JARS v61n1 - Greenhouse Freeze Alarm System

Greenhouse Freeze Alarm System
Richard Cavender
Sherwood, Oregon

Anyone who grows vireya rhododendrons, or any other tender plants, in a greenhouse in a cold winter area needs a cold temperature alarm system. I found this out the hard way many years ago when my heater malfunctioned and I lost a big part of my collection. I did have an alarm but it malfunctioned as well.
An alarm system is not elaborate and anyone with a bit of mechanical skill can install one. The system described below will work with any type of heat system and the supplies should be available at any good hardware store.
A bit of background first. I have two greenhouses that I heat. One is a 95-foot long by 14-foot wide "poly tunnel house." It serves as a cool greenhouse in winter and a shade house in the summer. It has two layers of poly and a small (1/64 hp) squirrel cage fan that blows air between the layers. This inflates it and keeps the poly rigid. It has been very wind resistant and quite energy efficient. I heat it with an 80,000 BTU propane space heater. The heater looks like a jet engine and does a good job. I have a 500-gallon propane tank that supplies both houses. This heater could be supplied by a much smaller tank; however, I would recommend at least 15-gallon. I have used a kerosene heater of the same type but it requires refueling and that can be a problem during a long cold spell or if you are out of town. Kerosene is also costly if you buy in small quantities.
My other greenhouse is a commercial steel and fiberglass structure. This house is 24 by 40 feet with 9-foot side walls. As it has only a one-layer covering, it is much less energy efficient. This house is heated with a 150,000 BTU hanging greenhouse heater. The heaters in both houses are thermostatically controlled.
There are a number of commercial greenhouse alarms on the market. Really fancy systems will even make a phone call! I have two ThermalarM monitors; however, I have several complaints with them. One, the numbers on the dial face fade out in sunlight after a few years. Two, the contacts corrode and fail to complete the circuit. This was the cause of the failure years ago. I have since rewired the one in the greenhouse and it has worked fine since then. My monitors are some 15 or 20 years old so these problems may have been corrected. These monitors are available with an outside horn but they require 120v power. My system is battery operated.
I like a battery operated system because there is no shock hazard and it will operate even if the power is off. This can be critical! The lantern batteries will last for several years because they see very little use. When that bell goes off, you are out of bed now and turn it off. If it operated five minutes a year, that would be a lot. Admittedly, this system will not do any good if there is no one home but it has served me well for over 15 years. I always test the system every fall by adjusting the thermostat or monitor to be sure the batteries are okay and all parts work.
After the failure years ago, I purchased a 2-stage thermostat for the poly house. The first stage turns on the heater and the second the alarm. The interval is adjustable. I like this thermostat as the switches are sealed in plastic and the only parts exposed to the environment are the wire attachments. Much less likelihood of corrosion. Really, any good 2-stage thermostat will work. Our home heatpump has two stages, heating and cooling, but you can connect the thermostat any way you wish. Be aware that thermostats are not always calibrated very well and a little experimentation may be required.
Wiring is rather straightforward. I used 2 conductor outdoor telephone wire. It is solid rubber with two 28-gauge wires. My poly house wire runs into the greenhouse and is connected, in parallel, to the monitor in there. That monitor is mounted next to the heater thermostat. From the greenhouse, the wire is strung overhead to the house. I entered the house through a gable vent. I then fished a wire down through an interior hall wall next to our doorbell. These two wires meet in the attic and are wired in parallel to two 6v lantern batteries. I found that I needed two batteries as one was insufficient to power the bell.

Components for the freeze alarm
Components for the freeze alarm: From the top, clockwise, lantern battery, thermostat
with the plastic cover off, a cheap bell, a buzzer (silver), wire that runs from the
battery to the thermostat, and wire that runs from the thermostat to the bell or buzzer.
Photo by E. White Smith

My interior bell is really just a doorbell. It operates on 6 or 12 volts. It has a nice plastic cover that the wife does not object to and does not stand out like a sore thumb. I did modify the bell by installing a switch on it so that it could be turned off while I went outside to find the problem. The mounting location is about 10 feet from our bedroom door and let me tell you, you will not sleep through it!
The mounting location for the indoor bell is not really critical - any location where you can get wires to it, even the ceiling. In my system, one bell serves both houses. However, phone wire comes in 2, 4 or 6 conductor and several buildings could be served by one wire. A separate indoor bell could be used for each greenhouse as well. Several alarm locations could also be served such as your house, work building, etc. Larger batteries may be required but a little experimenting will soon tell you.
I really do hate it when that bell goes off but I would hate it even more to find the contents of my greenhouses frozen. The hundred bucks and days work I spent on this system really does let me sleep better at night!
Addendum from E. White Smith at Bovees Nursery, Portland, Oregon
After talking to Dick Cavender we have installed a freeze alarm in our greenhouse and in our plastic houses. The only hard part of the alarm is finding a thermostat that will go down below 40F. We set our natural gas heaters at 40F, so if the power is off or something else happens and the temperature falls much below 40F the alarm will sound, and I will go and find out what is wrong (I used a buzzer in the bathroom next to our bedroom).
How important is the alarm to us? If the greenhouses happen to freeze we and the world would probably loose the biggest collection of vireya rhododendrons, over 700 clones. Many collections have been lost because of freezing when the growers did not know what was happening at the time.
Okay, yes, we have tried the fancy radio alarms but they are not always reliable. The battery, thermostat, bell or buzzer always works and is cheap to make up ($50). The thermostat we are using is from "Farmtec", stock number CR2095, for $24 plus shipping.
If you are growing tender plants in a greenhouse get with a program and install a freeze alarm to protect yourself.

Richard Cavender is a member of the Portland Chapter. E. White Smith is a member of the Tacoma Chapter.