Thermostat info

I recently bought a Robertshaw 9725i2 thermostat that is very programmable. Unfortunately, I feel like the manual does a poor job of explaining some of the settings. I did an internet search and it looks like I'm not the only one who feels that way.

There seems to be a lot of confusion about "stage differential" and "stage delay". The 'differential' relates to temperature and the 'delay' relates to time.

First stage differential - The difference between what you set the temperature at and when the furnace kicks on. (I'm referencing everything to heat in these examples).
Second stage differential - The difference in degrees between when the first stage kicks in and the auxiliary heat starts.

Example: Your thermostat is set to 70*. The first stage differential is set to 3.0*. The second stage differential is 2.0*.

Your furnace (in my case a heat pump) will shut off at 70*. It will turn on again when the temperature reaches 67*. (Set temperature - first stage differential, or 70 - 3 = 67). If it is very cold and your heat pump can't do the job the second stage heat (electric heat coils in my case) will start at 65*. (Set temperature - first stage differential - second stage differential, or 70 - 3 - 2 = 65*). This is how the 'differential' setting affects the thermostat operation.

Second stage delay - The amount of time the first stage of heat runs before the second stage kicks in.

Example: The second stage delay is set to 30 minutes. If the heat pump has been running for 30 minutes the second stage heat will kick in to help it out. This is the more normal occurence for causing the auxiliary heat to come on.

Note: The thermostat I have will control a third stage of heat. In my case it is a second set of electric heat coils. The 'delays' and 'differentials' for the third stage work the same way but just in relation to the second stage heat.

In a normal situation the differential settings won't come into play unless there is a drastic situation like (1) it is super cold outside and the heat pump just won't cut it or (2) you come home from vacation and the house has been setting idle and is very cold. The 'delay' setting is the one that will actually activate under most situations.

Something else I've learned...

When I installed my heat pump 10+ years ago I noticed a decrease in the heat bills. However, the last few years my heat bills have been very high and I didn't understand why. Recently, I think I figured out why. A few years ago I bought a Honeywell thermostat because it was more programmable. You could use an outside temperature sensor to also control when the main and auxiliary heat came on. As a nerdy engineer I thought this was neat. I set the high control point to 40* and the low control point to 28*. This means that at temperatures under 28* only the auxiliary heat is used. Between 28* and 40* the heat pump is used with the auxiliary heat coming on as needed (see above). Above 40* on the heat pump comes on. It seemed like another nice way to control what the furnace was doing...plus you can look at the thermostat and see what the temperature is outside.

However, this is what I eventually came to realize. The heat pump will work, although less efficiently, at very cold temperatures. Quite a bit lower than the 28* I had chosen to turn it off at. When the heat pump portion is running the whole system uses about 10 amps of electricity. When the auxiliary electric heat coils are operating the whole system draws about 50 amps. Starting to get the picture?

I did a little research using my Quicken records and found that when I started using the outside temp sensor to control the furnace my heat bills increased a lot. I disabled that feature and I now use the heat pump all the time and the auxiliary heat only when the thermostat deems necessary. The heat pump runs longer than only using auxiliary heat but it only uses 1/5 the power. My heat bills have dropped accordingly. Hopefully this experience helps someone.

Note 2: I switched from the Honeywell thermostat to the Robertshaw because the first stage differential for the Honeywell is automatically set from the factory at about 0.5* and is not changeable. I didn't like this. It was constantly on...off...on...off...on...off...you get the picture.

Update: Although I unhooked the outside temperature sensor I figured it would still be useful as long as realistic temperatures were chosen for the cutoff. This past winter was colder than usual. I had the opportunity to monitor the performance of the heat pump. I found that at temperatures below 5* or so was where the heat pump could no longer do its job in a reasonable amount of time. That is where I set the heat pump cutoff temperature. The 28* cutoff I had used earlier was based upon information on the internet.


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