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Old 10-27-2009, 08:37 AM   #1
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Reducing BTU's


I have a Lennox G26Q series sealed combustion natural gas furnace which is oversized for my newly insulated older home.

Is there a way to reduce BTU output?

Thanks
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Old 10-27-2009, 08:54 AM   #2
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Reducing BTU's


Not safely or without potentialy damaging the furnace.
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Old 10-27-2009, 04:23 PM   #3
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Reducing BTU's


Especially with that one, very sensitive to draft thru the heat exchanger.
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Old 10-27-2009, 05:39 PM   #4
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Reducing BTU's


Would it work if the NG were diluted to reduce its heat of combustion? Does anybody make such a dilution device?

Or the extra BTUs could be diverted to preheat water for a water heater by using an air/water heat exchanger, or be converted into electricity.

How many BTUs need to be diverted?

Last edited by Yoyizit; 10-27-2009 at 05:51 PM.
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Old 10-27-2009, 07:08 PM   #5
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Reducing BTU's


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Originally Posted by Yoyizit View Post
Would it work if the NG were diluted to reduce its heat of combustion? Does anybody make such a dilution device?
Nope. that would make matters worse. By causing condensate in the heat exchanger.
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Old 10-27-2009, 07:11 PM   #6
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Reducing BTU's


I think a small DIY gas turbine with a generator would do the trick. Wonder if Popular Mechanics has plans for one.
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Old 10-27-2009, 07:48 PM   #7
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I think a small DIY gas turbine with a generator would do the trick. Wonder if Popular Mechanics has plans for one.
I asked them, and they referred me to you

If you can store the excess BTU/hr in something with a large heat capacity
http://en.wikipedia.org/wiki/Thermal_mass
and then return the heat to the air in the house, the air won't cool very fast so the furnace may have the same "on" time but a very long "off" time.
Whether it's practical depends on how many BTU/hr needs to be stored and returned. It may not be pretty.

In Germany they use masonry blocks to accomplish this.

Last edited by Yoyizit; 10-27-2009 at 08:07 PM.
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Old 10-28-2009, 07:59 AM   #8
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Reducing BTU's


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Originally Posted by tedhermitage View Post
I have a Lennox G26Q series sealed combustion natural gas furnace which is oversized for my newly insulated older home.

Is there a way to reduce BTU output?

Thanks
Okay I'll bit, how do you know it is over sized?
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Old 10-28-2009, 11:42 AM   #9
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the furnace is oversized as a result of a computer generated heat loss/gain run on the structure and confirmed by the short cycling of the furnace during operation and my experience with the heating capacity of a 100k BTU furnace and a well insulated 1450 sq ft two story home.
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Old 10-28-2009, 12:38 PM   #10
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Reducing BTU's


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Originally Posted by tedhermitage View Post
the furnace is oversized as a result of a computer generated heat loss/gain run on the structure and confirmed by the short cycling of the furnace during operation and my experience with the heating capacity of a 100k BTU furnace and a well insulated 1450 sq ft two story home.
So how many BTU do you have to divert to get a reasonable duty cycle on most days?

Some of this from several sources may be relevant \/

Let's say a normal cycle for some system is 45 min on, 15 min off, duty cycle is 75% and total cycle time is 1 hour.

What is short cycling?
30% duty cycle?
Total cycle time of 30 min?
55 min on, 5 min off?
5 min on, 55 min off?

I think the key here is how it achieved the required run-time. You could get a 50% duty cycle by running alternate minutes, or even seconds, but you'd quickly fatigue the system. Many things like a minimum off time, and if you turn it on again before that period, you probably aren't using it efficiently.

Vaque question. But to use an HVAC example> A system which is too large for the area will "short cycle". Meaning it will run for a fairly short time, perhaps only a few minutes. The thermostat will be satisfied and the unit will turn off. But one function of cooling is to dehumidify the air, which makes people more comfortable. Having the room air circulated through the cooling coils for a longer period of time accomplishes this, whereas in the short cycle mode, less dehumidification takes place. ]


So it all depends on whether you are talking about HVAC

If the heater is oversized it will overshoot the thermostat because the blower turnoff is normally controlled by the plenum sensor. In a normal installation you account for this by the anticipator setting on the thermostat, but if the furnace is too large this doesn't work well and you overshoot the setting. The user turns the stat down below the desired temp to prevent the room from getting too hot due to the overshoot and the end result is big swings in temperature.

