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Sly
Premium Member
join:2004-02-20
Tennessee

Sly

Premium Member

[HVAC] Limit switch to protect compressor

I'm wanting to add 2 limit switches to my indoor HVAC blower cabinet. One switch will be a high limit and the other a low limit. I want to wire these switches so that they will turn off the compressor in case the indoor fan ever fails. Both switches will be wired in series to the 24vac transformer on the secondary side. In case the fan fails, the coil temperature will either rise or fall depending on if I am in heating or cooling mode. Once the temperature moves outside either limit, the switches will open and turn off the compressor.

My question is what temperatures should I shoot for? I'm guessing that the lower limit switch will need to be set higher than 32F to prevent coil icing. Probably around 35F or so... What about the upper limit switch? What is a safe upper limit for the condenser when in heating mode?

I have a variable speed indoor fan control to help control humidity. The blower is old but still appears to be in good shape. I have cleaned and oiled the fan and try to keep everything maintained. However, I know that varying the fan speed can put added stress on the motor windings and to be safe, I would like to add some limit switches that will protect the system if the fan should ever fail.

Also, I am having trouble finding limit switches (or snap disc thermostats) that are curved. Most are flat, designed to mount directly to an air plenum. I want to mount these limit switches to the refrigerant tubing so that they can monitor the line temperature. I have seen some limit switches that are designed to be mounted directly to tubing (that have a concave curve for mounting) on defrost controls. But I am having trouble finding one that operates above 32F.

Long story short, I need help finding 2 manual reset limit switches that have a curved mounting surface for tubing and I need to know what are the safe upper and lower operating ranges of an HVAC system before the switches need to open. Thanks.

mackey
Premium Member
join:2007-08-20

1 recommendation

mackey

Premium Member

Honestly an airflow switch might be cheaper, easier to install, and easier to find/build. It would simply be wired into the yellow "compressor run" wire.

/M

Msradell
Premium Member
join:2008-12-25
Louisville, KY

Msradell to Sly

Premium Member

to Sly
There is already high and low overpressure protection for the compressor that is built in as part of the original design. These will trip out the compressor prior to failure if you're inside blower fan ever fails. If no need to add more protection and it would just make things more confusing to troubleshoot in the future.

enon
@50.153.114.x

enon to Sly

Anon

to Sly
Even better than convoluted temperature limit switches or airflow switches is a simple current relay. You'd put it in the return path of the blower motor circuit. If there's no current flowing, it opens up the control line to the compressor.

mackey
Premium Member
join:2007-08-20

mackey to Msradell

Premium Member

to Msradell
said by Msradell:

These will trip out the compressor prior to failure if you're inside blower fan ever fails.

Not always. At work we had a piece of insulation come unglued inside the unit and completely block the coils which then fried the compressor.

/M

Lurch77
Premium Member
join:2001-11-22
Green Bay, WI

Lurch77 to Msradell

Premium Member

to Msradell
said by Msradell:

There is already high and low overpressure protection for the compressor that is built in as part of the original design.

Not common on residential systems, except for the high end units. But they are very easy to install.

Sly
Premium Member
join:2004-02-20
Tennessee

Sly

Premium Member

The problem I may have with a current relay or a airflow sensor is that I don't know if it would interface well with the variable speed fan control. I have an ICM controls CC750-230: »docs.google.com/viewer?u ··· 52-3.pdf

It delays turning the fan on until the refrigerant temperature reaches the set-point. When the system initially comes on, the indoor fan doesn't power on until the line pressures stabilize. So for about 10-15 seconds there is no fan. I'm afraid that with a current sensing relay or an airflow sensor, they will detect the fan is not running and prematurely turn off the compressor.

However those are great ideas. Maybe I can add a time delay device so that the relay or sensor can't kick out the compressor until at least a minute has passed. This would, of course, add to the complexity.

The outdoor unit does have a low pressure switch but not one for high pressure. I'm not sure if the low pressure switch is adequate to prevent floodback to the compressor in the event of a fan failure. I really don't want to rely on that to offer the only means of protection.

Tex
Dave's not here
Premium Member
join:2012-10-20

Tex

Premium Member

Is any of this stuff still under warranty?

