Replies

1. 
   walta100 | Aug 27, 2021 02:56pm | #1

   My wild guess is this happens when one of the other zones is calling for heat and the outdoor unit’s min speed exceeds what the head that is calling for heat can use, requiring the system to dump the extra heat into the other heads and this head is in a smaller room so it overheats noticeably quickly.

   I wonder if your system is much larger than an accurate manual J would call for because the installer wanted to get a head every place they would put a register in a forced air system.

   Disabling the fans low speed if that is what the tech is suggesting does not seem likely to help much as the unit will still be producing heat in the small room trapping the heat in and around the head.

   If you have good insulation and air sealing you may be able to shutdown 2 of the outdoor units on all but the coldest days, can't hurt to try.

   Walta

2. 
   jameshowison | Aug 27, 2021 03:23pm | #2

   I agree that it sounds similar to the situations I saw with a multi-head minisplit system, these are pretty well known. And I agree with Walta that it is probably happening when the large unit is calling for heat, but not at a high level. You have a 30k btu outside unit, connected to 24k btu of indoor heads. So the system is already throwing away 6k btu of its modulation ability. So the outside unit has a minimum capacity (not modeled by mitsubishi's software, btw) which is probably around 15.5k btu. So when the large unit calls for something in the range of 50% capacity (say 9k btu) while the small room does not want any heat. Then the outside compressor turns on and produces 15.5k ... now where is that other 6.5k btu going? Some of it (if I understand correctly) goes into the little tank inside the outside unit, but most of it goes into the refrigerant in the linesets.

   For a reason not entirely clear to me the valves on the smaller unit never shut entirely, so there is some high temp refrigerant circulating and the fan is blowing, and the small room overheats. The wireless thermometers help a bit (they do show the actual temp, not a really high temp caused by the refrigerant circulating by the unit) and at least that excuse is removed. I ended up getting sensors from wirelesstag.net to record things to make my point. I don't know the difference between connecting to unit and connecting to the cloud, my experience of adding those wireless sensors is that they add to both at once.

   Long story, yes, "thermal off" is a good option. No, it makes zero sense that this is a hard to reverse "clip a capacitor" setting, it could very easily be software. You'll have the small unit turning on and off more, which might be annoying due to irregular noise and airflow, but you'll have much less overheating. You might still get some if the large unit is actually heating the small room. You can kind of simulate things by turning the small unit off and seeing what happens without it?

   Anyway, I now think of these multi headed systems as akin to ducts where there are limited range dampers. It's almost impossible to design these systems to meet the two key criteria: indoor units match the outdoor unit (to get the modulation advantages) and indoor units match the requirements of the room (to meet the promises of independent zone control). Now think about doing that as the weather changes and the sun moves .... It's as though one were designing a duct system but you only had 10" and 14" ducts/registers and very limited dampers.

   btw. why are these two connected to a 30k exterior unit and not a 24k exterior unit ... both of those are 3U (ie 3 ports), this sort of oversizing usually happens when more heads are needed than ports. I bet there wasn't a 24k available that week?

3. 
   biggytre | Aug 27, 2021 04:38pm | #3

   Thanks for info.
   @Watler - Thanks. Trying to shutdown 1 or 2 of the outdoor units during the shoulder seasons does seem like a reasonable idea. Below is my system configuation FYI.

   Condenser 1 (24k Btu) serves 3 indoor units totalling 24k Btu: 6k Btu finished basement, 9k Btu 1st floor living room, 9k Btu 2nd floor master bedroom

   Condesner 2 (24k Btu) serves 3 indoor units totaling 18k Btu: 6k Btu 1st floor office, 6k Btu 2nd floor small bedroom, 6k Btu 2nd floor medium bedroom

   Condenser 3 (30k Btu) servces 2 indoor units totalling 24k Btu: 18k Btu 1st floor main living space/kitchen, 6k Btu 2nd floor above garage bedroom

   Basement has 1 indoor unit served by 1 condenser, 1st floor has 3 indoor units served by 3 different condensers, 2nd floor has 4 indoor units served by 3 different condensers

   @James - Thanks. When you say "Thermal off" you mean turning off the smaller 6k Btu unit when you don't want it to give out any additional heat, correct? When you say "it could easily be software" what do you mean by that exactly? Are you suggesting I could install some sort of 3rd party software system that would automatically turn the 6k Btu indoor unit off when it reaches set point? Good question as to why the contractor suggested 30k BTU compressor instead of 24k BTU compressor. I atually had to wait longer for the 30k BTU compressor as they had a 24k BTU compressor "on the truck". They said a 24k Btu condenser could also work, but they had some argument for going with the 30k Btu that I'm now forgetting. Isn't there some hypothetical efficeincy advantage in certain scenarios of having a condenser that has a higher capacity thant the combined capacity of all the indoor units it feeds? (I could be totally off on this, but I feel like I've seen that argument before).

