High Energy Consumption with High Efficiency Furnace


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Old 02-02-12, 07:17 AM
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High Energy Consumption with High Efficiency Furnace

Last October I had a XV95 furnace installed (Michigan, house is 20 years old, 2800 sq. ft) along with TCONT802 stat, humidifier and XL16i A/C. In couple months I have noticed that my gas bills are much larger than I expected. New setup replaced 25 year old furnace and a/c and when I compare the bills with old and new system its about 2 times more now with this 96.7% effiecient furnace. Last week I had Trane local tech support people in my place to evaluate furnace performance. Initially the company who installed the system provided me with load calculation and it came out to be 60K for the furnace. But he installed 100K BTU furnace for 2800 sq. ft. house. Trane people told me that furnace is oversized (assuming initial heat loss calculations were correct) but in order to make a/c work installer had to do that to make it compatible with cooling requirements in the summer. Also they told me that estimated BTU from the furnace is approx. 90K BTU but there is about 30% heat loss due to inadequate air ducts, specifically cold air returns. Based on that they recommended to disable t-stat control and use furnace board timer feature to control 1st and 2nd stage sequence instead. I agreed to this experiment even though it does not really make sense from my perspective. Not sure how this is going to improve my energy consumption/usage? Iím still puzzled why my energy consumption and so far I really have not received knowledgeable explanation for this. It appears that I paid for something that will not be achievable? Anyone have any suggestions/recommendations, the comfort level is not there plus there is enormous gas usage?
 
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Old 02-02-12, 10:07 AM
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If everything else stayed the same I would expect gas usage to drop with the more efficient modern furnace. Yea the ducting may not be perfect for maximum efficiency but the old furnace also used the same so I'd expect the new to do better than the old.

Do you feel air blowing out of all the registers in your house? Have you inspected the ductwork to make sure that something was not knocked loose during the new furnaces installation. I wonder if you have a duct or two blowing hot air into the crawl space and not into your house, or that the cold air return is not connected and is drawing cold outside air. Both situations should be very obvious to a professional though.
 
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Old 02-02-12, 10:47 AM
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The installation from old to new could very well have missed something as Pilot stated.

When the new furnace runs on a cold day, how long does it stay on? Does it run for 20 or 30 minutes or does it run in short bursts for only 5 minutes or less. That's on a cold day.

Bud
 
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Old 02-02-12, 10:56 AM
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On the cold day, when temp is in single digits the furnace runs all day without shutting down. When its 20-30 degrees it runs about 15 minutes. I did check all hot air supply locations and warm air does comeout, though 3 rooms upstairs don't have the same air flow as rooms downstairs and there is about 3-4 degrees difference in those rooms compared to 1st floor.
 
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Old 02-02-12, 11:35 AM
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One more thing to note on the continuous run of the furnace is, is it just the blower that is running or is it the burner running as well? If you do have insufficient return duct size, I would expect the blower to keep running, but the burner would be turning off and on to keep from overheating the fire box. Some of the HVAC pros may correct me as I'm in oil country and not used to gas.

The fact that it keep running when it is supposed to be oversized would indicate something is not functioning correctly, which you already knew.

Check that burner and let us know.

Bud
 
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Old 02-02-12, 06:17 PM
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Has the gas pressure been confirmed. Has the meter been clocked? Do you have a leak somewhere? Has the evap coil been confirmed clean? Filters clean? Blower speeds confirmed? Where is the stat located?
 
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Old 02-02-12, 06:51 PM
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Is the furnace in the attic?

If yes, check for leakage.

As hvactechfw said, get the gas pressure checked.

A 2-stage furnace in an attic can actually burn more gas than a single stage model as a result of higher duct losses. (longer cycles)

The solution is to seal up the ductwork, not stage the furnace based on time.

30% loss is huge; you effectively have a 60-70% efficient furnace.

Your furnace has an ecm blower which ramps up to compensate for restrictive ductwork; higher pressure = greater leakage.
 
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Old 02-03-12, 04:24 AM
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The furnace is in the basement. The gas pressure has been confirmed, the coil is clean, no problem with the filter, the blower speeds were also verified and we did not find any leaks. The stat is on the 1st floor in the great room with cathedral ceilings, seems to be in proper location. I'm going to check the burner today as per your suggestions to verify proper operation. If I understand it correctly, while the furnace is running, the burners should be cycling on/off with the blower running, correct?
 
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Old 02-03-12, 05:09 AM
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If you have your fan switch set to "auto" most furnaces will run the air circulation fan for some time after the burner has turned off to insure that you get all the heat from the metal heat exchanger. If your fan switch is in the "on" position the circulation fan will always be on and the burner will fire as needed.
 
