Imagine you are lying in bed on a winter night. Vooomm! The furnace starts up, and you hear the familiar rumble of the oil burner. Five seconds later, the furnace stops...
What's going on? Why did the furnace fire for only five seconds? Did something interfere with the flame? Is unburned oil spewing all over the combustion chamber right this minute?
This very problem happened to me last winter. Another problem is that I would hear the furnace fire for a few minutes, shut down, then start right up again a few seconds later.
I asked my furnace repairman if he had an explanation. He wasn't sure, but he installed a new flame detector just to see if it would make a difference. The flame detector is a photocell that kills the oil pump if no flame appears a few seconds after the burner starts.
The new flame detector did not make a difference. Next was the Aquastat. At $300+, I wasn't excited about the prospect of replacing it.
My house is heated by a hot-water system. There are five zones, including the domestic hot water tank. Each zone is monitored by an individual thermostat, which controls a little motor-operated valve. The valve opens up to let the hot water flow into the room that needs heating. When a valve is open, a big circulator motor turns on and pumps the water.
I figured a good place to start was to record the behaviour of the thermostats. The thermostats were 24 volts. So I got some relays on eBay, and connected them to the thermostats. A relay is a switch connected to a little electro-magnet. When there is voltage applied to the electromagnet, the switch closes, and you can power stuff with it. (I should have used transistors, but I've never been able to figure out how to make them work).
Then I fired up an old 486 PC that was collecting dust in the basement. It still had an operating system (Windows 3.1: gasp! were we ever so primitive?), and my trusty Borland C++ compiler. I found some sample code on the Internet, together with some schematics showing how to connect the parallel (printer) port to an external device. I wrote some code, and was soon able to display the on/off status of the five thermostats, as well as the circulator pump motor, and the burner.
The PC ran constantly in a corner of the cold, dark garage. It was fascinating to watch the furnace status scroll on the amber monochrome monitor. The relays would click away as the thermostats went on and off, and the motors whirred away. (Small things amuse small minds, as my wife would say).
And I finally solved the mystery.
The oil burner operates independently from the thermostats. The only thing the burner cares about is the temperature of the hot water. It fires up when the water temperature falls below the low point, and stops when the water reaches the high point. Also, the burner will not fire if no thermostats are calling for heat.
The furnace was only firing for brief periods because it was coming on at the end of a heating cycle!
The computer showed that a typical cycle went like this: when a room got too cold, the thermostat would click on. Hot water would pump through the pipes, and the room would gradually heat up. At this point, the burner motor would not yet come on. As the room drew more and more water, the temperature of the boiler would fall, until it was colder than the low point. Then the burner would come on, fulfilling its mission of keeping the water hot.
The timing of these two events is random. It typically takes three to five minutes to heat up the room. Occasionally, the low point would be reached precisely a few seconds before the room had reached toasty temperature. No sooner would the burner fire up, than the thermostat would say "Woops! that's enough, I don't need you any more". And the poor burner would have to shut down.
What about the brief shut-down between two firings? That is caused by a second room coming on just a few seconds after another room shuts down.
I did learn that the thermostats in certain rooms are coming on too frequently. I increased the anticipation interval (a little setting in the thermostat that controls how long it stays on) and hope to reduce my oil consumption.
I'll hook up the PC again this winter, just to see lights flash and hear relays click. Now if I just added a TCP/IP module, I could send the results to my office over the Internet...