1. 
   jameshowison | Aug 28, 2021 01:17pm | #4

   I don’t know an argument for the bigger compressor, but it is clear that one loses the ability to modulate down as effectively. So I think that was a mistake. That said even if they were exactly matched the same issue can occur. What are the Manual J loads for the two rooms on the 30k compressor?

   Sorry, by “thermal off” I meant cutting the wire in the board so that the unit stops the fan when it reaches it’s set point. I think that is what your contractor is proposing. That is what we had to do in two smaller rooms. I do think it will help (with a trade off of irregular noise and temp swings in that room, just what was promised not to occur with VRF).

   See this link too: https://www.greenbuildingadvisor.com/article/multi-zone-heat-pump-issue

   (Btw, nothing there is cold climate specific, despite the title)

   I said it should be software because I think Mitsubishi made a design mistake here. I’m just whining into the void :)

1. 
   biggytre | Oct 07, 2021 03:58pm | #5

   @James thanks for response and sorry for my delayed response here.

   On the Manual J load calcs for the 2 rooms served by the 30k compressor. The upstairs bedroom above garage came in at 7.4k btuh (6k btu indoor unit) and the downstairs main living area came in at ~18k-20k btuh (18k btu indoor unit). First floor is hard to pin down exactly because it’s semi-open concept and served by 3 indoor units with all of them sort of sharing the hallway/bathroom/stairway load in the middle of the first floor.

   Related to your cutting wire/“thermal off” comment. My question is – let’s say I go that route with the bedroom above garage 6k btu unit mentioned above. In the example you mentioned earlier with no “thermal off” (my current situation), you desribed a situation where the 18k btu unit iscalling for 50% capacity (9k btu), smaller garage bedroom 6k btu doesn't want any more heat, but compressor produces 15.5k btu and dumps the excess heat into garage bedroom. What happens with that excess heat if I go “thermal off” with the 6k btu unit? Does it dump the extra heat to the 18k btu unit even when it’s not calling for it? In my situation, this wouldn’t necessarily be a bad situation since that 18k btu unit serves such a large, open first floor area that is also the area in the house we use the most frequently. I’d much rather have the 30k compressor dump heat into the 18k btu unit than the 6k btu unit (rarely used room).

   Directly related to that above question – if I just turn the 6k btu indoor unit off right now before doing “thermal off” wire cut on it – will that accurately simulate what will happen under the wire cut scenario?

1. 
   jameshowison | Oct 07, 2021 06:20pm | #6

   Great questions. I hope others with more experience will chime in. But in my experience at my house, yes, thermal off appeared to accomplish the same thing as just turning the unit off manually. We had much less overshooting if we turned off the equipment, and then same when we gave them the ol' clip for thermal off.

   Now, you raise a great point that confused me as well ... why is heating leaking out of a unit that doesn't want any heat. Answer given is "the valves don't close fully" ... So I guess (and it's just a guess) that the refrigerant still circulates but without the fan blowing over it (due to thermal off) it doesn't put the heat into the bedroom (well, where does it go? Perhaps the outside compressor runs less frequently because the refrigerant circulating is hotter and thus the big space meets its set point sooner?

   What I'd really love is for someone to explain why thermal off isn't the default setting ... like why isn't this problem very clear in the documentation? I'm betting it is something to do with defaults and testing conditions that shows a higher efficiency.

2. 
   shrayd | Jan 21, 2023 09:03pm | #7

   Hi biggytre! I'm having a similar issue with my Mitsubishi setup (https://www.greenbuildingadvisor.com/question/is-my-mitsubishi-heat-pump-short-cycling).

   I'm curious what you ended up doing for this. I'll either get the MHK2 (probably $400/head) or Cielo ($120/head) to do the 'thermal off' via software.

