Managing Two Thermostats

Having a wood/oil combination furnace is a bit of a juggling act.

The idea is to burn wood: it’s cheaper. Unfortunately, at some point during the night, when I am not actively tending it, it runs out of fuel and stops producing heat. This is where the Oil comes in. When the wood furnace has run out of fuel and is no longer producing heat, I want the Oil to kick in and heat the place instead. Oil is expensive, so I only want it to kick in if absolutely necessary.

And I want it comfortably warm in here.

Traditionally this has been handled by setting up two thermostats and setting the oil to a lower temperature than the wood. Basically, the oil stays off while the wood is doing its job. Once the wood burns out, the temperature drops low enough to trip the Oil. Simple. Problem solved.

Sometimes I over-think things.1 As a Business Analyst, one of the my key duties was to identify inefficiencies in systems. I’m really good at that job.2

The way wood furnaces work is to burn and produce heat. The heat builds up in a Heat Exchanger which, once it reaches a critical level, forces the air through the ducts, into the house. You cannot control the heat being forced into the house. Once the heat reaches that pre-set level, it needs to be released from the Heat Exchanger, otherwise you risk a fire starting around the heat exchanger. Since while the wood is burning it is producing heat, the only way to control the heat is to prevent the wood from burning. This is done by starving the fire of oxygen until you want the heat.

Generally, once you get the fire burning in a wood furnace, you close the door and the fire consumes most of the oxygen in the combustion chamber, causing the fire to go out (or at least down). When the temperature drops to the critical level for “turning the furnace on”, all it does is activate a blower. The blower operates like the bellows at a black smith’s, forcing air into the fire and causing the fire to burn hotter. Hot enough to heat up the heat exchanger and cause it to force air hot air through the house.

What started niggling at me was the fact that the blower requires electricity to operate. If the Oil is set to a lower temperature than the wood, the house will never get warm enough to turn off the Wood Thermostat. This means that all night, after while the furnace has run out of wood, and the oil has started to heat the house, the little blower is blowing its little brains out trying to get a non-existent fire burning. Why am I wasting electricity on that crap.

Enter digital thermostats.3

Theoretically, I should be able to set the controls to indicate that the blower should give up at some point during the night. Basically this would be achieved by setting the wood furnace temperature to lower than the oil. Since the oil would keep the house warmer than the activation level for the wood, the stupd little blower would not come on.  I would only want this inversion to take place during a time when I am reasonably sure that there will not be wood burning (night time).

So, what is the schedule?4

[easychart type=”line” height=”300″ width=”350″ title=”Thermostat Schedule” minaxis=”10″ groupnames=”Wood Thermostat, Oil Thermostat, House Temperature” groupcolors=”36b70e,f1ac2a,aeaeae” valuenames=”0,1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23″ group1values=”16,16,14,14,14,14,14,18,18,18,18,18,18,18,18,18,20,20,20,20,20,16,16,16″ group2values=”15,16,16,16,16,16,18,18,15,15,15,15,15,15,15,15,15,15,15,15,15,15,15,15″ group3values=”18,17,16,17,16,17,18,19,18,19,19,18,19,18,19,18,19,20,21,20,21,20,19,19″]


  1. As I type this, Sharon and I discussing removing extraneous spaces from documents. She was giving me that look that says, “really? You are really bothering with that minutia again?” because I started to calculate the number of times you would have to replace double spaces with single spaces to result in no single spaces depending on the largest number of sequential spaces… Seriously, these are the things I ponder in my quiet time to myself. 

  2. I’m also good and fixing the inefficiencies, but that part is easy and not worth discussing. The solution is, once you have defined what the problem is, JUST STOP DOING IT THAT WAY! 

  3. When you think about it, electrical thermostats are a huge waste of electricity as well, Mercury thermostats used no electricity until they switched on, and never broke down (requiring a replacement)  

  4. Sharon just saw the graph… she’s rolling her eyes again. 

2 thoughts on “Managing Two Thermostats”

  1. The solution for me was to install digital thermostats — something a couple of the locals said was a dumb things for a wood furnace (see footnote 3) — and set-up schedules that forced the wood furnace to a very low temperature late at night — so low it does not come on (see graph).

    If you stick with the analogue thermostats you could just turn the blower off when you go to bed. Chances are you want the fire to burn very slowly anyway (to last all night), so blowing air into the combustion chamber is counter productive. Before this schedule, my bedtime routine was: lock the ferret cage, lock the door, and turn off the wood thermostat; effectively turning the blower off. If you have the analogue thermostat set just right, you could go to the blower itself, and push the on/off switch.

    So three solutions:

    1. Get a digital thermostat and set up a schedule similar to the one on the graph (not necessarily recommended)
    2. Turn off the thermostat every night
    3. Turn off the blower every night

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