Mitsubishi Minisplit fan operation in heat mode question

TAG

The fan on my wall unit never seems to modulate (much) when in heat mode like it does when cooling ... it falls behind often two degrees. Fan will change if I push it up to 80 or place the fan on max. I get lots of heat

Should it not work the same as when cooling? I know some here do a lot of these.

This is a 30k multi compressor attached to 3 heads -- one of them a wall with the eye and two ceiling units. I'm talking about the wall unit here. The ceiling units work differently.

I normally don't really use them for heat as the house has hot water panels and radiant -- but a leak in one of the manifold has the system down and I have been using the heat pumps in the house. Pre - covid we were not here all the time and we would use them as an additional boost when we arrived -- but in those cases I was bumping them way up to 80.

GW

There should be some sort of modulation with the fan speed. Your positive you have the remote on fan —-auto?

pecmsg

in heat mode there is a minimum to ALL heads. If there’s not enough load that fan won’t slow down. 

TAG

Have the fan on auto. Noticed it the other day in the AM -- think I had it on a 68 night schedule going to 72 at 7am. It was 69 at 8am ... so I hit the fan to max on the control and it went to high fan and quickly heated the space. I'm using the MHK1 (?) vs the hand remote to control the three heads. I have one for each and the internet link. I did note that both green lights were on the wall units and I think the second green indicates it out of temp by two degrees? When I got the ceiling units I believe the only way to control was using the MHK .... or maybe some wired.

I do understand the strange workings of the multi heads .... not always ideal I may add. We had to switch out the two ceiling units so the remote had the room temp and also have it set for no fan when satisfied. Otherwise the heat flowing all the time would overheat the rooms. Even on what they call super slow fan it was too much air. In heat mode when the temp sensor in the unit is used -- the constant flow of the refrigerant fools the unit and it never goes back on.

The wall unit I have set so the unit itself has the temp -- not the remote.

pecmsg

I’ve only noticed that issue on oversized equipment. How was the size determined?

TAG

It's properly sized. But -- why would a system not ramp up it's fan when it's 3 degrees low in the room .. especially if it's oversized? I could see if it was undersized and it was so cold out that the system could not provide enough heat to the heads on setback return. The other two heads were satisfied.

This unit takes care of two large rooms on the second floor of a cape with the ceiling units and the wall unit heats a new addition off the side of the first floor. 3x12k on a 30k compressor ... we actually had talked to Mitsubishi corporate about sizing back when it was installed in 15.

GW

"size" and 'Multi zone' are hard to say in the same sentence. If you gotta honker multi-zone and a small 6k head trying to heat a 12' x 15' bedroom- it's oversized right off the bat. No gettin around it. Because it's a multi- you may not see a low air flow, based on what else is going on (with the other two heads) and how fast the room heats or cools. Minis have a mind of their own

pecmsg

TAG said:

It's properly sized. 

# of indoor heads to the outdoor unit I’m sure. 

For the minimum heat output………doubtful 

TAG

Each of the rooms is about 25 x25 (one a bit more the other a bit less) ... the addition is a bit more. They all have a lot of glass. The upper two room have sliders with balconies and the addition is glass on two of the sides. It was really sized for AC as it's at the beach. The turndown of the compressor and heads worked ... going to a 9k head was about the same as the 12 in the ceiling units and 24k was questionable.

I just don't get the fan. If in AC mode and I walk in the room and I have it set on 75 and want it cooler ... moving it down medially causes a fan to increase/ ramp up

pecmsg

Sized for A/C. 

Where is this house located?
what size outdoor and indoor 

TAG

The house is on the coast of NJ ... 3x12K heads with 30k compressor.

As I said above we don't typically need the heat as the house has hot water .... we use the heat pumps in the shoulder seasons occasionally ... and maybe a boost for the addition.

If the coils in the wall unit are hot and the room is under temp -- I don't get why the fan is not working in "auto" like it does in in AC mode. If I manually place the fan on a higher setting it will cycle on and off and maintain the temp using that setting.

I wonder if the "eye" needs to see someone in the room?