1. 
   biggytre | Jan 24, 2023 01:48pm | #8

   @Shrayd I ended up having the tech do the "thermal off" wire cut for my 4 indoor units located upstairs. It has solved the issue. The units at first did make a slight annoying buzzing sound when they were calling for heat but fan was off, but I haven't noticed that sound recently. Basically, now, when an indoor unit on the same compressor calls for heat you will see the "fins" on the upstairs indoor unit drop, but the fan doesn't turn on so the indoor unit doesn't put out heat (and overshoot the set point).

   The tech was basically like "It's pretty dumb Mitsubishi set it up like this where you have to cut the wire. They could have just installed a switch or made it controlable via software/app." I didn't do the wire cut to any of my downstairs units, becuase I don't really care if the downstairs units are delivering "extra heat" (beyond set point) when the upstairs unit is calling for heat but the downstairs unit isn't. The opposite (bedrooms overheating at night) was really my concern.

   I don't know how this impacts overall efficiency of the three multi-zone systems I have. I'm **hopeful** what is happening is that with the upstairs unit wire being cut the "extra heat" that would be coming out of the upstairs unit is instead being "redirected" to the downstairs unit (since that unit actually has a fan on) but I have no idea.

   P.S. I got the original company that installed my 8 indoor units/3 compressors to cover the cost of one MHK2, but honestly I don't think it really made a difference. I treid to get them to cover the cost of 7 more but they wouldn't budge on that haha. My suggestion would be just to go with the cheaper cube-like Kumo Cloud temp/humidity sensors and do the wire cut for the upstairs/bedroom units (based on my experience).

2. 
   Deleted | Jan 24, 2023 02:34pm | #10

   Deleted

3. 
   shrayd | Jan 24, 2023 02:36pm | #11

   Hi biggytre! Thank you, this was super helpful!

   To recap, you saw better results from cutting the wire than from an MHK2, is that correct? And to double-check, the MHK2 was configured to sense the temperature (vs the head unit) and also to shut off the fan when setpoint was reached, correct?

4. 
   biggytre | Feb 01, 2023 11:10am | #12

   @Shrayd

   1) Yeah, the MHK2 didn't help the overheating problem. It was set up to sense the temp (opposed to the built in sensor in unit) and it was also programmed for "fan off" after reaching set point. None of that helped. The one room where I installed the MHK2 is a multi-zone system with 2 heads (one 18k BTU downstairs kitchen, 1 6k BTU in bedroom above garage). Bedroom above garage is where I installed the MHK2. When kitchen head called for heat, the bedroom head would spit out heat (regardless if it was 1, 2, 3, 4, 5, etc. degrees over the set point in the bedroom already). This was particularly an issue in this room, because we don't use it much and I just sort of leave it at 60F over the winter. I think having an external temperature is good (because the built-in units aren't super accurate), but it doesn't actually solve this multi-zone overshooting set point issue. Mitsubishi doesn't want to admit that, but the techs said this is a pretty common issue with this Mitsu multi-zone systems.

   2) I actually have Emporia Vue set up so I have second-by-second data on all my heat pumps. Unfortunately, I didn't do a close study of the "before wire cut" data, but overall - It definitely appears I'm using less electricity post wire cut.

5. 
   MattyT | Feb 24, 2023 05:12pm | #13

   Hello Shrayd. I have a three zone Mitsubishi system which overheats as well. I live in Portland, Maine and have used these heat pumps for about 4 heating seasons. These units tend to overheat bedrooms in a multi-zone system; that happens because the system is providing heat for one room (maybe a living room at 70 °F) while you only want 65 °F in a bedroom which is much smaller. The system works to satisfy the large room's heating requirement and ends up supplying too much heat to the bedroom. This is a well known engineering defect in Mitsubishi heat pumps. Some suggest cutting one of the jumpers on the board to stop the fan when not calling for heat. This sorta works...but not really. It caused one MSZ unit to have more erratic heating; it caused an MFZ unit to totally stop heating and now the fan never runs. I would say, DO NOT let anyone cut the jumper. If the installer does it, make sure they will replace the board if needed. I'm now getting at least one board replaced under warranty. I have a lot of experience with this problem; let me know if questions. (I used to live in Belmont)

6. 
   shrayd | Feb 28, 2023 09:21pm | #24

   Hi MattyT@, to connect the dots, I was able to successfully solve my overheating/overcooling and short cycling issues with new MHK2s for each head unit as well as cutting jumpers for the smaller bedrooms. See https://www.greenbuildingadvisor.com/question/is-my-mitsubishi-heat-pump-short-cycling for full context.

