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Old 03-07-2010, 09:35 PM   #1
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Need help from control guys or heat pump gurus (reversing heat pump defrost cycle)


I'm trying to design a system that will cool a room down to 55 to 65 degrees. I know high temp systems are not designed for this, but I'm hoping you guys are willing to help anyways. It will not hold any perishable items or cause any safety issues. It is just a project I need to complete. It will only run for short periods of time a few times a year. I was hoping someone could help me that has heat pump knowledge or control knowledge. I want to make sure I'm not missing anything.

As you guys know, there are problems associated with bringing the temperature down this low with a high temp designed system. I was originally going to just install a freeze stat on a straight cool system to prevent ice build up on the coils. I started to think that I might be able to incorporate a heat pumps defrost system into my application. I would basically run a heat pump defrost cycle in reverse. I'm working with an old Trane heat pump split.

Here are some of the problems I can think of:

-Trick the defrost board into thinking it is in the heat cycle.
O signal will not go directly to the defrost board. It will go to a relay (see reversing valve operation).

-call for defrost
Condenser coil sensor will be moved to the evap coil. The outdoor ambient sensor will be removed and a resistor mimicking a certain temperature will be installed. I don't really know what the exact purpose of the outdoor ambient sensor has with the defrost system.

-reversing valve
90-341 relay installed to control operation. Normally closed set of contacts will have 24v (R) going to reversing valve. The coil of the relay will be controlled by the orange output from the defrost board. When there is a call for defrost, the o signal will actuate the relay and open the contact points. This will cut power to the reversing valve putting it into the heat cycle.

-outdoor and indoor fan operation
The condenser fan will be wired directly and bypass the defrost board fan relay. The blower fan control wire (G) will go through the defrost board fan relay. It will break power to the blower relay when a call for defrost is made. The blower motor will be controlled by a 90-340 relay with a time delay board. I'm hoping a lot of the heat will be removed from the coil while the blower is on its delay, after the defrost cycle ends.

I just started thinking about this and haven't really thought about all the different issues. I might just stick with the freeze stat. Is there anything that I"m missing, or would it be better to just keep it simple?

Thanks

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Old 03-07-2010, 09:50 PM   #2
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Need help from control guys or heat pump gurus (reversing heat pump defrost cycle)


The ambient sensor is for the stat, has nothing to really do with the heat pump itself.

Easiest way to use a A/C or heat pump to get a low indoor temp like that. is to use a larger coil and blower.

EG: 3 ton condenser, 4 or 5 ton evap and blower. This will decrease the temp difference across the coil.
At 1600 CFM your temp delta will be down to 15, at 200CFM it will be down to 12.
Most cooler systems run a 10 delta across the coil.


Trying it your way. You need to add relays to the indoor fan and outdoor fan. And it will refreeze the indoor coil within a few minutes of coming out of defrost.

Most people just use a regular cooler evap for this. No defrost issues then.

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Old 03-07-2010, 10:33 PM   #3
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Need help from control guys or heat pump gurus (reversing heat pump defrost cycle)


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Originally Posted by beenthere View Post
The ambient sensor is for the stat, has nothing to really do with the heat pump itself.

Easiest way to use a A/C or heat pump to get a low indoor temp like that. is to use a larger coil and blower.

EG: 3 ton condenser, 4 or 5 ton evap and blower. This will decrease the temp difference across the coil.
At 1600 CFM your temp delta will be down to 15, at 200CFM it will be down to 12.
Most cooler systems run a 10 delta across the coil.


Trying it your way. You need to add relays to the indoor fan and outdoor fan. And it will refreeze the indoor coil within a few minutes of coming out of defrost.

Most people just use a regular cooler evap for this. No defrost issues then.
Thanks for the help. I've always noticed the outdoor ambient sensor attached to the defrost board, but I never knew you could hook it up to a T-stat. I thought you had to buy a separate trane part for this. I'm referring to older Trane defrost boards.

Why would a relay need to be added to the outdoor fan motor? There is already a relay for the indoor blower motor. The room is fairly small. I have a 2 ton heat pump to use for the outdoor unit. I have a few options for the indoor unit. How would I size a regular evap cooler coil for this application? I was hoping to get by with a freeze stat or reverse cycle heat pump because of the temp. I only need it to be 55 to 65 degrees. Thanks again for the help
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Old 03-07-2010, 10:46 PM   #4
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Need help from control guys or heat pump gurus (reversing heat pump defrost cycle)


for defrost at 55 degrees you would only need a timer to shut off the unit for defrost.
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Old 03-07-2010, 10:47 PM   #5
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Need help from control guys or heat pump gurus (reversing heat pump defrost cycle)


On older Trane HP's an outdoor sensor is connected to the stat's T terminal.

Depends which model Trane you have.

When the unit goes into defrost. The outdoor fan will stop, but it will actually be in heat mode, and need the fan to run to get heat across teh outdoor coil to warm up the indoor coil.

