Would it make sense to find another two-stage heatpump demand defrost board for another manufacturer's heatpump and rig it up?

It would obviously have to support reversing valve energized in cool mode and the same type of condenser fan motor. Some of the higher end 2-speed heatpumps may have a ecm condenser fan motor rather than two speed.

 

Would it make sense to find another two-stage heatpump demand defrost board for another manufacturer's heatpump and rig it up?

Well I was thinking of using that white rodgers single stage board (replacement for older rheem) mentioned in a previous thread here and just adding a relay for the second stage, but I honestly haven't decided if I'm going to use temperature to trigger a defrost, or differential pressure on either side of the coil. Trying to figure out which one is a better deal.

It would obviously have to support reversing valve energized in cool mode and the same type of condenser fan motor. Some of the higher end 2-speed heatpumps may have a ecm condenser fan motor rather than two speed.

Thank you. That's an excellent point that I never thought of. Mine may well be ECM. It starts fairly slow and winds up over about ten seconds. I'll have to check that.

 

Goodman 2-stage heat pumps defrost with compressor running on high to get the job done as quickly as possible.

 

How important is this feature: Timed vs demand?

I am not familiar with heat pumps and the system possibly slated for MIL's house is a Bryant and it only has "timed".

Is this more important in a high heat demand climate such as Canada? She is in Texas, south Texas.

Thanks

 

Digital:

Demand defrost is more valuable when you see significant amounts of run time below ~ 15-20 degrees F.

Reason being as outdoor temp drops so does humidity, and therefore less frosting of the coil tends to occur. In this case a Timed defrost system will often sense that the coil is cold and initiate a defrost. Demand system will know that although the coil is cold it is not frosted over, and will delay defrosting. That's kind of the simple explanation.

In defrost mode the system will energize the electric strip heat to temper the air. So the less often you have to defrost (basically running in AC mode) the less often you use the strip heat and that is where the energy $ savings come from.

 

Bob,

Can you post your model number, and even better a picture of the existing control setup?

 

Goodman DSZC160481

http://www.alpinehomeair.com/viewproduct.cfm?productID=453062630

 

I do not know much about heat pumps , but here goes .

If the condenser coil freezes up , that would diminish air flow , wouldn't it ? Could you control defrost , based on air flow ?

God bless
Wyr

 

Yes. That's what I was mentioning above. Two ways you can do it; differential temperature, or differential pressure. Can't decide which is better. Leaning towards temperature because unless you go with a more complicated electronic pressure sensor, you have to rely on a mechanical sensor which includes a rubber diaphram (rubber gets stiffer in the cold).

 

Wyr:

Pressure switches, looking at air flow change, were used in the past on some Demand systems. They were a bit expensive, and subject to issues like insects etc "clogging" them. A lot of different methods were tried, often because Mfgs were trying to work around existing patents.

Today all of the Demand systems on air source spits use two thermistors, one on the coil and one measuring outdoor air temp. Thermistors are pretty cheap, and the algorithms behind them have been fine tuned over the years and do a pretty good job.

Higher end and communicating systems utilizing ECM motor feedback and current sensor data probably offer some opportunities to further improve Demand defrost, but right now OEM's are kind of in a "it's not broke, so don't fix it" mode.

BTW, I enjoy your many helpful posts here!

 

Thanks for the complement .

We live in a pretty dry climate , so de-humidification is not an over whelming issue .

Likewise I am guessing , HP defrost is not as big an issue as it would be in some areas . But I have seen some units freeze into a " glacier " of ice . :-(

If there were such a thing as average conditions , at what Outside Air Temperature does defrosting become needed ?

I have quoted quite a few HP retrofit where the available electrical was limited & I had to get a commitment from the HVAC contractor that the electric resistance heat would NEVER be on at the same time as the compressor / compressors .

This approach is probably not optimal . But some times you must work with the existing conditions .

Do not know how he would accomplish that . Never really thought about it that much . Suppose you could do it with an ice cube relay .

