Wiring Nest to Mitsubishi PAC-US444CN-1 with radiant heating as primary heat source

JimmyT2319

Has anybody had success connecting a Nest (3rd Generation) to a Mitsubishi PAC-US444CN-1 where the primary heating source is radiant? Currently, my Nest is used to control my radiant heating and receives power from the current setup (wires: W1, C, RH). I would like to connect my Mitsubishi SEZ-KD12NA4 to the Nest. Because the Nest is already being powered by the radiant unit, I don't think I need to connect a transformer (is this correct?). Using the wiring example provided in the manual for single stage cooling with alternate primary heating source, I think I only need to connect wires Y1, G and W1 from the PAC-US444CN-1 to the Nest inputs Y1, G andW2. Is this correct? Does anybody have any other thoughts or wiring examples that have worked for them? Thanks in advance for the assistance.

ratio

You'll almost certainly need interposing relays between the thermostat and the Mitsubishi. Without them, the power from the two separate units will fight and the Nest will lose.

gennady

You will cross 2 transformers, one of them would die or damage will be done to equipment, depending on polarity. I would use relay to separate power sources.

Ironman

You don't wanna use a Nest with a modulating unit like the Mitsi: the Nest is trying to turn the unit off/on based on life style patterns; the Mitsi achieves its efficiency by modulating as low as possible for as long as possible.

The Nest is designed for bang-bang equipment, not modulating equipment. You will actually make the Mitsi much less efficient and less comfortable trying to use it.

We actually covered this in a class on modulating equipment today. Don't use a Nest on it.

Mitsi does offer a Honeywell wall mount option.

ratio

I see. It's a ducted head with a thermostat adapter. Ironman is right, you will sacrifice comfort and efficiency by using the thermostat adapter. However, only you can determine if the trade-off is acceptable, & you're right to want one control that will coordinate the two heating systems. It appears that you can avoid the relays by using the boiler power to supply the power to the thermostat adapter, that will eliminate most of the faults that would let the smoke out of the Nest. Be sure to observe polarity with the power wires, common/C should never be swapped with 24V/R/RH.

JimmyT2319

Thank you for the responses. If I can avoid the relays, do I still need a transformer on the Mitsubishi unit? This would essentially be a dual transformer setup, correct? I assume I need to connect the transformer to the PAC-US444CN-1 interface, then the RC terminal from the interface to the Nest and leave the common C wire as is since the Nest is already receiving power. Any thoughts are appreciated, thanks again!

ratio

Follow the instructions for "Example 4: Single-stage Cooling with Alternate Primary Heating Source" I believe. Remove the wires from R & C on the stat & extend them to the TR & TC on the adapter, then take R & C from the adapter to the Nest. That's four conductors. Wire the Y's and W's according to the example. That's another two or three conductors. ISTR that the Nest will support multiple fan speeds. Take a conductor from the G terminal on the Nest to one of the G terminals on the adapter. If the Nest does indeed support multiple fan speeds follow the Nest instructions & take that wire to another of the G terminals the adapter. Think about which speeds you want to have & land the wires with that in mind. That's maybe one more conductor. Set the DIP switches in the adapter as noted in the example. It looks like you'll need 7 to 9 conductors, you should be able to get an 8 conductor or 10 conductor stat wire.

JimmyT2319

This is great! So with this setup, I do not need to install a transformer or relay. I will purchase a stat wire with more conductors and try it. One more question (hopefully) - my current setup on the Nest is a wire to the RH terminal; should I wire it back to RH from the adapter or use RC instead? Not sure it matters, but I just want to confirm. Nest supposedly recognizes there is only one R wire connected so there's no need for a jumper. Thank you!

ratio

It's fine to leave it on the RH terminal.

HVACNUT

> @JimmyT2319 said:
> Thank you for the responses. If I can avoid the relays, do I still need a transformer on the Mitsubishi unit? This would essentially be a dual transformer setup, correct? I assume I need to connect the transformer to the PAC-US444CN-1 interface, then the RC terminal from the interface to the Nest and leave the common C wire as is since the Nest is already receiving power. Any thoughts are appreciated, thanks again!

