Norcold "3-way" temperature regulation... and a way to "fix" it?

[rickst29]

Since we use 12V on the road, and plug-ins at our destinations, the purely electrical fix works for me. Per an earlier post, my 2619 is still "trapped in the garage" by building supplies I'm using in a major remodel-- I won't be doing mine until at least May, I can't even get it out of the garage to open it up. But you asked a propane follow-up:

I think that your guess "the tubing contains only a tiny bit of volume, so it cant' matter much" doesn't apply in at least one very similar situation: The TXV valve on modern A/C installations.

All of the manufacturers (Danfoss, etc.) emphasize that the bulb's measurement will be significantly "polluted" by the air temp through the evaporator coil, unless you take care to insulate it all very well. (That's your indoor air, the stuff being cooled. It's much warmer than the exiting coolant gas.) The TXV adjusts the flow rate of incoming liquid refrigerant volume at your indoor coil, according to the outgoing gas line temperature. And just like our "backwards" propane regulator valves, they create more refrigerant flow when the temperature is "too high".

<OT>: TXV's are used in newer, high efficiency whole-house A/C systems. They replace the fixed-size valves used in the previous 40-50 years. They earn about 2 extra points of SEER rating for the system as a whole; while it is possible to meet the DOE requirement without one (13.00 SEER), EnergyStar requires 14.00 already -- and that makes it pretty much a mandatory component. </OT>
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But as I discussed in the blue emphasized paragraph at the end of the "massive missive", it appears that we don't need to reduce ambient effects on the regulator valve -- rather, somehow, we need to amplify any such effects. We want it to think that the fridge is hotter in the daytime heat, needing more refrigeration; and already much colder inside during the pre-dawn hours, when we want it to run much less.

And so, I've got a bad feeling that my entire "we're getting to much input from ambient, here's how to reduce it with insulation and relocating the bulb" idea would probably makes our problems even worse.

[rickst29]

Originally Posted by PopBeavers

You two should consider working behind the scenes on this. There may be a patent opportunity. If you share it all with us, you probably lose the chance at a patent.

OK, you owe me another new keyboard, you Rascal. Why is it, I never drink coffee when I need too, but then, late in the evening like this, I spew an unneeded cup all over the computer? There are many patents surrounding digital control of refrigeration systems already; I'm doing nothing new by implementing it "on top of" Norcold's dumb gas-bulb thingy -- which doesn't even behave properly under those primitive standards of "control".

A bulb thermostat should be "on" by no later than 43F, and pretty much "off" by no later than 36F. (That provides adequate "fudge factor" at the low end, and avoids switching too often.) But by going all-digital and having reliable thermocouple-based measurements, I can take my target down to a genuine 34F. Remember, you actually let it LEARN how the fridge continues cooling for a while after the Relay switches the circuit off. You specify the real target, without any fudge factor.

I suspect that the Norcold fails to run strongly, in response to increasing daytime heat, until the food temp has reached 44-46F. That's too late! Electrically, there's no major "costs" in starting sooner, and shutting it back down for a while if this decision turned out be be "too much, too soon". So I'll probably be setting my high limit at 38F, maybe even 37F (ammonia/water absorption, with no pump at all, is much slower to "get going" than a normal fridge with a compressor, a matching high-performance coolant, and "all that jazz".) This tight range gets cooling started much earlier in the morning, and refuses to let it get out of control BEFORE the heat of a really tough afternoon begins to spoil your food.

Norcold's system falls too far behind in the early part of the day, when it could work efficiently, and then tries to plays catch-up after the daytime heat has arrived. Starting to push the excessive fridge heat against a gradient of 30, then 40, and ultimately maybe even 50 degrees is a game which an ammonia/water absorption fridge absolutely it can't win! It needs to start playing aggressively in the first inning, rather than waiting for the eighth.

Wifey, and Puppy, and I are *not* gonna make any more "guesses" about backing off that dang thermostat to save our food (and critical medicines, and a really critical, really EXPENSIVE "durable" medical device which sits in the fridge -- in case the one I'm wearing fails) -- against freezing in low, early morning ambient temps while we're at plug-in campgrounds! (Guesses which we often forget to undo, until it's way to late....)

(OK, our dog is smart, but we don't really let him express an opinion on the decision "back it off to #2, or all the way to #1 ???" And he loves cleaning up exploded milk bottles and beer cans; if we let him have a say, he might vote as a "Traitor" to our greater interests concerning my medical stuff. )

[Bill]

Rick, you have addressed the electrical side of things in good detail, and purposely didn't address the propane side. In reading the Norcold Service Manual, I came across something interesting (to me, at any rate) re the propane side. Over the years, the board has discussed this a lot, and I don't think we ever came to agreement. Rick, I am thinking of your post #8 above. The manual states

"The thermostat changes the amount of propane gas that goes to the burner. This acts as a temperature control of the refrigerator. Number 5 is the coldest temperature setting. Note: The thermostat is not an automatic gas control. It does not automatically change the flame from high fire to low fire as do other RV refrigerators. If the cooling load changes, you must manually change the gas control to maintain the same temperature inside the refrigerator. When the outside air temperature is less than 50 degrees, the refrigerator may have a tendency to freeze food at the colder temperature settings."