Don't know if they changed how the fan works with the new ceiling design -- but the older type that I have does not have an "auto" fan setting and they don't work as the wall type. They are on/off at the picked fan setting. Even on low they blow too much for constant use ... mine also don't have the "eye" .

pecmsg

1000 Sq Ft or 10,000?

TAG

It's doing about 1800 sf of the house -- but the second floor also has a hall way and a large J&J bathroom -- the addition is off of the kitchen. In the summer with people and a lot of glass in all the rooms we needed the 2.5T ... the one bedroom was questionable with a 9k in the summer if the house was closed up and we needed it on a hot day. Mitsubishi had us do the 36k of heads on the 30k unit as it was within specification and the best way to set the system up. The heads don't perform as well as if on a single compressor and 24k was not enough on the hottest days. It's a beach house -- so we entertain and it's hot.

My new house is going to be three individual units as the use and layout don't lend to a multi the way it will be used. The beach house always has all three "on" even if the upper rooms are not used -- we leave it up high (78/79).

Again --- I don't understand the oversize concern. It's not overheating -- it's failing to reach new higher set points fast enough.

GW

Try killing the power for five minutes, maple battle wake it up a little bit

pecmsg

That’s twice the sized for A/C and heat. 

TAG

pecmsg said:

That’s twice the sized for A/C and heat. 

Even if you think it is ..... it still does not explain the fan. I'm all in with mini splits and the minimum output of the compressor and minimum of the heads. With the constant circulation of the refrigerant the heads just don't perform the same. I would agree with you if the spaces were overheating ... They are not

Can get the smaller room to slightly overheat with higher fan set speed if the other room is at full set point on a mild day ... but there is no reason to set it that way. Was playing around with it.

The need for constant circulation of the refrigerant to all of the heads makes sense when it's explained. The problem is the manuals don't explain the operation properly ... even the diamond dealers don't seem to have people that full grasp the way they work. I went through this last year on my new project -- they were telling me something I knew not to be correct.

pecmsg

You aren’t running enough heads to meet the minimum heating requirements.

Has the installing contractor contacted tech support, pulled and weighed the charge?

TAG

The heads are running ... it was 28 degrees out the other morning. The problem is the wall head is not turning up its fan on "auto" when the schedule on the thermostat bumps up the temp in the room.

If I set the fan on high ..... it will blast the room with hot air and turn down and then blast again -- keeping the proper set temp. Same if I pick medium .... it will cycle on and off. It's not modulating the fan on auto.

My memory was in cooling it would go down to 6000k and with heat it was a bit more -- under 25%. The problem is in cooling it does not make full 30k .... it's in the range of 28. So when everything is running my memory was 9k ... but, in reality it's going to be acting more like zoning so no one space will need everything.

All of them are oversized in heating when it 50 degrees out when you look at the specifications -- I think the 30 can make 38 or something like that.

I have almost the same set up in another house ... but, I will not be there for another couple of months.

pecmsg

These units operate differently in heating then cooling.

Has the installing contractor been there, you may have a charge issue!

pecmsg

There has to be a minimum refrigerant flow in heat mode for proper oil return. To accomplish this they open the EEV’s to ideal heads. If that’s not enough then they run the calling had fan higher in an attempt to remove the heat. 

GW

6000k min- that’s what I once thought. Mitsubishi rep told me that was a bit of a error in the data. That 6000 means “the smallest size head”, not the minimum capacity of the outdoor unit. 

Look at the data on a big eight zone unit. Same 6000 as mentioned.

The rep told me the minimum BTU capacity on the multi zone outdoor unit is very roughly 50% of the total capacity. Below that, it’s going to cycle. 

That’s why we sell a bunch of single zones, the performance of a single is superior to that of a multi. I get it, some people don’t want a gaggle of units sitting outside their house. Pick your poison

TAG

GW: That's strange .... why would the multi no be able to run at the 25% All of the others can and even lower. We were talking to engineering back in 2014/15. Not the local rep -- that's who gave me the information. Performance charts .. etc. Living with these system I can't see how I'm getting a minimum of 15k of cooling or heating out of it all the time. It would be too cold or too hot. I have played with the ceiling units to keep the fan on all the time and obviously doing that the unit is dumping anything it has into the space.

pecmsg: understand they are doing that (flow to all heads all the time to some degree). If indeed it is trying to get rid of heat ..... it is not thinking correctly because my problem is the fan is not ramping up.