If the unit is properly sized it will run nearly continuously (95%) on the coldest design day.

n a heating system, for example, if it normally takes 30 minutes to go from burner on, to shut down, and it starts taking 5 minutes for it to happen, that is short cycling, and it could happen for any of several reasons, but one result is usually that the customer is uncomfortable because it is NOT heating properly.

Sounds like your furnace is at least 3-4x oversized, and your AC 1.5-2x oversized. (On design day your heating system only sees the full load for a handful of hours, but the daily average load on design-day will typically be well over half the full-load.)

Short cycling is about more than duty cycle. There's a fixed amount of loss that occurs on every cycle, so maintaining a reasonably long minimum cycle is as important for efficiency as increased duty cycle. For a typical hot-air furnace it won't hit it's full steady state thermal-efficiency until it's been burning for at least a coupla minutes, and the first 30 seconds are about half it's full-rating. (For cast-iron boilers it's more like 6-10 minutes, due to the higher thermal mass.) On AC compressors it's a similar issue.

Minimum cycles can be stretched by the amount of hysteresis in the controls (thermostat, humidistat, other), or by adding thermal mass to the system (hard to do when air is the heat-transfer fluid.)

An ~80% AFUE- type hot air furnace running a 25% duty cycle on design day is really only hitting about 60% actual as-operated AFUE efficiency, even if it's not short-cycling (meaning all burns are at least 8-10 minutes or 4-5x the ramp rate to steady-state.) See:

http://simulationresearch.lbl.gov/dirpubs/42175.pdf

Look at figure 5 for derating your AC. If you're at 50% duty cycle on design day, you're running between 90-95% of steady-state efficiency, on THAT day, but most of the season you're much further to the left on the curve, something like the 10% of full-load zone.

For the furnace, see fFigure 6. Unless it's something special (sealed-combustion & forced draft condensing, etc.) , assume it's part-load performance is more like the SDL-C111 curve. If on design day you're at the 25% of full load mark, you're efficiency will be about 85% of full steady state thermal-efficiency rating. AFUE is typically 3-5% below steady-state efficiency assume steady state is 83-85%. 85% of 85% is 72%. But over the season most of the fuel is going to be burned during 5-15% of full-load region, so figure ~55% as-used AFUE if you have a standing pilot, ~60% if you have electronic ignition. If it's a fully sealed-combustion system (condensing or otherwise) with electronic ignition & automatic flue dampers, etc, it'll look more like the Bonn 85 Induced Draft curve, which isn't bad.

FWIW: Bonne was the guy who did the best most-accurated baseline measurements of this stuff back in the '80s while studying short-cycling issues, which is why the DOE regression analysis models try to fit his measured data. But if the equipment is actually short cycling a lot of the time you can toss those curves out the window- assume it'll be at least 10% worse, possibly much more.

Matching the equipment output to the load is the first most-important piece of designing for efficiency. The tendency in the trades over the years has been to always err to the overcapacity side to avoid the mid-winter morning call from the irate & freezing customer, but it's almost never the right thing to do. That's changing slowly, but there's still a wealth of ignorance out there- 3x oversized heating systems are almost the NORM in my neighborhood. AFUE presumes a 1.7x oversizing factor. By right-sizing it you can beat the AFUE numbers in a well designed system. But at 3x oversized and up you're slipping over an efficiency-cliff. (AFUE has many arbitrary and usually wrong assumptions- don't get me started! :-) )

Even if undersized slightly, most of the design-day heating hours occur while the occupants are in bed, and they never actually get cold even if the output lags the load for a few hours between 3-6AM. Similarly, if the AC is slightly undersized, even if it doesn't keep up with the full sensible-load on design day the fact that it's running at 100% duty cycle dries the air sufficiently that it's rarely a comfort issue, even if the temps run a few degrees high for an hour or three.


Last edited by Yoyizit; 10-28-2009 at 12:43 PM.
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Old 10-28-2009, 02:39 PM   #11
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Reducing BTU's


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Originally Posted by tedhermitage View Post
I have a Lennox G26Q series sealed combustion natural gas furnace which is oversized for my newly insulated older home.

Is there a way to reduce BTU output?

Thanks

You could call the manufacturer and explain your situation...they may be able to provide you with conversion equipment to bring down the btu's (ie. Smaller burners, orifices, or a mixture of the two)
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Old 10-28-2009, 05:55 PM   #12
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Reducing BTU's


I would NOT do that on that particular furnace. I am a Lennox tech and can tell you that the increased velocity to air mixture will be a huge problem and you cannot alter that part of it.
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