Heatpump outdoor unit fan running when on fossil fuel

wvriem

Hi, hope anybody has a like setup and can shed some light on this issue!

I have a hybrid system (oil fired hydro combined with heat pump).
Air handler is from First co, heat pump is by trane (Xr15). Boiler is by Energy kinetics.
It's all controlled through 24v signals managed by a Honeywell zone panel (hz432).

The zone panel is set up to 'dual fuel' which means it will switch to oil heat if attached outdoor sensor goes below 35ºF.

There is a W linking back from the compressor back to the air handler and so on - this is in order to switch on oil heat when the heat pump switches to defrost mode. If this happens, the two way valve reverses and the oil boiler fires because the HP tells it to by sending power to W. The compressor keeps going (pumping heat back outside) but the fan stops to allow the ice to melt off.

So far all as planned.

-BUT-

If the outdoor temp is below 35º the panel makes oil heat kicks on, but in the outdoor unit the FAN starts blowing too! Not the compressor, only the fan.

It has recently been installed by (well known) local installer and I'm chasing them to come back and solve this issue as it can't be good for the outdoor fan to blow for no reason whatsoever. Obviously with winter upon us the system will switch back to oil heat more and more.

I showed this to the installer recently, they looked very surprised and would try and contact trane/honeywell to find out what could be the problem. Haven't heard back from them yet, unfortunately.

I suspect the 24v on the W (oil heat), which makes its way back to the compressor via the 'defrost heat' signal, somehow kicks the train into 'fan' mode? As this is the only line powered when there is a call for heat in 'oil' mode.

Anybody got any clue what might result in this weird situation, where the zone panel's call for oil heat on W kicks the outdoor FAN in to spinning?

All other wires are straightforward 24v - Y, O, G, W, R, C, and seem to be wired correctly as far as I can tell.


Thanks!

Ironman

Well first all, it’s not gonna hurt the motor to run by itself, though it’s wrong.

Second, there’s no “fan mode” for the outdoor unit. A call for either heating or cooling energizes the contractor in the heat pump and that closes the contacts through which both the compressor and fan are energized. So, you have an issue which I’ve never seen in my 50 years in this trade. Unless the compressor is dropping out because it has a bad capacitor or it’s locked up or a wire is burnt off.

The boiler needs to have an isolation relay between it and the air handler. I’d try that first, but I still can’t conceive of any control wiring scenario that would cause the outdoor fan to run without the compressor since they’re both powered through the same set of contacts.

HVACNUT

I bet there's 24v on W at the condenser when just the boiler should be running. 
W is a 24v output for defrost. If its getting a 24v input, it could be back feeding to the defrost board and making it wacky. 
You'll need to isolate W from the condenser when in boiler mode. 
Basically, if W at the equipment side of the zone panel is wired in parallel with W to the condenser and the boiler isolation relay, then they need to be separated. 
Like @Ironman, I've never seen that, and would need a complete (legible) wiring diagram to really see what's going on. But it seems like a back feed.

EBEBRATT-Ed

Think @HVACNUT is probably right

jinbtown

are you sure that the fan isn't supposed to be running when in defrost to clear the coil of melted ice? microchannel can really hold water because of surface tension

wvriem

@Ironman, @HVACNUT thanks for the reply.

Compressor is working fine, in heating mode. (Can't test cooling now, but recall this was tested on install).

Yes, I agree it is likely W messing with the compressor. But the installation manual (This diagram is from the Trane Xr15 install guide, for example, below) show the W from the compressor directly linked to the W (and W2) at the air handler, and that again connected to the thermostat. Replace the thermostat in my case with the Honeywell HZ432, it still basically switches 24v to W to get the boiler running.

Or, in defrost mode, the condensing unit does it.

(in my case, the boiler is switched on by the W at the air handler, through a separate board - I expect to separate the oil boiler on/off control electrically).