Umm. Didn't you read the reply from @Ironman?
Wiring the Mitsi to the Nest is defeating the purpose of the Mitsubishi ' efficiency.

ratio

It may very well have a measurable difference, but will it have a noticable difference?

I believe it's much more important to coordinate the two heating sources than squeak an additional 10% [edit: I made up this number. It may not reflect reality] out of the cooling. I can't believe any gains resulting from running the system in full modulating mode can offset the occasions when the heat and cooling are running simultaneously.

Ironman

@ratio
I'm gonna have to respectfully disagree. I don't know where you got the 10% number, but the figure passed down to us was much higher. When you take into account that the system won't dehumidify near as well, that also means less efficiency and comfort.

The logic of the Nest is counter productive to the logic of the Mitsi. The Nest is designed for non modulating equipment.

Also, I've seen quite a few two system, two thermostat houses and buildings in my short years and having the stats fight each other is never an issue if the occupant understands that the heat must be set lower than the cooling. If they don't ... We send them to D.C.

If the OP really wants a wifi stat, then I'd recommend that he choose one that doesn't try to manage the system based on life style.

Still, the most economical use of the Mitsi will be to set it and forget it. An after market stat will not make it more efficient.

ratio

Sure, the 10% number was made from whole cloth. All I meant was that I don't expect it to be lots and lots. I could be mistaken, but if running it fully modulating vs a standard split AC meant saving 100's of dollars a month, I'd've expected to hear more sales pitchs about that.

Also, I'm trying real hard to not sh!t all over Nest stats. They seem like a fine product if you like shiney, and most of the scheduling stuff can be turned off. I wouldn't have them in my house for love or money, not least the privacy concerns, but the OP didn't bat an eye re your earlier comment so I figured it's a done deal.

I think your customers are more intelligent than mine. I've been stamping out fires on a project of ours, the owner of the corp. insists that the reheat on his delivered air VAV system may. not. run. Ever. The heating setpoint is 60, I get called because it's 68. Dial up the DAT, then I get called because it's 74 in there when it's set to 73. <sigh> As long as the company keeps sending me out this isn't going to go away.

JimmyT2319

@ratio
Thank you for your help. I finally got around to wiring the system as you suggested and it worked perfectly! I understand it may not be as efficient, but I am in area that does not require AC more than a couple months out of the year. And the thought of having to deal with 2 thermostats was driving me crazy. Thank you again, much appreciated.

ratio

I'm glad it worked for you. I'll check into the efficient hit before I recommend it again, but electrically speaking it's kosher.

And two thermostats would drive me batty too!

JimmyT2319

@ Ratio (or anybody else who can assist) you were so very helpful with your instructions on connecting my Nest to my Mitsubishi units using the PAC-US444CN-1, that I had one more request for help. I successfully connected 3 Nests to 3 separate units, but now I have one final connection that is a bit more difficult - how do I connect 2 units to one Nest? The reason for this is the area is one large open room (kitchen, dining room and living room) so has one zone for radiant, but required 2 units for cooling. Ideally I would like the 2 units to work simultaneously, but if that's not possible, maybe I can have one be the primary stage for cooling while the other works as the secondary stage. If I go that route, I assume I would have a problem using both units for heating, unless I only choose one to be secondary since the radiant is primary. Any assistance would be greatly appreciated. Thank you again in advance!

ratio

You'll need a double pole 24vac relay. Tie the coil in to Y1 & C on the thermostat, tie each PAC-US444CN-1 in to the relay, one on each pole, R from the PAC to the common of the relay pole, Y1 to the NO of the relay pole. That will bring them on simultaneously, but pay attention to the DIP switch settings—some may be significant. IDK if the fan will do the right thing, it may need to be tied in to a relay in an identical manner as well.

JimmyT2319

I will have to do some additional research and probably come back with some more questions. Do I need to anything with W1 from the PAC to the W2 on the thermostat? Or just scratch the secondary heat and only go with the radiant? Thanks again.

ratio

Oh, yes, I was thinking cooling-only. You'll need a relay for that as well.