4. 
   Deleted | Jan 24, 2023 02:23pm | #9

   Deleted

5. 
   biggytre | Feb 25, 2023 10:26am | #14

   @MattyT what do you mean by "more erratic heating"? I'm curious. Also, isn't the jumper cut remedy official Mitsubishi protocol/recommendation, so it wouldn't void warranty in any scenario? Have you heard of other folks having issues with the jumper cut remedy? I've only seen good things online, so I'm curious.

1. 
   MattyT | Feb 25, 2023 10:45am | #15

   Hi, By "more erratic heating", I meant that if you cut a jumper and continue to sense the temperature for the room on the temp sensor which is inside the case of the wall unit, it will get falsely elevated readings. Therefore, there will be a lag in restarting the fan; the room will get colder than it should before the fan restarts and the range of max & min temperatures in the room will be wider when the jumper is cut. If you read that service bulletin #1036 carefully, note on the last page in the paragraph, it says, "This will also require the temperature sensing be done remotely from the indoor unit." In my case, Mitsubishi advised to cut the jumper to stop the fan but they did not install a remote sensor as recommended in bulletin #1036. As a result of this change, my main bedroom temps (an MSZ wall) tend to run lower by about 6 degrees in winter with a +/- range of about 4 °F; this actually is not within specifications for the unit. In the guest bedroom which is an MFZ unit, the fan basically never runs as a result of the jumper cut; that causes the gas fired boiler to call for heat and the heat pump is not working at all. Just this week the installer was suggesting that I spend $500 on MHK-2 sens0rs to resolve the issue. I'm not going to spend that much to fix what is really an engineering issue. Consequently, I've been in touch with the Consumer Protection Division at Maine's Office of the Attorney General. Maine has a law which should help to resolve this in my favor.

1. 
   biggytre | Feb 25, 2023 11:10am | #16

   Ahh, I see what you're saying. I never had that issue because, as step #1, my HVAC installer installed the PAC-USWHS003-TH-1 external temp/humidity sensors on all my wall units (except the one that got the MHK-2), so none of my wall units are reliant on the internal temp sensor (which I've heard it just notoriously finicky). I believe Mitsubishi covered the cost of the PAC-USWHS003-TH-1 (I certainly didn't pay for it). My installer was Mitsubishi "Diamond Contractor" for whatever that is worth.

   Have you tried the PAC-USWHS003-TH-1? All four of my indoor units that got the jumper cut and are paired with the PAC-USWHS003-TH-1 have been operating perfectly for the past two winters. I also don't have any MFZ units, all of mine are MSZ so I can't speak to that.

1. 
   shrayd | Feb 28, 2023 09:44pm | #25

   FWIW, my installer is a Diamond contractor as well and they covered the cost for all 5 MHK2s.

   I always assumed that the jumper cutting strategy would involve external temp sensing (either MHK2 or PAC-USWHS003-TH-1), and based on my experience, I wouldn't recommend cutting jumper while using the temperature sensor inside the head unit.

2. 
   maine_tyler | Mar 01, 2023 10:55am | #30

   "my HVAC installer installed the PAC-USWHS003-TH-1 external temp/humidity sensors on all my wall units (except the one that got the MHK-2), so none of my wall units are reliant on the internal temp sensor (which I've heard it just notoriously finicky)."

   Bigtree or others,
   Are you saying the PAC-USWHS003-TH-1 can provide external control like the MHK2 but for a significantly reduced price? What's the reason to use an MHK2? My goal is simply external thermostat control.

1. 
   biggytre | Mar 01, 2023 11:37am | #31

   I have both (an MHK-2 on 1 unit and PAC-USWHS003-TH-1 on all 7 other units). Others who know more can chime in, but I think the main difference is that the MHK-2 offers a visual display of temp/humidity readings and allows you to control the unit (fan speed, temp setting, etc.) on the actual MHK-2 (if you prefer that to the remote that comes with the unit for whatever reason). But for me, I don't really care about any of that, so the PAC-USWHS003-TH-1 works fine for what I'm looking for and that is: Provide an alternative to the built-in temp sensor that communicates with Kumo Cloud via the wireless interface (PAC-USWHS002-WF-1) phyiscally installed on the unit.