So according to this diagram it looks to me like it should be normal that W can be engaged by the thermostat and feeds 24v to the W on the compressor? Or am I misreading this? If this needs to be isolated for some reason surely it would show in the diagrams?




Exactly the same situation in the First Co air handler manual (60VBXB-HW). No mention of separating W anywhere.

wvriem

@jinbtown As far as I know on defrost mode the condensor keeps working, the valve switches to 'ac' mode (so pumping heat from inside to the outside unit) and the outdoor unit fan is off. That is wat happens with me, and as far as I can see is expected behavior. In order to keep the people inside from freezing the backup heat (often electric coils, in my case hydro heat) comes on to compensate, on a signal from W from the defrost control board in the compressor unit.

It the thermostat (or zone panel) is in 'no heat pump as it is too cold outside, just stick to fossil fuel' mode then the outdoor fan is engaged for some weird reason.

Ironman

Again, if you realize that there’s no HIGH voltage power going to the fan circuit on the defrost board unless the contractor is energized (unless it’s stuck closed), then you’ll realize that the compressor must also be energized at the same time. There’s no contacts in the defrost board that can bring the fan on unless the compressor contactor first sends power to it. If there’s HIGH voltage power to the board, then there MUST be power going to the compressor; they share the same contacts.
And again, there needs to be an isolation relay between the AHU and the boiler. They are two separate low voltage circuits that can’t be combined.
Control circuitry is something that I specialize in and I do the type of setup that you have frequently. I’m very familiar with the HZ432, the heat pump and the AHU.

wvriem

@Ironman Sorry I didn't mean to say you're wrong - Just that there is, as far as I can see, an isolation between the AHU and the Oil boiler. So that circuit seems to be separate - this looks to be built in to the AHU (first co).

No isolation relay between the AHU and the compressor and the HZ432, though. I'm trying to find out if that is needed or not? Diagrams seem to suggest it isn't.

I agree something is signaling the outdoor unit to do something, as there's no high voltage over the control lines. I was looking at W, but maybe I need to look further.

But what exactly do you mean with engaging the contactor (would that be Y?)

Is there some kind of combination of signals that would engage the outdoor unit but stop the compressor - like a signal on Y and W and that makes the outdoor unit go in to 'ac' mode but some thermostat blocks the compressor from starting too because it's too cold already?

That might point to a wiring issue, as opposed to a control board failure. There is a mention in the AHU install guide:

"When the Tstat is switched from “Normal” to “Emergency” the compressor circuit “Y” is locked out"


I am not sure what this means but could this be the AHU sending a signal to Y too when on emergency heat? Bit weird, but would that engage the contractor as you mention?

Thanks again for all your input! Just trying to get a feel for where the problem could be so I can get the installer to investigate and fix.

JUGHNE

Is the fan running full speed during this time?

Does this have off cycle comp heating via a single pole contactor feeding thru the motor windings. Just a WAG on back feeding partial voltage to the fan.

wvriem

@JUGHNE No way to be 100% sure but the outdoor unit fan sounds and blows just like in normal operation.

Do you mean off cycle compressor heating? No clue, I'll see if the manual says anything about this.

Ironman

JUGHNE said:

Is the fan running full speed during this time? Does this have off cycle comp heating via a single pole contactor feeding thru the motor windings. Just a WAG on back feeding partial voltage to the fan.

Trane quit doing that a good while ago because of its inefficiency.

However, I’m thinking along the same lines like a bad capacitor or wire burnt off.

Ironman

@wvriem
It's very simple: on a call for either cooling or heating, the thermostat energizes the “Y” terminal which feeds the holding coil of the compressor contactor. That causes the high voltage contacts to close and send 240v to the compressor and the fan relay in the defrost board. The contacts in that relay are normally closed and send power to the outdoor fan. The defrost control cannot start the fan on its own; it must first receive power from the compressor contactor. If the control board is receiving power, then so is the compressor and it should be running at the same time. If it’s not, then there’s something wrong in the compressor circuit.
 

