# Forum Home Renovation Heating & Cooling  Reverse Cycle A/C operation

## pcal

Hi guys, 
I have a 2 horse split system that is acting up. It works perfectly on the cooling cycle, but won't do anything when switched to Heat - which is what we want from it right now!... 
I have metered the signals between the indoor and outdoor units, and suspect a failure of the indoor control board. 
There are 3 x 240v control connections between the two, plus the main power to the outdoor unit. In cooling mode, the outdoor unit receives 240v signals to turn on the compressor and the blower fan, but not the solenoid. In heating mode, it doesn't receive any 240v signals at all. I'm guessing it should receive all 3 - ie compressor, blower, and solenoid. My assumption is that the solenoid is responsible for reversing the flow of gas, thereby reversing the cycle to heat rather than cool. 
I have electrical / electronic qualifications, but refrigeration is deep black magic... 
If I'm on the right track, then a replacement control board should fix my problem - if not, then I'm hoping there is some wisdom here that may light my path. 
Yours in anticipation, 
Pcal

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## Claw Hama

Pcal sounds like either the remote/controler or the pc board as you say. What brand unit is it?

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## pcal

Thanks Claw, 
It is a Mistral unit - Model MISS25SS. I'm fairly sure the IR Remote itself is OK, as I have another (smaller size) of these units in another building and the remote works on that one OK. 
I don't suppose you have any idea about a parts supplier? I've tried ringing the number in the book that came with it, and all I get is an answering machine that asks for my credit card number so they can send someone around. 
Pcal

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## Claw Hama

I have seen new units at Bunnings you could try and find out through them or try your local electrical wholesalers mistral is a brand that makes a lot of exhaust fans etc. that are sold through electrical wholsalers. Or some of the standard refrigeration wholesalers like Heatcraft, check the yellow pages. Good luck

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## patty

I have the same unit and I thought I had the same issue but I waited at least 5 minute delay for the unit to start up when selecting the heat mode I dont know what it is as Im no expert on A/c units or if it has anything to do with the heat exchange unit or what.... 
So turn it on to heat and wait 5 or 10 min to come on if it does not start then you obviously have other issues

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## Claw Hama

Hi guys, some reverse cycle units run through a defrost cycle before they go onto heat and others will not turn the indoor fan on until the indoor coil temp is up to about 40-50deg c so you don't get a cold breeze. Once the coil is hot the fan starts and you get warm/hot air.

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## kompsj

Here are some fault finding tips if you are slighlty technical minded.  *Yes* it can take a few minutes for the outdoor pump to start. 
Is the outdoor pump and fan switching on?  If it is, and no heat, the pump is usually stuffed *or* low on gas(leaking somewehere) *or* unit has been contaminated with air or moisture (needs pumpdown and re gas) 
The only difference on the board from heat to cool is it switches a solenoid to reverse the refrigerant flow.  So listen for a clicking noise on the outdoor unit.  The solenoid just slides from left to right.  Inside this part looks like a round cylindrical piece of salami with pipes going in.  The rest of the logic is the same.  The external pump turns the same way.  I would suspect your thermo sensors are faulty.  If they are sensing temperature (in heat mode) higher than what is on the remote, then they will not switch on the outdoor pump. The sensor is usually visible when you take the filters off the indoor unit.  Try putting something cold on the temp sensor like ice. 
Another thing to try is to unplug from wall, disconnect the terminals and re connect.  I have seen this happen once before on this model.  It was bizarre... 
Obviously, make sure the remote has the picture of the sun on and the set temperature is higher than the room temperature :Doh: .
----------------------------------------------------
Jim Komps 
Air Con Installer.

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## tricky4000

When we first had it installed, I tried flicking from heat to cool on my Mitsubishi Electric unit and it took over 10 mins to make the change.

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## kompsj

You are spot on. One more factor into your equation.  The temperature sensor in the indoor unit will measure temperature against the setting on the remote control.  If the temp in heat mode is lower than the remote setting, it will fire the solenoid, pump and fan!
Some hints if your electrically minded.
Pull boards out and (unplug power cable from wall first or shutdown power to unit)
a) smell them for burning
b) check for any burnt parts
c) check all the solder connections on the board as I have seen these go bad heaps of times.  The older fixed speed compressor boards are repairable if you understand electronics well.  The new inverter boards are usually a throw away as most are multi layer and micro soldered!
------------------------------ 
Jim Komps
Air Con Installer.

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## Timmo

Could be a dicky relay on the indoor unit's relay board, or, if there's 240v at the valve then the reversing valve could be stuffed. Also might pay to check the interconnecting cables.

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