2. 
   maine_tyler | Mar 01, 2023 04:47pm | #32

   biggytre,

   thanks, I started another thread because I have some more questions...
   https://www.greenbuildingadvisor.com/question/remote-sensing-options-for-mitsubishi-mini-splits?discussion=new

6. 
   MattyT | Feb 25, 2023 11:25am | #17

   Yes indeed. I've been thinking about the thermostat adapter too and it seems like a better solution. In my case I would pair it to an Ecobee thermostat and allow that to handle everything. My contractor was actually a "Mitsubishi Diamond ELITE Contractor" which is probably the same thing as your contractor's credentials.

   I actually had a breakthrough on this a few minutes ago. I've been playing with the Ecobee and the Flair Puck with HomeKit integration. I just created a rule in HomeKit automation which will monitor the bedroom temperature at night and turn down the temperature setting on the living room if the bedroom gets too hot (if the Living Room is calling for heat, the bedroom can get too much heat). I'm pretty sure this will totally automate the resolution. In this case, the Puck is relegated to simply serve as an IR controller for the Mitsubishi. The Ecobee does most of the work and the HomeKit automation rule acts as a stopgap to keep the bedroom cool. Stay tuned...

1. 
   shrayd | Feb 28, 2023 09:56pm | #26

   I was looking at App Note 3062 for this (http://mylinkdrive.com/viewPdf?srcUrl=http://s3.amazonaws.com/enter.mehvac.com/DAMRoot/Original/10006\Application_Note_3062_ME_-_Thermostat_Interface_2_Sequence_of_Operations.pdf) and will admit that it seems to introduce a non-trivial amount of complexity in the setup. Plus, more vendors/ecosystems/hoops to jump through and fingerpointing when things don't work as intended.

   On top of that, you'll need 1 Ecobee per head unit which isn't cheap - curious why not just get an MHK2 and stay within the Mitsubishi ecosystem? Or Kumo Cloud things?

   Lastly, while your solution totally works, have you looked into the efficiency/electric consumption side of things? It's generally best to let the heat pumps self modulate vs turning on/off or setting deep setbacks.

7. 
   lyoung_veic | Feb 25, 2023 11:29am | #18

   Hi biggytre. Wow this really picked up steam with shrayd's question. The phenomenon is called heat bleed and it's only partly related to oversizing. The MHK2 or other remote thermostat doesn't do anything for heat bleed. True the entire system circulates refrigerant when any zone is calling. All multizone heat pumps do this. I have talked to several of the big manufacturers and they all design this way. As I understand there's a need to make sure the lubricant stays suspended in the refrigerant and that's why deliberate heat bleed is designed in.
   Given that the three heat pumps together are oversized for the house, and in particular oversized during the part-load conditions when heat bleed would be most pronounced, you could try running the house with just the two 24k heat pumps. You wouldn't have quite as much control in the rooms where the indoor units are off, but in a well-insulated house you might be fine on comfort. When it gets cold enough that you need that third heat pump probably the heat bleed will be less of a problem.
   shrayd, we don't know as much about your house; perhaps you don't have more than one heat pump. But fundamentally the only way to entirely stop the heat bleed is to not call for heat on the heat pump that is contributing to heat bleed, so if you have more than one heat pump, see if you can get along without the problem one during mild conditions. Disabling the fan during standby does slow how quickly heat is delivered to the room, but it's not going to stop hot refrigerant circulating through that indoor unit.
   This happens in cooling mode too, but I would guess it just doesn't rank as a "problem" that one room in the house is cooler than the others. Probably helps with dehumidification.
   Finally, underconnecting a multizone heat pump with indoor units is very bad for efficiency. Better to overconnect (for example, three nines on a 24k outdoorunit). biggytre, I'd ask your installer to swap out that big heat pump.

1. 
   biggytre | Feb 25, 2023 03:04pm | #20

   @lyoung thanks for the info. My original post was from 1.5 years ago (before I had the jumper cut on all 4 of my upstaris bedroom units), so the heat bleed is no longer a problem in my case. Sure, there still might technically be some heat "bleeding" from my upstairs units when the downstairs units are calling for heat and the upstairs units aren't, but the upstairs units now keep the temperature exactly at set point becuase the fan is (forced to be) off due to the jumper cut.