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Old 02-03-12, 06:07 AM
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What I'm looking for is on a cold day we would expect a rather long run cycle whilr the furnace brings the house up to meet the set temperature on the stat. Then an off time frame where the house temp drops. Once it trips the set temp again the cycle repeats. During those long run cycles, I would expect the burner to be on constantly. If the blower is not moving enough air, then the heat exchanger will reach a high setting and the furnace will shut the burner off for a short period while the blower continues to cool the heat exchanger. At a lower heat exchanger temp the burner will fire again and this on/off cycle will continue during the warm up cycle until the house reaches the stat set temp. This short cycling of the burner could indicate an insufficient air flow, or an oversized furnace, but it is a symptom that will help in the trouble shooting process.

Do you use gas anywhere else in the house, water heater, cooking stove, other?

Bud
 
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Old 02-03-12, 06:53 AM
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I have fan set to on. As for the gas usage anywhere else, I do use gas for water heater and the drier, thats it. I should be able to simulate cold day to verify burner operation just by increasing tstat temp by number of degress, correct?
 
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Old 02-03-12, 09:44 AM
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With your fan set to on, it will of course run continuously which makes it a bit more difficult to judge need for cycling.
As for just pushing the stat to a higher setting, that doesn't really simulate a really cold day when the furnace would be struggling to keep up. On a cold day, the heat loss is greater and the return air colder, thus extending the run time. For now, this is just one piece of data we need so not critical. IMO, your use is so out of line with what would be expected, that the problem is much bigger than just an oversized unit. To put your numbers into another perspective, your total use includes a base amount for hot water and dryer. If those were estimated and subtracted from last year and this year, both numbers would go down presumably by the same amount, but the difference between them would remain the same. When you then look at your increase in relation to just the gas used to heat the house it will be a greater percentage. Add to that, our winter hasn't been as bad as last, at least where I am, to your east.

Here's where I would start. One, do a mental check to be certain your hot water and dryer use has not significantly increased. Two, put the fan back to auto and let the system run as it was originally installed. Like Muggle said, don't stage the furnace based on time. With the furnace running based upon the stat setting you will then get a better feeling of run time vs idle time when it is off. You can also then check to see if the burner is running steady during an on cycle or cycling on and off.

A 100K furnace is not a monster for a 2880 sq ft home and your old furnace was working with the same ducts, so what is different now? Are the humidifier and ac new?

It may seem unrelated, but it is important, why the humidifier? A dry home indicates a lot of air exchange which can be leakage or forced leakage from the ducts. Is the humidifier currently running and where is the RH inside the home?

Have you had the blower set to "on" ever since the system was installed?

Bud
 
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Old 02-03-12, 10:06 AM
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Pretty much same usage as previous year. The fan was set to on just recently, I can set it back to auto. The humidiefier was put in together with furnace and a/c, rh in the house before humidifier was around 30% and with humidifier it stays around 40-45% now. I'll remove the jumper from furnace board and monitor run time/cycling and report back my findings.
 
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Old 02-03-12, 07:31 PM
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If the furnace is in the attic, the fan should be set to auto to minimize duct losses between cycles.

70F air moving through pipes in a cold attic looses heat.
 
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Old 02-03-12, 07:34 PM
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A 100K furnace is not a monster for a 2880 sq ft home and your old furnace was working with the same ducts, so what is different now? Are the humidifier and ac new?

Read more: http://www.doityourself.com/forum/ga...#ixzz1lNeCcvGX
Keep in mind that a 80k input he furnace puts out nearly as much heat as a 100k conventional model.

100k @ 95% is large for a 2800 sq ft house unless it's poorly insulated or in an extremely cold climate.
 
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Old 03-12-12, 09:22 AM
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Took me awhile. Here is update: I have had regional Trane rep come out to my house to verify that install went according to manufacturing specs. All seems to be correct. I have also paid independent source to perform Manual J calculations. According to the report the furnace installed is actually oversized: the heat loss is 72360 BTUH and the heat gain is 31692 BTUH. The proper furnace should be TUH2B080A9V4VA, 80000 BTUH. The 36000BTUH condensing unit would have been a better selection too. Would you recommend I request to replace my system with 80K BTU furnace and 3 ton A/C? or should I keep existing system and optimize it with air-balancing? The ducts must be sized for 1276 CFM for cooling and 961 CFM for heating. I understand that if I keep existing system, 1600 CFM is the amount to be balanced. Any suggestions?
 
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Old 03-12-12, 11:22 AM
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Took me awhile. Here is update: I have had regional Trane rep come out to my house to verify that install went according to manufacturing specs. All seems to be correct. I have also paid independent source to perform Manual J calculations. According to the report the furnace installed is actually oversized: the heat loss is 72360 BTUH and the heat gain is 31692 BTUH.

The proper furnace should be TUH2B080A9V4VA, 80000 BTUH. The 36000BTUH condensing unit would have been a better selection too. Would you recommend I request to replace my system with 80K BTU furnace and 3 ton A/C? or should I keep existing system and optimize it with air-balancing? The ducts must be sized for 1276 CFM for cooling and 961 CFM for heating. I understand that if I keep existing system, 1600 CFM is the amount to be balanced. Any suggestions?
You might get by okay with the larger furnace sizing. However, I'd probably go with a 2.5-Ton A/C as you need the longer runtimes for the lower temp days when humidity is the major discomforting factor!

