee control, you will need thermostat wires from the valves to the stats. Then get the spec on the pneumatic valve to get a match in 24v.

Those may have been modulating valves also, not just on/off?

offdutytech

Typically a pneumatic FCU would use the pneumatic thermostat to modulate the heating and cooling with one pneumatic signal to the valves thus the "Y" fitting. The valves have different spring ranges usually 3-8 psi for heating NO valve 8-13 PSI for cooling NC valve. To use a Ecobee with on/off control for that set up will require a few EP valves (diverting air valve) and a few other pneumatic parts.

With that said that form of on/off control is really not going to work the best. You are going to have full heat and full cooling that will result in poor comfort. You can try and find a good pneumatics contractor to repair what you have. The second option is they make stand alone building automation controllers for FCU units. The easiest controls would be a company called Easy I/O. You could leave the pneumatic valves if you wanted to and use a AOP transducer, but modulating electronic valves would be the better move.

Jamie Hall

Um.. may I humbly point out that if you are leasing the space, anything you do must be approved by your landlord, who may have his or her own ideas…

Michael08

https://forum.heatinghelp.com/discussion/comment/1813589#Comment_1813589

The current pneumatic valves are two-way valves. It's a very simple system with just an isolation gate valve and a two-way NO or NC pneumatic control valve (and then a drain spigot) on the supply side before entering the FCU for each circuit (heat/cooling). There appears to be a check valve on each of the the return side pipes from the FCU, although I haven't removed the insulation yet to verify. The thermostat is pneumatic only with no electric/electronic controls. The fan in the FCU is powered through an Intermatic-style time clock that has no timer wheel trip arms, so the fan runs 24/7 as long as the circuit is energized from the load panel; this is how it was when we took lease of the space. There is zero control feedback from our space (or any other space) to the boiler/chiller system in the building's mechanical room.

The FCU for our space is fairly small (nominal 4 ton) and our leased space is only ~630 Sq Ft, whereas the entire building is quite large. The heated water and chilled water pipes to our FCU are either 3/4" or 1" pipe (I've forgotten off the top of my head) coming off the four 6" distribution & return pipes that feed/return the heated and chilled water throughout the building. I know the boilers are way over capacity because the original building design was to incorporate hydronic heating of the sidewalks for melting snow; but this was never implemented. There is a control panel in the mechanical room that monitors the supply side and return side of the heated water and chilled water circuits. Based on those monitored temperatures, either the boilers are fired in the winter, or the chiller is run in the summer to maintain the correct supply and return temps. The water/glycol fluid in the appropriate circuit is circulated throughout the building 24/7 by the large pumps in the mechanical room.

I do not currently know the Cv rating of the original valve, but I doubt there will be any water flow problem if I were to replace the existing pneumatic valves with 3/4" or 1" Taco Zone Sentry valves (Z075C2-2 or Z100C2-2) which have a 10.3 Cv or 8.9 Cv, respectively. Whatever we do with our FCU will have essentially zero impact on the rest of the building's heating and cooling system (which has already been modified in other spaces over the years by different tenants).

Michael08

https://forum.heatinghelp.com/discussion/comment/1813603#Comment_1813603

I believe the pneumatic valves are modulating valves because there is only a single pneumatic line from the thermostat controlling both valves. That said, the building only runs the boilers in the winter and the rooftop chiller only runs in the summer. When the boilers are deactivated in the summer, the heated water circulation pumps are not run. I believe that in the winter, the chilled water circulation pump is also deactivated, but I'm not sure about this. I'll have to check. (So now that I think about, simultaneously running chilled water for dehumidification and then heated water to warm the air, probably will not work unless the chilled water recirculation pump still runs in the winter with natural cooling of the water from the environment.)

I realize that, even with one of the circuits (hot or cold) not running, the modulating valves will still regulate the amount of water flowing through the FCU at any time (rather then being fully open or fully closed), so this will allow the air to be heated or cooled more "gently." But I believe that replacing the pneumatic valves with electric zone control valves won't make a practical difference on comfort in the space. There are four air diffusers in the ceiling and we never feel the air blowing directly on us. Running the coils in the FCU at full hot or full cold with the thermostat calling for heat or cold as needed to maintain room temperature should be fine. This is no different than the control of the furnace or AC in a home.

One reason for switching to standard electric control zone valves from the pneumatic valves (or modulating electronic valves) is simplicity, while gaining the advantages of a programmable electronic thermostat. Another reason is that the pneumatic valves use a sliding stem to control the flow of water, and the sliding stem & seal is more prone to problems from corrosion than a rotating stem & seal.

hot_rod

The issue with knowing the correct Cv is a valve with a high Cv has a low shutoff pressure, 20 psi (psid) or less.

A 1 or 2 Cv valve will have a high 50-75 psi shutoff, aka PSID.

So if there is a single large pump, flowing a bunch of AH units that pump develops enough ∆P (high head) to cause a high Cv, low shut off pressure, valve to leak across.

Really for that small of an AH I would just error with a 2 or 3.5 Cv valve and you are covered for adequate flow and adequate shut off pressure.

We supply zone valves to many AH OEMs and they always order 1 or 2 Cv valves to assure they have enough shut off capacity, not knowing what pump they will be attached to at the jobsite.

Switching to electric ZVs makes the most sense to me.

EBEBRATT-Ed

Does the AHU take in any outside air? If so does it have a freeze stat to shut down the fan and close the OA air damper.

If it does take in OA I would recommend staying with modulating valves. If it does not take in OA then on-off valves are probably not an issue.

Michael08

https://forum.heatinghelp.com/discussion/comment/1813768#Comment_1813768

I believe your concern is that the valve mechanism will not be strong enough to shut off flow against the high head pressure of the pumps, correct? I can see how this would be a problem with zone control valves that use a paddle or other part that must close against a seat. But the Taco "Zone Sentry" valves are ball valves, so that should not be a problem. Caleffi also makes motorized ball zone valves and I like how those have integrated union tailpieces for easy valve body replacement.

Michael08

https://forum.heatinghelp.com/discussion/comment/1813773#Comment_1813773

No, our FCU does not take in outside air. It looks like it was supposed to — there's a 6" duct coming from outside our space and entering the return air plenum — but for some reason that 6" duct is connected to the return air plenum of another FCU in a different and completely separate leased space across the corridor. I have to wonder if that was a construction error, as I've never seen that before.

But even if our FCU took in outside air, could that small amount really freeze the coils once mixed with the much larger volume of warm room air from the return ducts? Also, the coils contain an glycol solution so that should also prevent freezing.

hot_rod

any if the ball type valves will be fine They are a bit slower opening, if that matters

Michael08

I see no reason why a slightly slower opening and/or closing speed would make any difference in this system. If anything, a slower opening should reduce inrush noise and, more importantly, a slower closing should prevent water hammer.