Also, can you provide some model numbers of the equipment?

Sly
Premium Member
join:2004-02-20
Tennessee

1 recommendation

Sly

Premium Member

It's an old Trane XL1200 2.5 ton r22 heat pump. Built around 1989. I've basically rebuilt it from the ground up. New scroll compressor, head pressure control, all new electronics, de-superheater to heat the house's water when in AC mode. Replaced the metered orifice with a temperature expansion valve...

The indoor air handler has an ICM CC750-230 variable speed fan control. It drops the fan speed whenever the de-humidistat calls for reduced humidity. Normally the unit operates at a 20*F temperature differential between return air and coil temperature. When the de-humidistat calls for humidity removal, the CC750 drops the fan speed to maintain a 27*F temperature differential. By keeping the humidity low, I don't have to set the AC as cold to be comfortable. I used to keep it at 74*F but now leave it at 79*F with reduced humidity.

The indoor fan is completely controlled by the CC750. The house thermostat is wired into the variable speed fan control and the microprocessor then controls the blower depending upon what the thermostat is calling for. The CC750 has two thermocouples that are used for monitoring. One is mounted in the return air duct and the other is mounted directly to the coil. When the system calls for cooling or heat, the fan is left off until the system pressures (temperatures) stabilize. Once the coil temperature reaches the setpoint, the fan ramps up smoothly to 100% to oil the motor and then ramps back down to about 25hz. From there it ramps up and down to maintain differential until the de-humidification call is satisfied. After that the CC750 goes into bypass mode to run the fan at full speed for maximum cooling.

In heating mode I have it wired into a whole house humidifier. When the system calls for heat, the humidifier is energized. If there is a need to raise the humidity, a signal is sent to the CC750 to reduce the fan speed. The reduced fan speed will raise the air temperature coming out of the system. The higher air temperature helps the humidifier work more efficiently until the humidity call is satisfied. After that, the CC750 switches back to bypass mode and the fan runs at full speed again.

In both heating and cooling mode, the fan is delayed for approximately 10 seconds. Sometimes more, until the coil temperature reaches proper operating temperature... In this setup, I thought that adding a couple of snap-disc thermostats would add a little bit of insurance for the compressor should the inside fan ever fail. As said earlier, this system doesn't have a high pressure switch, only one for low pressure. I think that the low pressure switch is designed to shut off the system in the event of a refrigerant leak. I don't know if it is sensitive enough to prevent flood-back to the compressor in the event of an indoor fan failure.

I think that if I used an air-flow sensor or a current sensing relay for the fan, I would also have to incorporate a time delay relay so that the sensors are kept out of the system until the fan actually turns on. This would be no problem as I already have an adjustable relay sitting around. However with the added complexity I still would like to just use 2 snap-disc thermostats to limit the high and low temperatures of the coil. The thermostats would not actually interface with the variable speed fan but would only monitor indoor coil temperature. If the fan were to ever fail, then the coil temperature would fall outside of the extreme and the snap-disc would open, cutting power to the compressor.

The heat pump is a Trane XL1200 model TWX730B100A0 and the air handler is a model number TWV030B140A0. The metered orifice has been removed from the air handler and replaced with a TXV and the variable speed fan control was added.

Cho Baka
MVM
join:2000-11-23
there

Cho Baka to Sly

MVM

to Sly
lol, I think that is a step further than model numbers.

Sly
Premium Member
join:2004-02-20
Tennessee

Sly

Premium Member

Yea, I figured it was tl;dr for most people. Just wanted to include some background for anyone interested. It's been a fun little project retrofitting this old heat pump. It runs more efficiently now that it did when it was new. Even better than many made today.
iknow_t
join:2012-05-03

iknow_t

Member

said by Sly:

:) Yea, I figured it was tl;dr for most people. Just wanted to include some background for anyone interested. It's been a fun little project retrofitting this old heat pump. It runs more efficiently now that it did when it was new. Even better than many made today.

if you still want to use klixons you can use the flat one's if you have an adapter made at a machine shop. have it made out of copper bar, flat on one end, with the concave shape on the other to fit the tubing. I wouldn't go over 1/8" thick for that..(distance between the copper tube and the klixon)