Click here to Find a Contractor in your area.
Welcome! Here are the website rules, as well as some tips for using this forum.

If you've found help here, check back in to let us know how everything worked out.
It's a great way to thank those who helped you.
Need to contact us? Visit

Two (or maybe 4) stage chiller control

I've been working on building a couple chillers for a year to replace a manufactured unit and i'm close to starting it up but realize i need a water temperature control strategy. The one i'm replace was a trane two stage reciprocating R-22 15 ton unit which was low on sophistication for managing lower load which is what resulted in losing one compressor and then there was a refrigerant leak in the piping inside the chiller barrel on that same side so we're giving up and going our own way.

I've built this up using two 5 ton two stage condensor units so that actually gives me 4 stages theoretically. The Trane was controlled by an analog two stage aquastat. The theory is sound but I could actually use logic that could manage up to 4 stages and I would like to modestly vary the water temperature targets vs. outdoor temp although that is not essential. I used to just manually adjust the targets during 'shoulder' season as I was doing with hydronics before various affordable reset controls hit the market.

So looking for recommendations. I could do something as simple as use two two stage controllers and create an effective multiple lead lag circumstance by setting one for more aggressive temp range, e.g. 38 degrees on the lead with 6 degree diffential and then set the lag for 44 or thereabouts and spend some time calibrating the probes and calls although it would be nice if a I had a single control that responded to a single temperature probe to keep the stages on the same page without a lot of field calibration. Just looking to see if anyone has made up any solutions or found a ready made control that might have the logic for this.




  • Mike_SheppardMike_Sheppard Member Posts: 657
    Honeywell T775P.

    I’ve used them to replace the old rotary control on Carrier recips.

    With expansion modules it can control 12 stages.

    Here’s an overview of the T775s general applications. Table of contents has a few pages for chiller control.
    Never stop learning.
  • EBEBRATT-EdEBEBRATT-Ed Member Posts: 7,036
    I know @Mike_Sheppard likes the Honeywell T775s but I hate those controls. Look at the Johnson 350 series I just think they are more versatile and easier to set up and controlling 4 stages is easy JMHO
  • Mike_SheppardMike_Sheppard Member Posts: 657
    The 350 series is good as well. I think the T775s are just about the easiest to install and set up control I have ever used though.
    Never stop learning.
  • ratioratio Member Posts: 2,425
    +1 for the T775, which will give you two analog & four relay outputs with one set & one reset input…all in the same controller…

    Another idea might be one of these, which may allow you to reuse the existing controls, which has its own appeal I believe. Also it's a lot cheaper than the T775.

  • archibald tuttlearchibald tuttle Member Posts: 682
    thanks for replies. i took a look and it looks like the T775R2019 or 2001 has got 4 outputs. Quick scanning the literature showed a kind of overlapped sample for programming to call the 4 stages. it actually seems to have contemplated just my situation, ie. 2 condensors each having a solenoid that reduces capacity.

    quick reveiw didn't make it clear to me how to engaged the reset in a T775R but i assume that just takes more reading and the reset can be programmed in either direction depending whether the control is being used for heating or cooling?

    Then i'm not really clear on the difference between floating and analog outputs but those appear to be different from what i would consider the be outputs also, i.e. 4 SPDT relays??

    and depending on whether the floating/analog distinction means anything to my use of those 4 staging relays, the lowest price 4 stage T775R I could find wasn't much more than the lowest price T775B so i wouldn't be paying much more for reset. That said i might hit ebay and see what is just laying out there in dusty or changed out stock in which case beggars won't be choosers and the 4 relays looks to be what i need although the documentation says that the floating output eliminates the need for half the relays outputs which confuses me, but maybe there is some way to use a two relay unit to somehow control 4 stages , or maybe the other outputs can serve as relays?

    also looked at the johnson system 350. That looks to be modular, so you just buy a stage control for each stage, but i'm not sure if all you need at that point is just a transformer unit and some one of the individual units is set as the overall brain controlling the others or if there is some master brain unit i would have to buy.

    thanks for folks who have looked at this hitting me back.

  • archibald tuttlearchibald tuttle Member Posts: 682
    and BTW, i would think a unit like this could be programmed for freeze protection with a manual reset provision ????

    might still be better part of valour to use an additional analogy safety for that purpose but it would be a minor addition to the programming. is that function available in the logic of one of these controls?
  • ratioratio Member Posts: 2,425
    Floating is for like an actuator, energize this wire to wind open, energize that wire to wind closed, deenergize to hold position (NB, some floating actuators are spring-return—they have a constant power too.)