These retrofits were often driven by the desire to minimize the electrical consumption of the resistance heat strips .

I once priced a situation at an apartment complex , that had a chill water system . And maybe a boiler , can not remember .

The underground piping was shot & I think the chillers were , also .

The idea was to abandon the whole system and install HP's for each apartment .

Each apartment only had a small loadcenter of very limited ampacity . It was very challenging to come up with a workable approach . Privately , I was glad we did not get the project .

Anyway , back to defrost .

Another idea would be a digital camera focused on the condenser coil with AI software analyzing the image . When sufficient frost build up is observed , defrost is triggered . When the frost was gone , defrost would be shut off ( maybe with a little more time to " dry " the coil ) .

The hardware is not that $$$ . The software may be an issue . But who knows , in the future .

God bless
Wyr

 

Bob,

That's about as complex a control board as you'll find. A few things to consider as you plot to replace it:

1. It can support EITHER a 2-speed PSC or ECM fan, depending on how it's wired. You'll have to keep digging in to make sure on this one.
2. It has an integrated contactor, so you'll have to add a contactor if you use any other control board
3. It has a relay controlling the crankcase heater around ambient temperature
4. You'll have to come up with an output to drive the second stage compressor solenoid
5. Control board has inputs and monitors both the high and low pressure switches during very specific running and temperature conditions
6. Control board has current sensors for faults and protection of the scroll compressor

Can it all be replaced and "upgraded"? Sure, but this is a much bigger project than the other thread that was dealing with a basic 1-stage unit with PSC fan motor.

Hey, this is a DIY site, I'm not saying don't try if you feel like it. Just know that it's a very complex undertaking and the cost to do it may well take many years to recover.

 

Hey, this is a DIY site, I'm not saying don't try if you feel like it. Just know that it's a very complex undertaking and the cost to do it may well take many years to recover.

Thanks for the input.
Right now I'm just playing with the idea, and it may not even be needed. The board can adjust for up to 120 minutes defrost time so I think the best thing is to simply operate it as-is for this first year and see how much time it spends defrosting. If it's going to need defrosting every 2 hours or less for the majority of time anyway then there is no sense worrying about it. Demand defrost isn't going to save that much money.

 

For the second stage compressor, can't Y2 go straight to the scroll's second stage solenoid?

Low and high pressure switches can be done in series with the contactor's coil, no?

 

When there is a Y2 demand the compressor will need to see it, as will the OD fan motor. Some Copeland scrolls take a direct, un-modulated AC input, others have an in-line DC rectifier and use a pulsed signal. As long as you know what you have.

Sure, pressure switches can be wired in series with the contactor. All I was saying is the control board has times when it does not "look" at the pressure switch...for example during certain defrost conditions. This is to avoid nuisance trips. Certain low ambient conditions as well. This logic would all be lost.

 

Nevermind

 

I've been reading up on this new "timed" control board Goodman is using. Apparently I may be looking at nothing more than splitting hairs by changing this board over to demand.

The board is time dependent but there is an on-board temp sensor which measures ambient temp. The defrost cycle is locked out until a certain temp. I think it was 28 degrees... or something like that. The time is also based on run time and not clock time so if you set it for 2 hours then it has to RUN for a total of 2 hours before a defrost cycle kicks in. I thought it was clock time and it defrosted every two hours regardless.

If this is all true then I don't see demand defrost saving all that much more money.

 

You may have something there. It's always measured in run time BTW. I still have too many unneeded cycles at the higher temps also. I think mine stops defrosting around ~50F.

 

Indeed if I look at the wiring diagram there is an outdoor temp sensor as well as a coil temp sensor.

I know that the coil temp sensor has something to do with controlling the length the defrost cycle because there is a dip switch to select the temp (50* or 70*) at which the defrost cycle ends.... but it may be double acting and monitoring the coil temp prior to defrost as well. I've read some other threads in other places and other seem to think that the timer is more about triggering a "CHECK" for frost build up. That's 3rd party guessing though.

This board may be quite a bit more sophisticated than I thought it was.