Read more: http://www.doityourself.com/forum/ne...#ixzz1ovVEWddI

The heat loss of 72,360 BTUH and the heat gain is 31692 BTUH; but are usually padded to varying extents.

I also believe there is a problem causing the furnace to not deliver near its capacity to the rooms.


The summer design in Detroit is only 88-F db & 72-F wet bulb or 46% relative humidity. You need long runtimes for that humidity level & I would say probably no more than a 2.5-Ton A/C; if the heating load calc was close to correct.

Also, most 60,000-Btuh furnace's can be selected to deliver 1200-cfm or, 3-ton of air for the A/C. So, it appears they installed a 4-Ton A/C; way more than for MI conditions.

You need to use the CPH setting in both heat & cool modes, especially during milder weather in cooling mode, to get the length of cycle run-times that are most efficient.

There really isn't any reason to oversize gas or oil furnace heating as it is simple & low cost to buy portable supplemental electric heat, if ever need be, for the rooms you are in on those very few coldest nights.

Additionally, you can dress warmer on those rare extreme times.

Something is NOT Right with the initial install; are the Return Air filter areas sized for 300-fpm velocity at 1600-CFM of air flow? A manual D should be performer on that duct system whether you downsize or not!

Could the Return be pulling cold air from the attic; what kind of temp-rise is it delivering at specific CFMs to he rooms, or losing supply air somewhere. Did he check actual CFM delivered to the rooms? That is a must do!
 
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Old 03-12-12, 12:07 PM
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So you believe I could leave the furnace alone and just downgrade to 3 ton A/C? As for the duct system, initially the contractor did not do any calculations what so ever. On average I get about 35-45 deg. temp rise. When I paid independent contractor he provided some basic schematic. I'm not sure how to attach images here.
 
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Old 03-12-12, 01:13 PM
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So you believe I could leave the furnace alone and just downgrade to 3 ton A/C? As for the duct system, initially the contractor did not do any calculations what so ever. On average I get about 35-45 deg. temp rise. When I paid independent contractor he provided some basic schematic. I'm not sure how to attach images here.
Read more: http://www.doityourself.com/forum/ga...#ixzz1ovwew0in

Can the airflow be adjusted so it gets around a 60-F temp-rise?
Multiply 64.8-F times the air flow, taken just after the A/C coil, to get the Btuh the furnace is delivering. Now take the temps at the supply register outlet to see how much is being lost.

You Divide the Rated Btuh output 90,000 by the delta-t, temp-rise to get the CFM.
35-F X's 1.08 is 37.87-F temp-rise. The furnace output 90,000 Output (?) / by 37.87 = the CFM 2376-cfm (No Way; something is wrong);
depending on where you took the temps;
if just after the cooling coil that furnace wouldn't be delivering 95,000-Btuh;
if at the SA outlets then it's losing way too much heat from the duct system!

Okay, 90000-output / 64.8-F temp-rise is 1388-cfm.

Example; a 57,500 output / using 64.8-F temp-rise = 887-cfm.

So, what is happening...?
 
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Old 03-12-12, 02:04 PM
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Ok, at the furnace inlet temp is 74, outlet temp is 126, delta is 52. Then when I measured the room on 2nd level, which is the far away from furnace compared to all other rooms here is what I got: inlet temp for return grill is 69.5 and floor register temp is 106.5, delta is 38. Then what I calculated next is this:
----------------------@Furnace-------------------@System
Actual airflow-------=1678-----------------------=1678
Temp. delta---------=52--------------------------=38
Times 1.08----------=56---------------------------=41
Delivered BTU-------=93968---------------------- =68798
Equipment BTU-------------------------------------=93968
System BTU-----------------------------------------=68798
BTU Duct Loss---------------------------------------=25170

So I do understand I have very high duct losses and I plan to seal it, at least I will attempt to the best of my abilities. The question is do i replace my 100K BTU furnace or just leave it alone and only replace the 4 ton A/C?
 
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Old 03-13-12, 06:16 AM
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More importanly I wanted to confirm that if I replace my condenser, how do I match it to evap.coil? Exact match, evap coil slightly bigger? Current condenser is 4 ton with 5 ton evaporator coil. Estimated condenser size according to Manual J is 3 ton. So can I keep my coil or is it too large for 3 ton? Thanks
 
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Old 03-13-12, 10:56 AM
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More importantly I wanted to confirm that if I replace my condenser, how do I match it to evap.coil? Exact match, evap coil slightly bigger? Current condenser is 4 ton with 5 ton evaporator coil. Estimated condenser size according to Manual J is 3 ton. So can I keep my coil or is it too large for 3 ton? Thanks
Some 3-Ton condensers can be matched with a 5-Ton evaporator coil; have them check it out or, go on the Internet & search for possible matches.

It will need a TXV refrigerant metering device that will handle a 3-Ton condenser; check that out, it might be okay.

Read more: http://www.doityourself.com/forum/ga...#ixzz1p1KgarwI
 
 

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