Switching from a mercury to a digital thermostat

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Old 12-14-09, 04:23 PM
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Switching from a mercury to a digital thermostat

Hi,

I recently changed my old Honeywell mercury dial based T87F thermostats with Honeywell RTH7500 thermostats. I have an oil furnace with forced hot air and air conditioning for cooling.

Since changing them I have noticed this difference:

Mercury Thermostat (T87F) in Winter
With the mercury thermostat the air came out very hot instantly and would only run for 30-60 secs before switching off.

Digital Thermostat (RTH7500) in Winter
With the new thermostats in place the fan runs for much longer although the air coming out is only slightly warm.

Is this how it should be? Even though the fan is running longer, the fact the air is not as hot mean I am saving lots in oil? Is it done on the basis that electricity is cheap and oil is expensive?

Appreciate anyone's thoughts or experiences - thanks!
 
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Old 12-14-09, 04:54 PM
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Originally Posted by inforr View Post
would only run for 30-60 secs before switching off.
Probably less than 10 min. "on" time is short cycling.
 
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Old 12-14-09, 05:01 PM
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It sounds like you are judging things by the fan operating cycle and not the best way to control the firing of the furnace you have.

Dick
 
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Old 12-14-09, 05:43 PM
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@hesaidshesaid - Is short cycling good or bad? Sorry I am a newbie to this.

@Concretemasonary/Dick - Sorry don't understand your point. Are you saying that the way it is now is much better for the furnace?
 
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Old 12-15-09, 08:44 AM
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The new t-stat has nothing to do with the fan running on the furnace..

There's a fan limit control on the furnace that comes on a X˚ temp, and off at X˚ temp. so, if you had a tune up or service on the furnace lately, the tech may of adj the fan limit control.

With the longer run time after the burner shuts off is better cuz it removing the heat from the heat exchanger rather than shutting down early and having the heat waste go up the chimney.

On the advance setting menu, I suggest to set the system type/cycle per hour to 3.
 
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Old 12-15-09, 08:48 AM
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Originally Posted by Jay11J View Post
The new t-stat has nothing to do with the fan running on the furnace..

There's a fan limit control on the furnace that comes on a X˚ temp, and off at X˚ temp. so, if you had a tune up or service on the furnace lately, the tech may of adj the fan limit control.

With the longer run time after the burner shuts off is better cuz it removing the heat from the heat exchanger rather than shutting down early and having the heat waste go up the chimney.

On the advance setting menu, I suggest to set the system type/cycle per hour to 3.
Thanks Jay11J - I have the RTH7500 tstat which is mentioned regularly in the forum.

What you are advising is that I change the function:
0240 - Heating Rate Cycle
from 5 - Gas or oil furnance
to 3 - Hot water or high efficiency furnace

Is that right?
 
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Old 12-15-09, 09:50 AM
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Correct.

With this setting, you may get longer run time, and gives the system a chance to warm up and run more effeceint than the stop and go you may be seeing now.
 
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Old 12-16-09, 12:47 PM
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Originally Posted by Jay11J View Post
Correct.

With this setting, you may get longer run time, and gives the system a chance to warm up and run more effeceint than the stop and go you may be seeing now.
I am not actually seeing an increased frequency of the fan going on and off. It is just the fan is running a lot longer since I changed thermostat.

I just wanted to understand the theory behind this because prior (with T87 thermostat) I would just get short hot heat blasts lasting 30 secs or so - but as a result the temperature of the house would fluctuate a lot.

Is running the fan for long periods much more efficient than short heat blasts? Just conscious of electric/filter costs vs oil costs.

Sorry to draw this out but I have come from the UK where I am used to central heating with radiators.
 
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Old 12-16-09, 06:13 PM
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Originally Posted by inforr View Post
I am not actually seeing an increased frequency of the fan going on and off. It is just the fan is running a lot longer since I changed thermostat.
Not sure why you see a change in the fan. The t-stat has nothing to do with the fan on/off. Just turns on the burner, then the furnace's fan limit control turns the fan on and off as what the limit control set to.


I just wanted to understand the theory behind this because prior (with T87 thermostat) I would just get short hot heat blasts lasting 30 secs or so - but as a result the temperature of the house would fluctuate a lot.
Ok, so if you were getting a short heat, I now can see why you had a short blower run after the burner shuts off. The heat exchanger hasn't had a chance to really get warmed up.

Now with the new t-stat, the burner stayed on longer and the heat exchanger is hotter, so after the burner shuts off, you have a lot of heat left on the heat exchanger.

Is running the fan for long periods much more efficient than short heat blasts? Just conscious of electric/filter costs vs oil costs.
Running the burner longer is going to be more efficent than the short blast.. The heat exchanger takes 5 to 10 minutes to get to peek efficiency than a short blast.

Don't worry about the longer blower run, it's easier for the motor to run than the stop and go stop and go.. stop and go..

No need to worry about anything.

How long have you been in this house with the oil furnace?
 
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Old 12-16-09, 07:35 PM
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Thanks Jay!

How long have you been in this house with the oil furnace?
We just moved in a month ago and decided to replace the old thermostats with newer ones to take advantage of 7 day scheduling and avoid the temperature fluctuations.
 
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Old 12-16-09, 07:47 PM
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Since you just moved into the house, do you know if the past owner had the furnace checked?

Another word, Oil furnace should be checked every year. Oil filter changed, burner, heat exchanger cleaned and checked. Burner should be tested for Smoke, CO, and draft test to make sure all is well.
 
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Old 12-17-09, 07:42 AM
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Duty cycle:
if your furnace puts out 1 therm/hr, and you only need 0.7 therm/hr because of the weather, it needs to be on 70% of the time.

With an on time of 10 minutes, the off time would be 4.3 min.

A house has a large thermal time constant. For a lamp filament with a thermal time constant of a few dozen milliseconds, an on/off time of 10/4 milliseconds is more appropriate.
 
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Old 12-17-09, 12:58 PM
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Now that you are conversant with duty cycles, you can right-size your next furnace using Excel.

Your furnace puts out 100,000 BTU/hr and you pick several typical winter days, and you get readings like below

btu/hr outside temp
25000 55
40000 45
50000 30
70000 10

-956.5217391 slope
79728.26087 intercept
-0.992214801 correlation

so, btu/hr = -956T+ 79700
so for 100,000 btu/hr
the 97.5 percentile temperature is
T = (Btu/hr - 79700)/(-956)
=-21 F

If the 97.5 temp. happens at 80,000 BTU/hr then your furnace is oversized. If it happens at 150,000 it is undersized.

Real world data will very likely give you a lower correlation coefficient than in this example.
 

Last edited by hesaidshesaid; 12-17-09 at 02:06 PM.
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Old 12-17-09, 12:59 PM
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Problem resolved!

Originally Posted by Jay11J View Post
Since you just moved into the house, do you know if the past owner had the furnace checked?
I actually had the furnace checked yesterday and no major problems were found.

Interestingly today I found that the system wasn't reaching the set temperature 67F even though it had been on for hours - it was stuck on 64F. So I called them out thinking it was something that had been changed during the tune-up.

It turned out that a zone valve was sticking. This explained why the heat coming out was not as hot all the time and guess it just so happened to start after I changed the thermostat.

Now the fan isn't on as long anymore and the air coming out is always hot.

Glad to get to the bottom of it!
 
 

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