What I’m trying to get you to see is there’s no way that the defrost control board can “turn the fan on” in and of itself. It doesn’t have that capability. It can only turn the fan off during a defrost cycle.

motoguy128

Ironman said:

Well first all, it’s not gonna hurt the motor to run by itself, though it’s wrong.

Second, there’s no “fan mode” for the outdoor unit. A call for either heating or cooling energizes the contractor in the heat pump and that closes the contacts through which both the compressor and fan are energized. So, you have an issue which I’ve never seen in my 50 years in this trade. Unless the compressor is dropping out because it has a bad capacitor or it’s locked up or a wire is burnt off.

The boiler needs to have an isolation relay between it and the air handler. I’d try that first, but I still can’t conceive of any control wiring scenario that would cause the outdoor fan to run without the compressor since they’re both powered through the same set of contacts.

Nope. many newer unit control the fan off the defrost board with a relay not the contractor.    During defrost the fan shut down.  Or if it’s a ecm motor just a 24v call.  It’s 240v is energized all the time like all ECMs.  

motoguy128

I wonder if the installer tried to use a dual fuel kit rather than just using a proper dual fuel thermostat with outside sensor and programming it correctly.  

motoguy128

Defrost isn’t practical with dual fuel since fossil fuel furnaces take over a minute to get warm. A defrost call is sometimes over by then.  Better to set the balance point to 40f so it rarely defrosts.  

Ironman

Furthermore, even if there’s power to “W” from the zone panel to the outdoor unit, that won’t cause some mysterious back feed. That is a normal situation anytime the backup heat is energized on a heat pump system.

You’ve either got an improper wiring/control setup, a stuck contactor or something wrong in the circuit between the contactor and the compressor. 

wvriem

@motoguy128 Well, it's a zoned setup and the HZ432 takes care of the dual fuel management - based on its outdoor sensor it engages the HP or switches the Oil boiler on. It's fairly basic, in that is about all you can configure. You can combine it with HP thermostats or conventional thermostats. In my case it's HP thermostats (Honeywell too).

re: defrost: True, it takes a good few minutes for the hydro to heat up and compensate for defrost blowing cold air. Possibly useless as defrost is over by the time hydro heat is available. As I believe the trane has a temperature sensor so defrosts only when needed. Hasn't seen it happen much.

wvriem

Ironman said:

Furthermore, even if there’s power to “W” from the zone panel to the outdoor unit, that won’t cause some mysterious back feed. That is a normal situation anytime the backup heat is energized on a heat pump system.

You’ve either got an improper wiring/control setup, a stuck contactor or something wrong in the circuit between the contactor and the compressor. 

As the wiring is color coded (yellow, white, red, blue, green, black) so pretty straightforward to check; I've looked at the diagrams and how the wires are connected and (apart from a 'flood check device' cutting power when the condensate pump is failing) it all looks exactly as per diagrams.

Control setup is also rather simple - the HZ432 has a menu system if you select 'dual fuel' it basically only allows you to set up balance temp to switch to oil heat, and DATS max and min. Number of zones, type of thermostat and some fan timers and that's about all he options. Looked ok to me.

So -IF wiring and setup is correct- it looks like the culprit is inside the Trane compressor unit, you'd think?

I'll get back to chasing my installer to come back sooner rather than later to find out what is going on.

JUGHNE

Can you post a picture of the wiring diagrams inside the AC/HP control panel?

wvriem

@JUGHNE No diagrams inside the panels, but this is the diagram for the dual fuel setup for the zone control panel:

wvriem

Well, re-checked all the wires and the HZ432 settings. All looks correct.

But somehow, the signals from the air handler/thermostat get messed up and the outdoor unit fan starts up, when it should not. As I said, just the fan, the compressor is not running. (It might be engaged, but blocked for some reason?).


There are just 5 wires going to the outdoor unit: Red, Blue, Yellow, Orange and Black.

Red is 24V
Blue is common
Yellow is compressor
Orange is changeover valve
Black is return signal for heat when defrosting.

