Sunday, December 4, 2016

Adventures With My Freezer (the one in the house)

Over the years I've been doing modifications to the freezer in my kitchen. It all started way back in 2014 when I decided to not only measure the power usage of this device <link>, but to also control when the defroster runs <link>.

A tiny bit of background for folk that haven't been following my silliness for very long: I'm trying to lower my electricity bills here in the Arizona desert, and have a power company that offers a lower rate during off hours. What I do is try to keep all my major usage in the non-peak hours, and having the defroster run whenever it wants to sometimes leads to defrosting in the middle of the peak period. Also, I bought this freezer because it is simple, really simple. There's no huge electronic package in there that is designed to save power, hook to the internet, take pictures, call my Mom, etc. It's just the basic freezer, albeit a high capability, and repairable model that looks really good.

I also added the ability to monitor the temperature inside the freezer <link> after my 'wonderful' defrost controller crapped out. While I was fixing that I decided to combine the two relays on the Arduino shield I was using to parallel the contacts, hoping to extend the life of the relays.

Don't ever do that !!

I totally forgot that doubling the number of components also doubles the possibility of failure, as well as doubling the ways they can fail. This is a great story of doing the wrong thing, so I'm going to teach you a bit about the defrost cycle on a freezer so you understand what I got myself into. Since I have the ability to chart the actual action of my freezer, you get to have real examples of my (seemingly infinite) ability to screw up.

This is what a normal defrost cycle looks like when you can observe both temperature and power usage.

Ignore point A, that's the icemaker running to dump ice in the tray, and the tiny bump next to it is the lights in there when I open the door.

Point B is the start of the defrost cycle, this happens on a timer that closed a SPDT set of contacts that shuts off the compressor and kicks on a heater that is actually attached to the evaporator at the bottom of my fridge.

Point C is where the heater is running, and it will run until a temperature sensor (also mounted on the evaporator) reaches a built-in temperature that the manufacturer thinks is the best to clear the frost away. In my particular case this thermostat is 37F.

Point D is where the heater shut off and the freezer just sits there until the timer runs out after 30 minutes. This is to allow the water to drain out the bottom into an evaporation tray and away from the freezing area of the freezer.

Point E is where the compressor kicks back on, the spike is the usual one you get from the run capacitor letting a lot of power through to get things going.

Point F is the normal compressor run cycle. It runs until the Freezer gets to operating temperature and then shuts off. On freezers the compressor runs a long time, but that's OK because these things only use around 100W. In the days before LED lights in appliances, the lights used a lot more power than the compressor did.

So, the heater pulls around 400W, but should only run for around 15 minutes, however once in a while it because of high local humidity, someone holding the door open too long, trying to freeze a gallon of hot water, or something like that, the heater will run the entire 30 minute cycle time. When this happens, the next defrost cycle will take care of the left over buildup.

Unless something is broken, or you've got someone in the house that messed things up.

I noticed that the temperature in my freezer was gradually rising and the compressor seemed to be running too long. Here's a chart of what I saw when I looked at it:

Once again, the sharp peaks are the icemaker doing its thing, but let me step through the other parts I pointed out:

A is where the defrost heater started. This part appears perfectly normal.

B is where the defrost heater should have leveled off at around 400 watts. Notice it's up around six hundred and climbs after that.

C is the defrost heater shutting off after the full thirty minutes of time allowed by the timer. It didn't shut off early and coast.

D the big spike of power should have shown up, it didn't. The spike there is the ice maker dumping ice.

What was happening? After a bunch of troubleshooting and scratching of my head, I figured it out; one of the relays had a frozen up contact and the compressor was running during the defrost cycle. The other relay was working just fine, but it couldn't stop the compressor with the other one bad.

I gutted the relays to double check (sorry I forgot to get a picture) and one of them had the contacts welded together and was the culprit in this mystery. I pulled it loose and let it go. About two days later, the remaining relay gave up the ghost as well. So much for those relays. I've got my eyes on one of the 30 amp relays now and have an order off for a circuit board that will hold it and an XBEE in my own designed Arduino shield.

Yes, I feel a bit stupid. But, isn't the ability to chart the performance of my freezer cool? Looking at the database of freezer data I was able to tell exactly which day it happened and how much the temperature rose over time as the defroster competed unsuccessfully with the compressor. Even the fancy internet connected freezers can't do that.

Right now, the freezer has the old mechanical defrost control in it that has had to be replaced twice because the mechanical clock in it doesn't hold up over time, and the defroster turns on whenever it feels like it. I had to get out the heat gun and melt the frost off of the evaporator by hand to give it a head start, that wasn't hard, but it did let the ice cream get soft.

More later, I'm not giving up on this.


  1. Cool that you made your freezer "smart".

    Out of curiosity, how much have you cut your bills (percent wise) since starting the projects? Have you even reduced the total energy consumption (since you for instance cool your house when the ac heat sink works in cooler conditions)?

    1. My total energy usage is about the same over the years, except for the huge amount I cut with the variable speed pool motor. That cut about 15% right off the top. The other stuff is not cutting my power, it's moving the usage period to a cheaper time.

      No, I'm not being a good green citizen, I'm just trying to keep my money in my pocket.

      I also save another hunk with the solar water heater. In my area, the AC units, pool and water heater are the big consumers of power, so I guess I have made a pretty good hit.

      But, in direct answer to the real question, how much am I saving: My last bill was $124, and my neighbor up the hill who also has a pool, paid $325. In the summer my bill is almost always one third of his.

      Of course, there have been the occasions where I mess up, or something breaks and I get a bill closer to normal. I see about one of those a year or so.

      I actually compared my house to one of the neighbors that has a leased solar system and my bill was a couple of dollars less than his combined lease payment and residual power bill. I laughed a bunch at that.

      I'd still like to have solar power though, just can't cost justify it.

  2. To reduce the bill by 2/5 by adjusting when things run is really impressive. It's a substantial annual saving.

    Here in Sweden households don't have demand billing so instead we just have to find ways to reduce power usage. An increasing number of households use different kind of heat pumps for heating and warm water. They have quite good COP now but the initial cost is substantial if you don't go for the air versions. But those should work quite well where you live I guess.

    Always fun to read about your projects

    1. Can you give me a pointer to the kind of heat pump you use. Heat pumps for heating water are really nice, but it can't beat using the Arizona sun. My problem has always been that I take my long leisurely bath in the evening...

      If I didn't have demand billing, I'd change my approach entirely.

    2. These are quite popular here, and But there seems to be a real jungle with lots of stuff that's either crap or just not made for our climate.

      My father have the Nibe F750 with a SAM40 to reduce the otherwise all electric heating's impact on the power bill. Unfortunately the company that installed it miscalculated and it will need an additional accumulator tank to function more efficient.

      The ground source heat pumps are probably most common, either like your neighbour had done (despite tough soil conditions) or by drilling an energy well.

      Sounds like you just need a larger accumulator tank with a arduino controlled pump regulator though :)

    3. Yes, I do need a larger accumulator tank, but the expense of a bigger tank with all the insulation and other associated costs (like where to put and plumb the thing) keep it out of my price range.

  3. Good adventure! you have try one more trick to make your freeze as solar freezer and save electricity.