 

Yeah, my coil Klixon opens at 50F and closes at 32F so if the coil gets cold enough to close it, the defrost cycles will continue. I would think there should be times that defrost is needed below 28?? Does your board also cycle a defrost every 8 hours or whatever for oil return?

 

Does your board also cycle a defrost every 8 hours or whatever for oil return?

No, It says nothing about that. I've heard that before and it's never made any sense to me anyway. An AC doesn't reverse to return oil so why would a heat pump need to?

 

Crap.
When in doubt.... read the instructions. :whistling2:

DEFROST CONTROL
During operation the power to the circuit board is controlled by a temperature sensor, which is clamped to a feeder tube entering the outdoor coil. Defrost timing periods of 30, 60 and 90 minutes may be selected by setting the circuit board jumper to 30, 60 and 90 respectively. Accumulation of time for the timing period selected starts when the sensor closes (approximately 34°F), and when the wall thermostat calls for heat. At the end of the timing period, the unit’s defrost cycle will be initiated provided the sensor remains closed. When the sensor opens (approximately 60°F), the defrost cycle is terminated and the timing period is reset. If the defrost cycle is not terminated due to the sensor temperature, a twelve minute override interrupts the unit’s defrost period.

It also goes on to state that a defrost sequence usually lasts for around 3 minutes... which is nothing.
At any rate the coil sensor does have the ability to override and reset the timer if the coil for some reason should go up in temp during a normal run.

If everything works out here as Goodman claims... I don't see demand defrost saving much money

 

Crap.
When in doubt.... read the instructions. :whistling2:
It also goes on to state that a defrost sequence usually lasts for around 3 minutes... which is nothing.
At any rate the coil sensor does have the ability to override and reset the timer if the coil for some reason should go up in temp during a normal run.

If everything works out here as Goodman claims... I don't see demand defrost saving much money

Thats the standard method a time and temp defrost control uses.

On demand saves money at lower temps when there isn't enough moisture in the air to frost up or build up ice on the coil. And when its between 36 and 40 degrees outside, and the sun melts the frost off the coil.

 

That's nothing more than a crappy old timed defrost.

 

Trane, Carrier , York and Lennox have been making heat pumps for 40 yrs so by now they have figured out the timing method very well. Goodman has very good technology now that they got married to Amana. In the old days Amana was built like a Mercedes. I doubt you can improve on all the research and yrs of real world tweaking they have done so I would use it as is.

And yeah, it does not defrost every 2 hours like yer fridge.:yes::laughing:

 

I doubt you can improve on all the research and yrs of real world tweaking they have done so I would use it as is.

Yeah... It's beginning to sound like demand defrost is going to be more trouble than the nickels and pennies that it MIGHT save.

 

See if you can find out what the OAS does. Could be just a convenient sensor for a thermostat. We don't know if the "heat pump only" remark is just for the bottom sensor or both? My guess is just the bottom sensor so that would be a clue that the OAS is not controlling the defrost cycle in any way.

 

Okay, first you are correct. The OAS is simply the sensor for the comfortnet thermostat and provides the balance point for the AUX heat.

S-26 TESTING TEMPERATURE SENSORS
(COMFORTNET READY MODELS ONLY)
The ASXC and DSXC ComfortNet ready air conditioner models are factory equipped with an outdoor air temperature (OAT) sensor. The OAT sensor allows the outdoor air temperature to be displayed on the CTK0* thermostat when used with the ASXC and DSXC models.
The ASZC and DSZC ComfortNet ready heat pump models are equipped with both an outdoor air temperature (OAT) sensor and an outdoor coil temperature (OCT) sensor. The OAT provides the balance point temperature in heat pump systems (air handler w/electric heat + heat pump) and dual fuel systems. The OCT sensor is provides the outdoor coil temperature and is used in determining defrost cycles.

Now having said that, there does see to be a difference between the legacy and systems and the comfortnet defrost system. The comfortnet system has the ability to reset the timer should the coil temp go up above 31*.