   What I was trying to say is that the remote temperature sensor PAIRED WITH the jumper cut does fix the heat bleed problem (at least on my Mitsu system). Alone, you are correct, the external tempearture sensor wouldn't do anything for the heat bleed.

   Honestly - I thought my system was oversized until it reached -13F here a few week ago. My system was having trouble keeping up with that (set point at like 68 and downstairs couldn't get over 62 in the morning). I actually took that as a GOOD sign that the system isn't oversized. That -13F was the lowest temperature in like 40+ years in Boston region. But as you said, my system is "oversized" for shoulder season heating. I may play around with only running 2 of the 3 compressors during the shoulder season.

1. 
   biggytre | Feb 25, 2023 03:15pm | #21

   P.S. I could be using the term "heat bleed" incorrectly. To me, there are 3 scenarios for heat delivered by the indoor unit:

   1) You want heat and the fan on that unit is on (normal heating, good boy heat pump)
   2) You don't want heat and the fan on that unit is on (high heat bleed, bad boy heat pump)
   3) You don't want heat and the fan on that unit is off because you cut the jumper (low heat bleed, not a big deal because it's not overshooting set point)

2. 
   Deleted | Feb 26, 2023 09:43am | #22

   Deleted

1. 
   biggytre | Feb 27, 2023 09:29am | #23

   Right, the system is definitely not undersized. I guess my point is that the system is not "dramatically oversized" (realize this isn't a technical/quantitative term). 99% deisgn temp is +1F. It has no trouble keeping up with set point in the -4F to +1F range, but the -13F day gave it some trouble, which I take as a sign that it's ballpark appropriately sized (but I am not an energy modeler!).

3. 
   shrayd | Feb 28, 2023 10:01pm | #28

   +1 to what biggytre said. I'm another data point where MHK2s + jumper cut addresses this issue (https://www.greenbuildingadvisor.com/question/is-my-mitsubishi-heat-pump-short-cycling?discussion=edit#comment-245142 for additional details).

   I think the key point here is that remote temp sensor needs to be PAIRED with a jumper cut.

2. 
   shrayd | Feb 28, 2023 09:59pm | #27

   Thanks! To close the loop, I was able to successfully address overheating/overcooling and short cycling by installing MHK2s per head unit as well as cutting the jumper on the smaller head units. See https://www.greenbuildingadvisor.com/question/is-my-mitsubishi-heat-pump-short-cycling?discussion=edit#comment-245142 for additional details.

1. 
   biggytre | Mar 01, 2023 03:43am | #29

   Nice! Glad to hear it worked out for you!

8. 
   jwasilko | Feb 25, 2023 01:12pm | #19

   link to App note 1036 about bleed:

   http://s3.amazonaws.com/enter.mehvac.com/DAMRoot/Original/10009/Application%20Note%201036%20ME%20-%20Applying%20MXZ-C%20Multi-Zone%20Systems%20-%2020190110.pdf

9. 
   BLinc117 | Feb 15, 2024 04:21pm | #33

   So I am dealing with this issue with the fact my MSZ-FS06 units bleed too much heat.

   Trying to resolve it i installed a MHK2 kit, (already have Kumo Cloud). I also cut the JR01 "jumper" (which is actually a resister?).

   The MHK2 s controlling the unit fine, however if its not calling for heat (say its reporting 70f and its set to 65f) the fan is still spinning slowly :(

   At a loss of what to do here, is there something I missed?

   1. Installed MHK1 unit - configured successfully
   2. Realized the Jumper thing was requiredD
   3. Powered down entire system
   4. Cut the Jumper JR01 (Actually a resister)
   5. Booted system up
   6. Tested
   7. Was sad

   PS I did go into Kumo cloud and made sure thermal off was set to "off in heat" - but my understanding is MSZ ignores this, also my understanding is that with MSZ there are not settings in the MHK2 for fan behavior (fan is set to auto).

10. 
    happyheatpump | Feb 28, 2024 04:35pm | #34

    I believe that with or without a remote sensor(MHK2 or kumo/remote sensor) once the jumper is cut, the fan will still run in standby mode until another zone "calls" then the fan will shut off while that particular coil is hot. This stops the bleeding and but allows circulation for the most part. I am using a kumo with remote temp sensor and on my cassette module it was a JRTU resister that needed to be cut. No more over heating on that zone when the larger unit calls for heat. I have heard some other locations such as JR24 and JR01 get mentioned and I am thinking it is simply dependant on model and generation of equipment