As everything else is working I'm thinking it has to be the one or a combo of the 3 signals on Yellow, Orange and Black.

Can anybody come up with a combination of signals that would activate the outdoor fan but the compressor does not/is blocked? Like signal on both Yellow, Orange and Black at the same time, and too cold to start compressor???

The only other thing I can think of is that the fan in the AHU runs and this changes some temperature on the inside coil which triggers a pressure switch somehow? Far-fetched, but I'm really lost what is going on here!

Unfortunately installer difficult to reach and I've been him leaving messages for some time now but no response yet...

jinbtown

I think that this point you need to break out a multimeter and reproduce the issue and start checking voltages, taking pics of the outside unit wiring, etc.

Ironman

As motorguy pointed out, if the fan has an ECM, it may have constant power to it and it turns on when “Y” is energized. But that essentially creates the same scenario as if it were a PSC motor that’s wired the way that I described. Why would “Y” be energized when the hz432 is turning on the boiler? If that is indeed happening.

Try two things:
1. Disconnect the “W” wire at the condensing unit. As mentioned you don’t need the auxiliary heat to turn on during defrost.
2. The next time your scenario happens, temporarily disconnect the “Y” connection to the condensing unit and see what happens.

wvriem

@Ironman Thanks for the input. Interesting results!


"When you have eliminated all which is impossible, then whatever remains, however improbable, must be the truth."


So I have disconnected the Black W coming from/going back to the outdoor condensing unit. Switched to fossil fuel, switched thermostat on the outdoor fan no longer spins up without need.

?!?

So I have an outdoor unit (Trane XR15) that doesn't like sitting idle and getting a signal on the W (call for [backup] heat) which it uses to call for [backup] heat when it is defrosting...? If that happens it starts its fan.

Now I'm really curious if this is just normal behavior for the Trane XR15 (possibly never expecting a dual fuel setup, but designed for electric backup heat, and NEVER expecting to be idle during a call for heat, even backup heat. Then you simply wouldn't notice the fan spinning during call for W, as the heat pump would be working anyway too.

If it's the Trane I want them to come out and replace the logic boards or whatever under warranty.

But if the problem is not the Trane, then what would cause this?

Is there *anything* either the AHU of the Zone Panel (HZ432) can do to mess with the W signal that would make the Trane go crazy? Like sending DC where it is expecting AC or 12v where it should be 24v?

Interesting challenge.


NB Chased installer several times but still no callback. Not impressed obviously.

Ironman

I don’t think changing the board will make any difference. And even if it would, what’s the point? As already stated, it’s useless to try and run backup heat during defrost with your setup. Just leave it like it is and don’t worry about it.
Don’t worry. Be happy!

wvriem

@Ironman

We found a workaround, if not the source of this weird problem.

I guess you are right in that once W from compressor disconnected it seems to work fine - without backup heat during defrost.

Installer came around and spent a good amount of time trying to figure out what was going on. Rechecked all the wiring. And bypassed the relais board in the AHU that switches boiler, also wired the zone valve 'zone open' switch to trigger the air handler fan. So the zone panel 'volt free' switches the boiler, and the zone valve 'volt free' switches the AHU fan on. (which is great, now I get a well timed purge with my energy kinetics boiler).

Started everything up. With the air handler open, the ECM fan in the air handler does very little work. But the moment the air handler is closed up, fan has to work harder and as if by magic - the outdoor compressor fan kicks in. Open the air handler case again - it goes off. What??

Somehow, somewhere, the load of the AHU fan, or the controller of it, messes up a signal on the W. Voltage drop is minimal, from 28 to 27 v.

Installer and me both baffled.

The only way we got it to work is by completely isolating the W (defrost signal) from compressor by means of an extra relay and the problem is, if not exactly solved, at least circumvented.


But it remains a mystery...

ratio

Use a variac to see if it's voltage related, then look for noise on the wires (If there's a µC in the defrost board).