For a regular cooler defrost. You just need to get one that matches the load of the room.
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Old 03-07-2010, 11:10 PM   #6
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Need help from control guys or heat pump gurus (reversing heat pump defrost cycle)


Quote:
Originally Posted by beenthere View Post
On older Trane HP's an outdoor sensor is connected to the stat's T terminal.

Depends which model Trane you have.

When the unit goes into defrost. The outdoor fan will stop, but it will actually be in heat mode, and need the fan to run to get heat across teh outdoor coil to warm up the indoor coil.

For a regular cooler defrost. You just need to get one that matches the load of the room.
The outdoor ambient sensor I'm referring to isn't really like the outdoor temperature sensor. It is just a thermistor that plugs into the defrost board. I thought it had something to do with the defrost, since Trane has a outdoor sensor for thermostats that is sold separately and is considered an upgrade. This just comes standard on units (the unit I'm referring to is a 02 XE1000).

Yeah, I tried taking that into account (see indoor and outdoor fan operation original post). The outdoor fan would be directly wired to the contactor. The control wire for the indoor blower would go through the outdoor fan relay (in the defrost board). Once the unit went into defrost, it would break power to the indoor blower relay. I have to do this project on a budget. I might try to upgrade the air handler size like you suggested. The room is under 400 sqft, so there will be a lot of air circulating in a small room.
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Old 03-07-2010, 11:13 PM   #7
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Need help from control guys or heat pump gurus (reversing heat pump defrost cycle)


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Originally Posted by 3 legged dog View Post
for defrost at 55 degrees you would only need a timer to shut off the unit for defrost.

I was just going to use a freeze stat and a time delay on the outdoor unit (to cut down on short cycling). Do you think the whole heat pump reverse cycle is overkill? I think my interest in controls is driving me to do it more than the functionality of it.
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Old 03-08-2010, 03:59 AM   #8
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Need help from control guys or heat pump gurus (reversing heat pump defrost cycle)


Quote:
Originally Posted by pomelo View Post
I was just going to use a freeze stat and a time delay on the outdoor unit (to cut down on short cycling). Do you think the whole heat pump reverse cycle is overkill? I think my interest in controls is driving me to do it more than the functionality of it.
Yes. A larger air handler is easier. Or a cooler evap.

A regular cooler evap has less fins per inch. So they don't freeze/frost up at 55. And what you should use.

An air handler having more fins will tend to freeze up because of the large temp difference across it.

Using a larger coil and blower, so that your temp difference is closer to 10 or 12 will prevent freezing of the coil. Since at 50 room temp, you'll still be around a 38 to 40 coil temp.
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Old 03-08-2010, 01:00 PM   #9
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Need help from control guys or heat pump gurus (reversing heat pump defrost cycle)


Quote:
Originally Posted by beenthere View Post
Yes. A larger air handler is easier. Or a cooler evap.

A regular cooler evap has less fins per inch. So they don't freeze/frost up at 55. And what you should use.

An air handler having more fins will tend to freeze up because of the large temp difference across it.

Using a larger coil and blower, so that your temp difference is closer to 10 or 12 will prevent freezing of the coil. Since at 50 room temp, you'll still be around a 38 to 40 coil temp.
Have you seen a cooler evap connected to a straight cool condenser (a/c) before? Most walk in refrigeration have low btu ratings. I believe an 8x12 would only need around 10k to 12k btu system. I would use heatcraft's engineering manual to do a load calc for a walk in and a manual j for a house, but I'm not certain how to size it for a mismatched system. I have a used 2 ton unit that I would like to cool the room. The room is only 400 sqft. If I put in a 3 or 3 1/2 ton evaporator, I will have a lot of air circulating in a tight space.
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Old 03-08-2010, 04:37 PM   #10
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Need help from control guys or heat pump gurus (reversing heat pump defrost cycle)


Seen several residential A/C condensers on walkin cooler evaps.
Just need to derate it for the indoor temp that the cooler needs. A 2 ton A/C will not have a 2 ton capacity at an indoor temp of 55F. At an outdoor temp of 95 and an indoor temp of 55 you might get 16,000BTUs total, if your lucky.

Regular cooler evaps generally move more then 400 CFM per ton.

If your only going to be using it for short periods of time, a few times a year. A high air volume shouldn't be a problem.
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Old 03-08-2010, 07:34 PM   #11
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Need help from control guys or heat pump gurus (reversing heat pump defrost cycle)


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Originally Posted by pomelo View Post
I was just going to use a freeze stat and a time delay on the outdoor unit (to cut down on short cycling). Do you think the whole heat pump reverse cycle is overkill? I think my interest in controls is driving me to do it more than the functionality of it.
Sounds like a fun project. I would recomend lots of insulation. And yes, I think the gas defrost is way overkill. Also use a good air filter because the fan will be running constantly.
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Old 03-08-2010, 08:13 PM   #12
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Need help from control guys or heat pump gurus (reversing heat pump defrost cycle)


Thanks guys. I decided against using the heat pump defrost system. It would have been interesting to wire up and watch it go through a cycle. I'll try and do some calculations and look for an over sized air handler or cooler evap.

Thanks

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