From: John De Armond Subject: Re: Better furnace thermostat Date: Wed, 13 Oct 1999 04:20:57 EDT Newsgroups: alt.rv,rec.outdoors.rv-travel Erik wrote: > > "Dave Thompson" <bdthompson@home.com> wrote: > > The current model is probably OK. Adjust the heat anticipation slider > > until you get what you want. > > Could you tell me in more detail what the "heat anticipation slider" is? > (I am not sure mine has one.) This is a little adjustable heater inside analog thermostats that supplies a small amount of heat input to the thermostat. This causes the thermostat to open slightly before the room reaches the setpoint. It "anticipates" the extra heat recovered from the furnace as the heat exchanger cools after the fire is out. In the other direction, since the heater is off when the thermostat is off, the thermostat comes back on a little before it normally would. This "anticipates" the room getting cold before it actually does. Physically, it is typically a small resistance wire with an adjustable slider. It will have a calibrated scale. This scale should be set to the number of milliamps the thermostat circuit pulls. Measure this with a suitable DVM. Turning the anticipator to a LOWER number will INCREASE the anticipation effect and tighten up the deadband (span between turn on and turn off) Go too far and, as someone else has said, and the furnace will cycle rapidly. That said, I can't imagine fooling with an analog thermostat when digital ones are so cheap and work so well. There are two major variations - thermostats that are powered from the loop voltage and those that contain their own battery. Loop-powered thermostats will not work on millivolt (self-powered) furnaces. Such furnaces only supply about 700 millivolts to the thermostat circuit. If you don't know what kind of furnace you have and don't have a suitable meter, buy a digital thermostat that is battery powered and is rated for both 24 volt and millivolt circuits. Read the label. Even if an e-stat has an internal battery, if it is not rated for 700 mv service, it may not work. This is because relays not designed for low EMF switching can sometimes accumulate film on the contacts that the 700 mv cannot punch through. They work fine on the 24 volt systems. Relays rated for low EMF switching have high pressure contacts that will cut through the film even at low voltages. Other advantages of digital thermostats include: * Digital indication of room temperature. * very accurate and capable of being calibrated if you have a calibrated reference thermometer. * adjustable deadband (can hold the room temperature to the setpoint +- 1 degree) * Learns the thermal characteristics with intelligent anticipation. * Programmable - either 5+2 days or 7 days. Of limited use in an RV unless you're living in it. * cheap * position insensitive. For the programmable versions, the two variants are 5+2 and 7 day programmable. The 5+2 holds 2 programs, one for the weekdays and one for the weekends. Pretty useless unless you work a 9-to-5 job with little variation. These are the cheapest of the programmables so if one program for all days is OK, say, warming the RV at 9:00AM every day, then it will do the job. Non-programmable ones are a little cheaper for this application, however. The 7 day programmable 'stat will hold a program for each day of the week. They usually have shortcut keys to emulate 5+2 mode if your lifestyle fits. Many digital 'stats have two control circuits so that two appliances can be controlled. This is nice if you want to control your overhead AC and your gas furnace on the same 'stat. Many of the high end 'stats have auto-crossover. That is, it will automatically cross over from heat to cooling or vice versa as the conditions dictate. This is nice for those times when heat is needed at night but AC is needed in the day. I have a Hunter brand e-stat that does all the above except the auto-crossover. I bought it a Sam's for about $25 about 3 years ago. Battery life has been about 2 years. Only minor problem is that it loses its program when the battery is changed. I use a top-of-the-line White-Rogers Digital Comfort-Set II e-stat in my restaurant. This does all the above and in addition, can stage up to 3 cooling and 2 heating stages or units and has a remote sensor so the thermostat can be locked in the office where no one can tamper with it. The auto crossover is very useful because the heat load varies so much between meal times and prep times that AC is needed one hour and heat the next in the winter. At about $200, probably overkill for an RV. OTOH, it could be used on a large RV to stage 2 or 3 AC units according to the heat load. John |