    You don't need a power module for the 350 series, they'll run off a 24 VAC module just fine. They have a separate reset module as well, and just gang relay modules as necessary. I'll eventually be converting one of my boilers to staged operation by adding a 350 slave relay module, (among other things).

    I don't recall offhand if either systems has freeze protection built in.

  • Mike_SheppardMike_Sheppard Member Posts: 657
    Floating is a way of controlling an actuator. You use two of the relays in the T775 to control the actuator rather than the mod outputs (analog). Analog is a voltage or current signal: 0-10v, 2-10v, 4-20ma, 135 ohm.

    If you are controlling stages you will be using the relay outputs. Look through that link I sent you and make sure the T775R will be compatible with what you want to do. T775R is indeed for reset. It can reset in either direction based on how you set the settings. It makes you choose outdoor settings and their corresponding water temperature settings, then draws a line between them. For example:

    outdoor max: 90
    water min: 44
    outdoor min: 60
    water max: 60

    These settings would mean at 90 degrees outside the water temperature will be 44, and at 60 degrees outside the temperature will be 60. And it will interpolate for you between those two points.

    If you are going to be selecting stages just by energizing relay, on/off basically, then ignore the floating control. Don't buy a floating T775 it's not going to do what you want. What you want is relay outputs, one relay output per stage. You can get an expansion module that will add more relays if you need it.
    Never stop learning.
  • archibald tuttlearchibald tuttle Member Posts: 682
    @ratio getting it re the floating, what i can't figure though @Mike_Sheppard is if Mod 1 and Mod 2 which are actuator operators in the T775R diagram are actually controlled by relay 1 and 2 so if you hook up an actuator then you don't hook anything up to the relay contacts for those relays?

    e.g. here is as sample layout of wiring connections although i'm not sure precisely which model this is:

    I ask academically because I have no actuator to worry about and it looks like several of the T775Rs have 4 relays and cost about the same as 4 individual johnson relays but take up less space and give you reset control so in this application i might be shading honeywell.

    and on the sensor connections it comes with two and there are just two sensor connections. i'm assuming those are outdoor temp and supply water temp normatively although they are designated a and b i think.

    and as to choosing the right one, the specs on relays seem pretty clear and as long as i'm not using an actuator i believe then i can just take the relay count for granted? Then I think i would choose the T775R2001 for cost and availability. here is the T775R family listing:

    so any other interpretive comments ,guffaws and thoughts appreciated but i can see you are steering me in the direction i was trying to go and i really appreciate it. and @ratio
    i would like to learn more about the johnson setup. they must have some kind of line pamphlet/install stuff. i seem to find bits and pieces of the system but not the whole system. offerings.

    thanks all,
  • Mike_SheppardMike_Sheppard Member Posts: 657
    You need the T775R2019. You will not be using floating outputs.

    Floating is another form of actuator control. The mod outputs are for voltage or current modulation. Where it says “each floating output eliminates two relays” it means that each floating output will use two of the relays. For example, controlling a floating actuator, relay 1 would be used to drive it open and relay 2 would be used to drive it closed. Relay 3 and 4 would be used for the second floating output. You cannot control on/off stages, like you are trying to do, with floating outputs.

    Yes, sensor A is water temp and sensor B is outdoor temp.
    Never stop learning.
  • EBEBRATT-EdEBEBRATT-Ed Member Posts: 7,036
    The 350s you would need and A350 (first stage), and 3 S350s (2,3rd & fourth stage. That's the minimum and you feed it with 24 volts, the water temp sensor comes with it. You can get a Y350 120-24 volt transformer if you want one. A D350 display module shows the temp setting, nice to have but not necessary.
  • ratioratio Member Posts: 2,425
    Regarding the 350s, @EBEBRATT-Ed's got it right. They will snap onto a DIN rail & connect together. You can actually use two of the D350s, one on each end to give you set (process) temp & reset (outdoor) temp at the same time.

    I just download the installation manuals from the internets, it gives you enough information to figure out what's going on, they're pretty simple, single purpose devices, not like the T775. IIRC the manual shows a module list in it somewhere.