Legacy systems:

Solid State Defrost Control
During operation the power to the circuit board is controlled by a temperature sensor, which is clamped to a return bend (3/8" coils) or a feeder tube (5 mm coils) entering the outdoor coil. Defrost timing periods of 30, 60, or 90 minutes may be selected by connecting the circuit board jumper to 30, 60, or 90 respectively. Accumulation of time for the timing period selected starts when the sensor closes (approximately 31°
F), and when the room thermostat calls for heat. At the end of the timing period, the unit’s defrost cycle will be initiated provided the sensor remains closed. When the sensor opens (approximately 75° F), the defrost cycle is terminated and the timing period is reset. If the defrost cycle is not terminated due to the sensor temperature, a ten minute override interrupts the unit’s defrost period. The new upgraded defrost control has a 12 minute override interrupt.

Comfortnet:

DEFROST CYCLE - COMFORTNETTM MODELS
The defrosting of the outdoor coil is jointly controlled by the
UC PCB and the outdoor coil temperature (OCT) sensor.
The OCT sensor is clamped to a feeder tube entering the outdoor coil. Defrost timing periods of 30, 60, 90 or 120 minutes may be selected via the dipswitch settings on the UC PCB. In a communicating system, the defrost timing periods can also be selected in the communicating thermostat user menu. During operation the microprocessor on the UC checks coil temperature via the OCT sensor every 30, 60, 90, or 120 minutes when there is a call for heating. If by the time the microprocessor checks the coil temperature, and it's low enough (approximately 31°F), and if there is a call for heat from the thermostat, the PCB will initiate a defrost. When the microprocessor detects the coil temperature to be high enough (approximately 75 0F), or 10 minutes of maximum defrost cycle time has elapsed, whichever occurs first, the defrost cycle is terminated and the timing period is reset. The field service personnel can also advance a heat pump to the defrost cycle by simultaneously pressing the “TEST” button and the “RECALL” button on the UC board.

 

Okay, first you are correct. The OAS is simply the sensor for the comfortnet thermostat and provides the balance point for the AUX heat.


Now having said that, there does see to be a difference between the legacy and systems and the comfortnet defrost system. The comfortnet system has the ability to reset the timer should the coil temp go up above 31*.

Legacy systems:


Comfortnet:

They are still describing a standard time/temp defrost for both systems. Just blowing a little smoke for the Comfortnet description.

BTW, which system do you have?

 

How much moisture is in the air in your area at -10°F. Shouldn't be much moisture at that temp. So not much water to drain.

 

Just calculated out the defrost cost for a month. Defrosting every 2 hours for an entire month (5 minute defrost time including a 5Kw heat strip) will cost me $17. I'm not even sure that's worth worrying about.

 

Did you include that in 5 minutes of defrost, the heat pump will remove about 2600 BTUs more heat, then a 5 KW heater will put in, so the heat pump will have to run a bit longer to recover the loss.

PS: With only 5KW of aux during defrost. The air temp coming out of the registers will be around 18°F colder then the air entering the return.

 

Good points. I recently staged mine from 20K to 10K during defrost. At 10K it just barely holds the supply temps that the pump alone was producing.

 

I wonder if anyone has experimented with coil color? It would seem easy to fab up a sensor to detect white

 

Interesting concept, although I may dream up a way to fool the coil sensor... I would rather not get rid of the comfortnet board. Maybe a differential pressure sensor connected to a small heating element just big enough to keep the cold off the sensor. The pressure drops due to ice then the element switches off and the sensor cools.... dunno... just playing with ideas right now.

 

Gas is cheaper.:yes:

The minimum standard is now 95%. Too bad you pisssed off Centra gas or MB Hydro yrs ago and cannot go back.:whistling2:

I could of sold you a Pulse and side vented it and you would have saved a fortune. They were 94-96% efficient back in the early 80's when I put Mom's in. Saved a FORTUNE over the 30 yrs it ran.

Ah, the road less travelled.

 

I can go back to gas anytime... gotta pay the service fees. Don't want gas is the point.