The branch box systems have more control ... but, they scare me. We have a guy in town that has made a good living installing the Fujitsu branch box system in many of the huge Victorians in the older part of town. I have not heard any horror stories .... but -- boy, what a lot of pipe.

My new place is an odd stone buildings and the addition is open on three side and ductwork was impossible -- it also had a small room way up in the old building that we could not get ducts to property . Mini-splits are a great solution for difficult setups. Unfortunately the placements on either side of the building and the application forced using three single units. Finding space for them and the Carrier is a design problem when trying to hide services.

pecmsg

It all has to do with maintaining oil return al low loads. What exact algorithms are used I can't say. A tech needs to be on site with all the model and serial #'s and in contact tech support. The'll be able to say what's exactly going on. I still believe its just oversized.

TAG

Weather has been warmer than normal. leaving the wall unit set for med fan on the remote has the room at set point each morning and no issues through the day. It's going back to it's normal supper low fan speed after reaching the warming temp in the morning.

Since I have the wall thermostat -- it's programed to be the sensing location (default). Googling around it seems I'm not the only one who reports this operation -- I'm also not seeing any feedback on why.

With the wall units you don't get into the 101 - 128 setup table for fan control .... so, I know it's not that. The ceiling units have been working as normal all along and keeping the spaces at 72.

The reset (power off) did not seem to change anything.

TAG

Pecmsg -- Even if the unit was over sized. why would the fan fail to come on when the unit requires heat and the coils are hot. Any system designed to heat at 5 degrees outside is going to be oversized when it's 45 degrees outside. They still pump out heat? It's not too much ... it's not enough.

GW: I went back and looked at my notes from 2014 and Mitsubishi Engineering did tell me and the diamond dealer (we had the regional rep here as well) 6k/ 7.2k on the 30k head. They were the ones who told us to do the 3x 12 heads on the 30k compressor. I get that all too often these things get installed and sized with people adding up the heads and matching the compressor .. something that engineering specifically told us is incorrect. I remember them saying if you want to zone with really small heads in small spaces the way to do it is move to a branch box system.

On mild days in the summer I can see the fan running slowly and the heads are running and it does not overcool ..... one thing we did notice after it was installed was how great it was with humidity control. They work great. My other house is in SC ... so that's mostly AC as well. Both of these houses are old historic that have been added on to with no good way for ductwork extensions ....

pecmsg

Im not there
We have no pressure or temperature readings
No readings from the board(s)

You need a contractor on site.

TAG

pecmsg ..... you keep saying that and I'm just trying to find out if the units work differently (fan wise) in heat mode vs cool.

If I turn the ceiling units up to 80 they all start to blast and they will heat the spaces -- if I do the same with the wall it will not ramp up the fan (like it would if the unit was cooling). It will heat the space if I manually set the fan higher -- it will cycle on and off. So with all of them running even when it's cold out they are getting very hot.

If someone said ...... The wall heads will do that (not ramp the fan) if slightly low on refrigerant .. OK.

TAG

Called the installer ..... confused on the phone. System (outside) is properly charged and running correctly .. he had no answers for me ... he made a few calls. They have to contact tech and understand how it works when ceiling units are in the mix. ?

wmgeorge

As a old guy who when working specialized in electronic / computer HVAC controls, taught the stuff and now has to live with a Mitsubishi Mini HP in Iowa I installed. First slapping your gauges on these critical charge units is not what you do. If you suspect a charge issue, recover weigh in the proper charge.

There are diagnostics build into both the inside and outside units. My son who works as Service Manager / Estimator in commercial HVAC/R like did sells a lot of this brand and does not have issues. I think they only do single zone or two zone maybe?

These Mitsubishi HPs are way over engineered and I wonder IF anyone including the people you talk to at factory service really understand. My unit thermostat is not close to being accurate and others have commented the same looking for better.

I gave up on the Auto cycle fan and use strictly one speed but vane directions are set not to blow directly on the occupants. I would think the fan speed depends on the coil thermistor temperature.