Mitsubishi heat pump not keeping up

ksm

I have a 3 zone 30,000 Btu Mitsubishi mxz3c30nahz4. Heads are msz gs18na, msz gs12na and msz gs 06na. Most of the time I just run the 18,000btu unit which heats the entire first floor (about 800sq ft) and keeps the upstairs pretty warm as well from the heat rising up. Lately I’ve been putting the 12,000 Btu unit on at night to make the bedroom upstairs a little warmer but when I do so the 18,000 Btu unit downstairs starts struggling. By the morning the downstairs can be as cold as 64 degrees when unit set to 70 with both lights on indicating thermostat not satisfied and is blowing air like crazy with fan on auto trying to catch up. As soon as I turn the 12,000 Btu unit off the 18,000 Btu unit catches up pretty quick and settles down. If I don’t put the 12,000 Btu unit on in the bedroom, the downstairs will be 72 degrees in the morning when set to 70 and the 18,000 Btu unit will be cruising like it’s no problem. I never really put the 6,000 Btu unit on because it is a guest room. Is it normal for one unit to struggle this much when putting another unit on at the same time?

ratio

How old is the install? Did it ever work right?

I think I'd start by checking the charge. Tech Support can tell you it's not right by taking some pressures & temperatures, but the only way to know it's right is by sucking it all out and weighing it back in.

ksm

It is a little over a year old. It is hard to say if it was ever working right because the first heating season I had a firmware issue causing defrosts every 20 minutes. When I finally got the new circuit board the heating season was mostly over and I never really ran multiple units at the same time. Only the 18,000 Btu which worked great on its own. Then cooling season didn’t seem to be any issues running all units at the same time.

EdTheHeaterMan

Your home most likely needs a larger outdoor unit. Your indoor units add up to 36,000 BTU and your outdoor unit is only 30,000 BTU. and that is rated at an outdoor temperature is at about 40°F. The heat pump will deliver much less the colder the outdoor temperature gets.

I think it is time to ask the original installer what criteria was used to select the 30,000 BTU outdoor unit? Since the system is clearly not large enough to heat the home, what are they going to do about it?

Kaos

@EdTheHeaterMan That unit is a cold climate unit, this specific one delivers 2.5 tons of heat at 5F. They can be paired with up to 130% capacity indoor heads, but you won't get more than what the outdoor unit is capable out of it.

https://ashp.neep.org/#!/product/152220/7/25000/95/7500/0///0

@ksm Crank the thermostats on all 3 indoor heads. max heat, max fan. After about 15min measure the air temperature coming out of it with something semi accurate (digital meat thermometer works). You should see about 105F out asssuming 70F room. If it is significantly less than that, the unit is down on refrigerant which means you have a small leak.

GW

it’s possible you’ve lost some juice. it’s possible you have a leak.

Long lines? After 98 feet the installer needs to add juice

Can you get a basic temp rise? 30 degrees is good. 40 degrees is nails.

Once you pas your 28000 btu, you can’t expect all the heads to provide full bore power.

Mitsubishi allows for 130% connected load. You can connect 130%, but you sure won’t get 130% blowing into the home

EdTheHeaterMan

https://forum.heatinghelp.com/discussion/comment/1839188#Comment_1839188

@Kaos Said: @EdTheHeaterMan That unit is a cold climate unit, this specific one delivers 2.5 tons of heat at 5F. They can be paired with up to 130% capacity indoor heads, but you won't get more than what the outdoor unit is capable out of it.

I tend to disagree with the 2.5 ton (30,000 BTUh) at 5°F statement. It is just the way that Heat Pumps operate. you see the colder the outside temperature is, the less heat there is to take from the air and compress it to make the inside warmer. Now a Heat pump that is rated for 2.5 ton at 45°F outside temperature can not possibly generate 30,000 BTU heating capacity at 5F. It is a physical impossibility. Here is the specification sheet from the MXZ-3C30… it clearly states that the heating capacity at 17° outdoor temperature is less then 2 ton.

When it gets to 5° outside temperature, does some magic low temperature fairy come down from the sky and make it produce more heat? I think not. I still stand by my statement that the 30 is too small for the space and that the 36 may have been a better choice.

If you need to understand more about how heat pumps operate, I will be happy to learn ya some basics… Just send me a PM

pecmsg

https://forum.heatinghelp.com/discussion/comment/1839252#Comment_1839252

And add into that is the # of defrost cycles while operating at 5°F.

tim smith

Ed, this unit is NAHZ4 not NA3 as you show on the product data sheet. Different unit specs. Here is correct one. Not quite full capacity but much closer than other spec. Again max capacity does have some limitation but can reach it.

Original poster, what’s the od temp at times you’re having problems. Also of course would be necessary to have load calcs of your house. You probably don’t have that.

GGross

https://forum.heatinghelp.com/discussion/comment/1839252#Comment_1839252

You are looking at a different model Ed, many heat pumps have their rated heating capacity near 0f these days, typically we would call them "cold climate" I think mitsubishi calls them "hyper heat" models. I believe @Kaos linked to the data sheets which show 28,600 btu/hr at 5f COP1.91, which is a fairly standard output for a 2.5 ton cold climate modern inverter heat pump (mini split ODU)

just double checking the brand I am more familiar with, LG, their cold climate (RED) multi ODU 36k model, is 36k btu/hr output at -13f, obviously more heat output as the outdoor temp rises I can link the spec sheet if needed but don't want to confuse the conversation by throwing in a different model link here

ksm

I have had this issue when the outdoor temp is as high as 35f. If I only run the 18,000 Btu by itself it has kept the downstairs at 70-72 when set at 70 when outdoor temps are into the low teens to single digits. This is why I’m so confused. I understand the machine has limitations but to completely lose it at 35f outside and only running 2 inside units seems ridiculous for a hyper heat condenser. I have measured discharge air temp with a meat thermometer at up to 110 f when only running the 18. When I turn the 12 on simultaneously the highest I’ve measured is in the 90s

pecmsg

you need to get the charge on that your check.

You need to have.

it’s a very complicated machine. I need somebody that knows what they’re doing and with the tools to do it.

tim smith

I would not be surprised if low on charge from start or a leak. Someone will need to weigh out charge and then calculated what’s needed and weigh back in. Talk to installer and see what he charged with? Maybe just used pre charge weight? Knowing what it started with will help determine if leak after current charge weighed out. Takes competent refer tech.

yellowdog

This really sounds like a design issue to me. You probably don't have any of the heat loss data for the rooms or the outdoor design temp they were calculated at. Get the charge checked and weighed to see if it is correct. If it is, you were swindled from your installer from the start. if it is low, you had a sub-par installer.

Kaos

This has nothing to do with Man J or heat loss. The unit is simply not running properly.

Mitsubishi publishes a correction factor for defrost. It is actually not that much once you get bellow freezing:

@ksm If you can't get hot air out of all heads when cranked, the unit is down on juice. 99% of the time a multi split operating strange is a charge issue.

bjohnhy

Its definitely not functioning as designed. Hopefully just a charge and not a valve problem.

My house has the non hyper heat version with 3 indoor units. It has no problems like that (keeping all heads roasty toasty) when outdoor temps in teens. Efficiency is another issue though.