Not a big fan of heat pumps as the only source of heating, At least around here (central Ohio) they give up right when you need heat the most, i.e. below 0. They work heat during the swing seasons though.

Check & see if the nest will let you set a balance point, above which the heat pumps run & below which the boiler runs. That'd give you the best of both worlds.

JimmyT2319

Ok, now I think I am starting to understand (please correct me if I am wrong). I will probably need 3 double pole single throw 24vac relays (cooling, heating and assume for the fan). But here is where I need help - I assume I split the C from the Nest (still need to power the Nest itself) to each of the 3 coils, but will that be enough power for 3 relays? And do I do the same with the R from each PAC - split it 3 ways to each of the 3 relays? Do I need to connect a C back to the PAC as I did with the original single unit configuration? If not, are any transformers required? Thank you again for all your help. I love these types of projects!

ratio

The PACs can be powered from a single transformer of sufficient capacity. MAINTAIN POLARITY. The Nest can be powered from one of the PACs as noted in the manual. Each function (heat, cool, fan) will need a relay, each PAC will need a pole on the relay, so 3 double pole relays with 24VAC coils. Tie the coil common to the C on the power that feeds the stat (a wire nut is fine), tie the other side of the coil on each relay in to the Y1, W2, & G terminals.This will give us three functions each with two isolated outputs. Take the R from each PAC to the COM terminal of the same contact on each relay. Take a wire from the NO terminal to the Y1, W2, or G terminal of the corresponding PAC.

That sounds a little confusing. I'll see if I can't draw up a schematic, but I just got a new computer and not all of my software is installed yet.

ratio

Does this help any?

JimmyT2319

WOW!!! This is awesome - I cannot begin to thank you for your help on this project. I totally understand what needs to be done. Just two questions: 1) since I already have C and R coming from the radiant setup, can I use those as I did in the original, single unit setup and delete the need for the transformer? 2) Also, it sounds like I would need 3 DPST relays, but your schematic shows DPDT relays. I know DPDT relays are easier to find, so there simply would be no connection to NC, correct? Thanks again!

ratio

1) Probably. The only limiting factor is the power available. If you know where the transformer that powers the radiant system is, see if you can read the label on it. You're looking for a number that reads something like "40 VA", hopefully higher. How many other thermostats are being powered from those wires?
2) Correct.

JimmyT2319

I don't see a label, but it's a Taco SR504-EXP with 4 thermostats connected to it - two of these thermostats are each powering a PAC and one will be powering the 2 PACs and 3 relays in this project. Do you think I am good? As always, thanks again for your assistance.

ratio

Can't really tell from looking at the manual I found on the internets. Try it, set all the stats to cooling, & see what happens to the 24 volts. If it stays around 24 volts, it's fine. If it sags to 20 volts, it's not fine.

tim smith

Jimmy T, one more thing to add, you can now use the kumo cloud from Mitsubishi, If memory serves me right you may have to add one item, then you can actually set up true auto switch over from heat to cool. This is the one pet peeve I had with the pacus when I got the first ones a year and a half ago. I have 8 nests set up on the old app which the only way to switch over was go to all the stats and manually switch them all to the mode you wanted then control from the app. Much better now.

basa820

> @JimmyT2319 said:
> @ratio
> Thank you for your help. I finally got around to wiring the system as you suggested and it worked perfectly! I understand it may not be as efficient, but I am in area that does not require AC more than a couple months out of the year. And the thought of having to deal with 2 thermostats was driving me crazy. Thank you again, much appreciated.

Hi Jimmy, what was the final wiring for the Nest? My mitsu is cooling only, no heat. It may differ from your setup

mattshields

Hi Jimmy,

I think I'm trying to do the same thing you were doing. Did this work?

thanks.

motoguy128

For efficiency, just a reference, the EER on most minisplit is only 10-11, even if they have a 18-26 SEER rating. At full load, it’s basically a split window unit. Rotary compressor and undersized condenser coil.

I believe the MFG do all they can to hide that little inconvenient truth. The good news is most multi head systems end up being oversized and with diversity due to zoning, they rarely see full load.