Optimizing temperature control

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  #1  
Old 02-06-13, 08:39 AM
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Optimizing temperature control

I recently purchased a new home and the existing gas furnace is original. It is a Trane XE60 and it is controlled by an old school thermostat. I had planned on installing a new programmable thermostat but I am holding off until we actually move in. Don't want to find out it is defective and come back to frozen pipes. Anyway, while working on the house I noticed that the temp stays very stable but the furnace is constantly cycling. From poking around here it seems for older furnaces this is quite normal and actually suggested for it to run 6x per hour. Where as newer high efficiency units should run 3x's per hour. Just seems like this constant cycling is not only wasting energy but also putting extra strain on already old components. Is there a reason to not set this to 3 cycle per hour other than the wider temperature swings? Is there really a benefit in efficiency or is it just my impression? Even though the unit is constatnly cycling it isn't using as much gas as I had feared. We have been keeping the temp in the low 60's and outside temps have often been in the teens recently and bill indicated around 8 therms per day. So just wondering if making this change will actually save any money, if not it isn't worth hearing the wife complain about the temp swings.
 
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Old 02-06-13, 12:51 PM
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Personally, I would leave it as it is.

I doubt that the additional cycling is costing you much at all.
 
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Old 02-06-13, 01:36 PM
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FYI, your furnace is 60% efficient. For every $100 in utilities cost $40 is going up the chimney.
 
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Old 02-07-13, 11:35 AM
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Personally, I consider HVAC Tech estimate of fuel efficiency to be greatly exagerated.
 
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Old 02-07-13, 03:10 PM
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XE60 = 60% afue...............................................
XE70 = 70% afue............................................
XE80 = 80% afue..............................................
XE90 = 90% afue.........................................
 
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Old 02-07-13, 03:17 PM
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page 3 https://docs.google.com/file/d/0B0nk...it?usp=sharing

I'm sorry, XE60 = 62 - 63.2% afue for the product line............
 
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Old 02-08-13, 02:24 PM
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Phooey on AFUEY!


The inefficiencies of a standing pilot can be greatly reduced by shutting off the pilot after the heating season.
 
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Old 02-08-13, 09:13 PM
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I disagree.

One thing to remember about those old furnaces is that the real seasonal efficiency varies greatly depending on the length of/number of cycles.

The thermal mass of those heat exchangers is quiet high -> they take a lot longer to warm up at the beginning of each cycle. Having the fan come on only at 140F (instead of 30-45 seconds on a new furnace) compounds the problem -> heat could potentially pour up the chimney for 2-4 minutes at the beginning of a heating cycle which lasts less than 10 minutes.

When the burners shut off, a lot of the residual heat (originally absorbed when the heat exchanger warms up) flows right up the chimney; sure, the fan runs for a while, but there's nothing to prevent the chimney to continue drafting and pulling heat out of the furnace.

From the spec sheets I've seen, natural draft units with electronic ignition are rated at 68% AFUE give or take. The use of an optional damper (trap moist, corrosive exhaust in the heat exchanger - great plan! ) puts the seasonal efficiency in the 70s.
 
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Old 02-08-13, 09:17 PM
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To answer the original question:

Setting the cycle rate below what's recommended by the t-stat manufacturer won't harm your furnace.

6 cycles per hour is a lot.

I would set it to 3 or 4.

The longer each cycle is, the less heat your furnace will waste. Just like a car in stop and go traffic, old furnaces run very inefficiently when they're constantly cycled. (see above post)
 
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