In other words, setting the knob at "1" gives a low flame, and setting the knob to "5" gives a high flame, but once you have set the knob, the flame size never varies unless you move the knob. This explains why the temperature inside the box tends to follow the ambient, getting lower at night and higher in the day, when running on propane. We kind of knew this, but some folks (including me) have been confused by seeing different flame sizes, while others insisted that it was a bang-bang (full on or full off) control.

Since the DC circuit also has no thermostat, it is apparent that AC operation is the only mode that is thermostatically controlled.

Another issue we have discussed is whether setting the knob to "5" makes it easier to light the refrig. The answer is apparently "No", since the knob has no effect on flame size until after the flame is lit. However, I do note that the manual says to start with the knob on "5", though it doesn't say why. Perhaps starting on a high flame makes the thermocouple warm up faster, so you can let go of the red button sooner.

Bill

[rickst29]

I kept guessing and guessing, from looking at little snippets of information, while you actually READ THE TEXT. That design is really bad (I'm sure that everyone here agrees.) The next question obviously becomes this one:

Gee, I wonder if a thermostat-regulated needle valve from one of those 'other RV refrigerators', could be put to work in the Norcold fridge (downstream from the dumb controller)?

If so, you'd have your Propane control! Just turn the the "dumb" Norcold to #5 for all three fridge power sources. My thermocouple/PID/relay setup would manage AC and DC properly, and the flow-control-capable thermostat would reduce flame size from Norcold's "full on" setting whenever it isn't needed. But right now, from what you read, it's as dumb as a brick on propane: You turn the "thermostat", and it simply adjusts the needle within the needle valve (mechanically) for more or less flow.

They list the maximum propane output at 640 BTU, and that's just about right for the listed number 10 orifice. (Tables I looked at show slightly higher BTU for this orifice, size .010, but it's close.) When you turn the thermostat "down" to lower poser settings, it squashes the needle further in, reducing flame size. I'm sorry, but it looks to me as if changing this into a temperature-controlled valve mechanism involves major effort.

BTW, for any of you which want to search for such a valve: This valve is already extremely small, .010 is typical for water heater pilot burners. (And in fact, that's probably what it is -- a propane "pilot burner", but with the thermostat giving you an option to create a pressure reduction from the 11 inches nominal when it actually gets to the burner.) In water heaters, a "big" pilot burner is no problem at all -- it all goes towards heating the water tank anyway. It would be nice to have something more sophisticated on our fridge, but I have no idea where to find such a low-output valve, with PID or thermostatically controlled needle adjustment, at a reasonable cost.

[ThePair]

Ok, so if I'm understanding this correctly, I'd do the following:

A) Order the parts.
B) Install the two relays, one each for AC and DC lines, just by putting the relevant boxes in line where indicated on the diagram.
C) Install the thermocouple someplace in the fridge by drilling a hole in the side and screwing it in?
D) Install the PID and hook up power -> power (leads 1&2), thermocouple -> (6&7), then run one wire from 9 -> relay A#3; relay A#4 -> relay B#3; relay B#4 -> 10. This would be considered in series, I'd imagine.

After this, it all comes down to programming the PID, then, right? So when the PID wants more cooling, it'll trip both relays by completing the 10V circuit, and both AC and DC will become "closed and active", but only one will actually get power, based on the switch setting on the front of the fridge. Do I understand this correctly?

In addition, I take it that "relay output" from the PID is unused in this scenario.

If I have this right, I intend to attempt this modification. The nice thing about it is that it's reversible/removable if I screw it up, somehow.

Any tips on the programming of this thing? I've read the instruction manual, but it's still not really clear to me what settings are appropriate for this application. IOW, what specific settings did you use?

[ThePair]

Very disappointed to do this conversion, only to find that the PID module is defective and won't power on

Nice thing is, since I'm camping tomorrow, I can reverse the process and "dumb-down" the fridge to normal just by reconnecting the leads at the relays, which I've already done. Once I get back, however, I'll be contacting the company for a replacement...

[rickst29]

When they send you another one, I'd try it out external to fridge first. In the house, on the kitchen table.

But first- upon reconsideration, I think that the "control side" of the two SSRs should be wired in parallel- so that each sees the full output voltage of the PID. Terminal #10 (PID SSR output "+" goes to the "+" terminals (terminal #3) on both of the SSRs. And PID terminal #9 (SSR minus) goes to the "-" control terminals (i.e., terminal #4 on both of them.) And I'll be using a wire nut to connect 3 separate wires together, rather than doing a through-wired single piece: If one of the SSRs eventually needs replacement, you don't want them strung together.