  • archibald tuttlearchibald tuttle Member Posts: 682

    But if i'm not using either of the floating outputs does that not leave the relays for standard operation? Or does the PC board wired pathways cross contaminate the simple NO/NC relay output?
  • Mike_SheppardMike_Sheppard Member Posts: 657
    I haven’t used the 2001 in a long time, but if I remember correctly, it’s programmed to be two floating outputs, as in you can’t turn floating off.

    I’m not 100% sure on that. You could call Honeywell and verify that.

    The 350s the other guys are recommending are good as well. I personally prefer the T775 because for me they are extremely simple and user friendly. But everybody is comfortable with different controls. They’ll both get the job done.
    Never stop learning.
  • archibald tuttlearchibald tuttle Member Posts: 682
    @Mike_Sheppard @ratio

    i get that the 350 approach is very modular and build up your own and i like the logic of that, buy what you need, mount on rail. But for this application 4 staging outputs in a compact discrete package with reset combines what i'm looking for in one control (possible exception of freeze lockout but you can't always get everything - is there a 350 manual reset module)?

    I assume the 350s cross talk so that only the A350 has the temp sensor inputs and the D350s get that same information, because 90% of what you are trying to accomplish is to have each control get the same signal from temp. sensors so you don't have calibration headaches with relatively narrow stage banding.

    I made some heads spin at honeywell when i asked them if either the floating or mod outputs would confound my ability to simply use the four relays C to NO if i was not wiring any load onto the floating or mod terminals. That was a headscratcher.

    Meantime, i found a T775L2007 that has 4 NC - C - NO relays and no floating or modular capability but is resetable. and it looks affordable and everything was beautiful until i asked if i'm able to apply reset to all relays relative to the setpoint and whether either the design temperature or upper end was adjustable per relay to move the curves in sync above and below one another. if the setpoints are increased, does it just move the curve up of do you get a diminished curve with the same effective off or equal point, e.g. 60 degree water at 60 degree outdoor temp. Because of course if i have setpoints of 38, 42, 46 and 50 at 90 outdoors for my 4 stages with a deadband of 3 and a reset to 58, 62, 66 and 70 at 60 outdoors then the lagging stages move out of the way, is that possible in your experience? That question caused a fair lot of head scratching at honeywell as well.

    Obviously i don't even really anticipate the 3rd and 4th stage calling at cooler outdoor temps but i need to reset them out of the way. That does also get me into the question of whether these controls support lead changes. Might be a bridge too far. I lead changed on the old 2 stage Trane with a user installed double pull double throw manual switch and just changed at the beginning of each season. TACOs PC702 and the Tekmar equivalent for instance change leads automatically for two stages although since some of my stages are solenoids and required that particular condensor to be called, in this case i will have to switch in pairs, i.e. 1 and 2 with 3 and 4, so i'm not sure something short of super custom logic would allow that to be done automatically every so many start cycles although it wouldn't be really an immensely difficult programming proposition.

    (and maybe my sample ranges are too narrow even working off the same sensor so i could increase the stage separation to 6 degrees and deadband to 4 to allow it a little space to recover before calling the next stage).
  • archibald tuttlearchibald tuttle Member Posts: 682
    and just heard back from honeywell and they say you set the curve on for the first relay which results in a setpoint but you set the latter relays as an offset from that setpoint. well i think that more or less would take care of the issue because the other stages are moving relative to the algorithm defined setpoint for the first stage if i'm understanding them correctly. sounds like you have messed with these controls so maybe that make sense to you?
  • ratioratio Member Posts: 2,425
    According to the manual, each relay/analog output can be individually configured for reset. (It looks like if an output is configured for reset, it's acts as an offset from the first output, and not a setpoint) There's also a setback that can be triggered by an internal time clock or a digital input. The outputs can be set up as synced, you can select floating or standard for the relay (paired, mind you).
  • archibald tuttlearchibald tuttle Member Posts: 682
    @ratio thanks for that study. that corresponds with the answer i got from honeywell. i see now if i had read all the fine print i would have got it. I was looking for just the simple answer that they use one curve and offset from it if configured for reset which appears to be an option for each relay in the "setup" mode. thanks,

  • ratioratio Member Posts: 2,425
    It's very intuitive. If you know how it works.
Sign In or Register to comment.


It looks like you're new here. If you want to get involved, click one of these buttons!