Using 1 condensor to operate 2 evaporator coils

[sz2kq1]

Looking to design a simple 2 pipe system so one VFD controlled heat pump can run 2 evaporator coils. The evaporator coils are EXV controlled with independent controls for each coils superheat. I would like to be able to run them independently so either one or both coils can run as needed. I do not want the ability to heat and cool at the same time, so the system will be in heat mode or cool mode. On the cooling side the exv valve could probably use it to control the flow but when in heating mode I want to control the flow of the liquid side so I only provide heat tot he unit running. The 2 coils are only 8 feet apart so the piping of the combined liquid and suction lines will be pretty easy. Since I am only wanting one mode available, either heating or cooling, can it be as simple as just installing a NC solenoid valve on the liquid line to each coil before the EXV valve and after the separation tube? Or am I missing something? Do I need a solenoid for the suction lines as well for heating mode? Anyone done this before?

[Poodle Head Mikey]

I don't know and haven't done it - but let's talk about it anyway.

The area stat would call for system operation along with the operation of it's associated indoor unit. The outdoor unit responds to some feedback - what is it? Suction pressure? Evaporator temperature? How does the outdoor unit know how many indoor units are active?

Oh wait: Is this system a matched set of condensing units and evaporator units? Or is this system to be grafted together from non-matched equipment? If un-matched; how would the control interfacing be accomplished? What will actuate the condensing unit? How is the drive speed determined? Based on what input?

It seems likely to me that the outdoor unit drive speed set point is supplied via a temperature sensor on the evaporator. It would be handy if it was a suction pressure derived set point - but apparently that only makes sense to me. <g>

As I rambling this along I have some questions - because all the precise details are still unknown - but how about:

Why wouldn't a single LLSV in each liquid line provide sufficient independent control?

Outline the overall control Sequence Of Operation first: _____________________________________

PHM
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Looking to design a simple 2 pipe system so one VFD controlled heat pump can run 2 evaporator coils. The evaporator coils are EXV controlled with independent controls for each coils superheat. I would like to be able to run them independently so either one or both coils can run as needed. I do not want the ability to heat and cool at the same time, so the system will be in heat mode or cool mode. On the cooling side the exv valve could probably use it to control the flow but when in heating mode I want to control the flow of the liquid side so I only provide heat tot he unit running. The 2 coils are only 8 feet apart so the piping of the combined liquid and suction lines will be pretty easy. Since I am only wanting one mode available, either heating or cooling, can it be as simple as just installing a NC solenoid valve on the liquid line to each coil before the EXV valve and after the separation tube? Or am I missing something? Do I need a solenoid for the suction lines as well for heating mode? Anyone done this before?

[sz2kq1]

First thank you for the discussion. A little more detail. The inside units are both IQ drive systems with pressure and temp feed back from the evaporator coil exit to a control board that controls the PID open/close to the EXV. The board is non communicating so it basically gets a start command from the thermostat then runs closed loop to maintain approx 6 deg superheat. There is also a temp sensor for the exit air temp to control the fan speed with. Both units are the same model but different coil size to match the area they service. A master controller will read both the temp and humidity on each floor (2 floor system) and determine the which unit comes on and the fan speed required for peak efficiency. This system also starts the condenser and determines the initial requirement for load stage1 or stage 2 and the startup HZ for the compressor (The start up cycle is a little more complex to control start amps and load but only until the unit reaches stability in pressures) Once the system is stable the PID for the VFD is controlled from the suction pressure with the ability to tweak the PID/suction pressure based on internal temperature.
I agree my first thought is just add a solenoid valve in each liquid line before the EXV valves and make sure the system faults quickly if there is a valve failure. It seems to me this would work but I wanted to get some other opinions since i have not done this before.

[poppa]

lennox did it back in the day with solenoid valves and two stats with there two speed compressor. worked great til it didnt. usually was a simple problem that some tech made compicated and out that system went.

[Fla.HP]

From my view point, he is on the ball with t'stats controlling L.L. solenoids. Pretty cool setup since variable speed C/U makes all the decisions...

[town]

I will say oversized the outdoor unit as 2 stage compressor with 2Selenoid On each set at the or close to the evaporator, this also will include one Zone control panel

[sz2kq1]

To clarify my understanding of your suggestion of 2 solenoids on each set. That would mean one solenoid valve for each liquid line and one for each suction line for each evaporator. This would be a total of 4 valves that would isolate each evaporator coil when not in use.

Thanks for the feed back.

[Fla.HP]

You just need the solenoids on the L.L. The variable speed C/U will take care of the rest...

[Elfshadow]

Sooooo we are creating a multihead minisplit system?

[Fla.HP]

Yea, basically.

[Elfshadow]

Sooo if you have EXV's you should be able to drive them closed to isolate the evap instead of adding LLS.

[Fla.HP]

Can you really modulate a txv fully closed? Never seen that done,but then there are many things that I have never seen! Lol!

[Elfshadow]

Can you really modulate a txv fully closed? Never seen that done,but then there are many things that I have never seen! Lol!

it depends on the txv... but they said they are using EXVs.

All the ones I have dealt with except a JCI model allowed you to drive them all the way closed.

Mind you I have no idea how they are controlling these so they might not have access to do that.

It's done on some larger commercial equipment... just to save the cost of the liquid line valve.

[Fla.HP]

My point is, if you use LLS, it doesn't matter what kind of metering device you have...

[Elfshadow]

My point is, if you use LLS, it doesn't matter what kind of metering device you have...

Yes... but I was refrencing the OP where he said they already have EXV. Why add the cost of liquid line valves if you already have electronic expansion valves?

Of course it being heatpump complicates this some what.

[Fla.HP]

Yea, there could be software issues. Like when the system completes load demand, it would act like a "pump down" system. Software would think there is a low pressure issue, and lock out. This microprosser stuff is hard to modify, with all this "proprietary" crap. You bring up valid points.

[Fla.HP]

On a side note, I tried, back in the late 80s to make an electronic metering device. Utilizing a Bosch fuel injector, driven by a square wave signal generator. It worked well, could vary pulse width and duration. Was even gonna program a "PIC"(programmable Integrated circuit) but there were two big problems that reared there ugly head.
1: fuel injectors see 50# of pressure, I was injecting 250# of pressure. They did not like that.

2: the noise. It was like a hum, variable with load, vibrating the LL, something you don't wanna hear at 2 am. These are issues you don't see until you build a prototype. R and D can get very expensive.

[town]

To clarify my understanding of your suggestion of 2 solenoids on each set. That would mean one solenoid valve for each liquid line and one for each suction line for each evaporator. This would be a total of 4 valves that would isolate each evaporator coil when not in use.

Thanks for the feed back.

The Zone control panel have a time delay when room temp is satisfactory. That will keep your SL open with enough time. When calling for ac will be energized controlling wherever coil or space you want to cool 😎

[town]

Are you familiar with the Zone control panel?

[pecmsg]

Can you really modulate a txv fully closed? Never seen that done,but then there are many things that I have never seen! Lol!

Heatcraft Does!