Yes, current at the PID will be higher, but it can still keep the voltage up. (The specs for the 7100 list 40mA current when the control circuit is shorted; since the SSRs probably accept less than 15 mA each, the voltage should stay high enough to switch both of them.) Whatever you do, DON'T plug power in at the PID until all the other connections have been made made! Use a 12v plug-in-the-wall power adapter to provide power to the PID (on your kitchen table -- no need to even open up the TM).

Get a glass of ice water, and another glass of "cool, but way too warm" water at 40F-50F. You'll need to supply both voltage AND load at the "load" terminals. I'll be doing this (carefully!) on the 120VAC SSR: Connect the "hot" side of some lamp cord (with just the plug and the two wires, stripped about 1/2" at the ends) to one of the terminals (it doesn't matter which one at the SSR). The hot is the one with ridges or other texture. The "neutral" wire is the smooth one.

The other terminal of the SSR will also be "hot" when the SSR closes the switch. So, connect a "hot" lead between this terminal and the "hot" terminal of a single-lamp housing. (On lamps, "hot" is the brass-colored terminal.) Connect the current-carrying "neutral" lead of the lamp cord plug to the "neutral" (silver-colored) terminal on the lamp housing. Make SURE that you haven't made a polarity mistake, and install a 100 watt light bulb in the lamp housing.

Since the lamp probably has a switch, test the resistance between the two terminals on the lamp housing (with the bulb in place, and nothing plugged into the wall) It should be around 100-150 ohms. If it's infinity, flip the switch, and it should now be connected. That resistance is within the light bulb.
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NOW plug in your PID, and try some settings. You could try SV=34, AH1=38, and AL1=36. Poor some 50F water into a glass with room for ice, and drop in the thermocouple -- the lamp should go ON, because you're above the "too high" threshold. If you move the thermocouple over to the icewater, the lamp should go out (almost immediately).)

Next, make room for some ice in the warm glass. Toss in some ice, and the thermocouple, and wait. The temperature will fall as the ice melts. At 36F, the lamp should turn off. This "ice test" will not behave exactly like the refrigerator (because the lamp doesn't do the cooling, and overshoot is controlled by the amount of extra ice -- nothing to do with turning the lamp off). But it verifies the on/off controls, while working pretty close to the temperature range you will really use.

If the PID and AC control relay both work, I'd probably do the installation right there- the DC relay needs substantial voltage (at least 5 volts) and load on the power circuit too, and I'd be inclined to use the TM fridge as the test platform. Too much hassle to assemble a test platform indoors. And of course, "training" needs to wait for a realistic environment.

[ThePair]

Wow.

I was just going to get my 12v DC battery and run two wires to see if the thing would power on, then rehook up the wires and be done. I intend to hide a power switch under the sink, which would help with whatever wireswapping needs to be done in the future. Also means power won't be applied until they're all installed. But I was not going to run so fancy a test as you're suggesting. That's a bit more than I realistically have time or equipment to do. I can't see how just putting current to the power terminals to see the numbers turn on could be an issue, then testing it installed?

Switching to a parallel setup isn't the end of the world if you think it better, I'll just need another couple wires, I guess.

[rickst29]

after a 'Google' check, the diagrams which I saw (for kilns, beer, etc.) showed parallel connections -- although none of them were being driven by our particular PID.

The advantage of a "full" test (at least with the 120VAC relay), is that you can verify functionality for cooling a fridge. I've got a feeling that nearly all of these are being sold to control heaters. But since you've already cut your mounting holes (when you installed the "dead" one) I agree with you: It will be pretty fast installation. The only thing which I wouldn't like is waiting all that time for the slow, slow, SLOW refrigerator system to cool down, verifying proper turn-off.

Now that I've slept on it, I think that we should perform the "training" routine under cool conditions: If you do the calibration during a hot day, when ambient is rapidly re-heating the fridge after the heater element shuts off, it will decide that there's almost no "over-shoot" at all when running the system. Then, at 4 AM, when some additional cooling does take place, your lettuce freezes.
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If you don't have a lamp socket available to use, you could just do a sloppy "wrap and tape" of the wires to the plug of a lamp which is completely intact. If the plug is polarized, "hot" is the SMALL connector, and "neutral" is the TALLER connector. IMO, it's insane that USA codes don't require 3-wire connections on all AC appliances. But that's going way OT...

[ThePair]

I'm not worried about testing time. I'll just let it run overnight and check it the next day. The way I look at it, we can just test if AC is present on the terminal block when starting up the system, and then make sure the AC is off when the fridge is at temp. Well, that will be confounded by the dumb thermometer too, but the way around that is two-fold: one can put the thermocouple in icewater while the fridge is warm and see if it shuts off (like your experiment) Or, one can just be pragmatic and see if anything freezes overnight. A thermometer with memory would help, but I don't have one of those.

As to the training, it seems that there's some fuzzy logic in place, so I'd guess that the system might have some additional learning or compensatory abilities. Since I'll have a cutoff switch, I may see how it does with training each time, which would then allow for the set to have the most current conditions to use each time.

As it so happens, there are some convenient places to tap in for the wiring. Since all that's done on mine already, I do trust a live field test more than a lab test