View Full Version : impact of improper venting
yesterday, I posted a thread about having much higher than expected gas bills with our new Rudd 92% furnace and possibly having my back contractor out to check the performance of the furnace. In looking though the archives here there are numerous posts about the importance of following mfg venting guidelines to the letter (especially for Rudd/Rheem), which my contractor really didn't.
Rudd Achiever 92% upflow 100k btu single stage. For both the intake and exhaust, he ran 3" schedule 40 from the unit. He used two long sweep 90's and 4 foot straights to get up to the joists and about 8 feet of straight out through the rim. This looks like it is to spec. However he ran the 3" out through the wall and then used two more long sweep 3" 90s to place the exhaust a foot above the intake. He did not reduce to 2" as the manual requires and didn't use the vane kit on the intake. Could this reduce the efficiency of the unit?
Thanks for any advice. I appreciate it very much.
01-27-2006, 10:49 AM
I don't see a venting reducing fuel usage, unless it is causing short-cycling.
Does it have long run times?
Is the furnace sized correctly for your house?
Is it multistage, and controlled with a multistage thermostat?
Is it installed/ducted properly?
Post the model numbers, and pictures (if possible) for more information.
Originally posted by djj
yesterday, I posted a thread about having much higher than expected gas bills with our new Rudd 92% furnace
Dennis, if you have some old gas bills or if your utility has a website where you can review your past usage/bills, you'll probably find that it's the cost of the gas that has gone up, not your usage.
Try to find a historical month (when using old furnace) that has close to the same number of heating degree days as the current bill. Compare the number of therms or ccf used in each (you might have to divide by number of days in billing cycle to get a therms/day, since most billing cycles can vary a few days). My guess is you'll find you used less gas in the latest bill. If so, be thankful...imagine how much more you could have paid! :)
Thanks for your replies. I will try and answer both your questions.
Just to be clear, this is a new construction so we don't have any old bills to compare to. However in looking at the heat loss calcs, we appear to be using about 30% more gas than the manual j would predict for the temps we have been having. Our local utility company does all the manual j's and are very thorough. Our heat loss calcs with a design temp of -15 deg (Iowa) are about 77k BTU/hr. Their recommended sizing for the 92% furnace was 85-100k BTU.
It is a single stage Ruud URGRJ model and is 105k BTU. My contractor said he went with the slightly larger size to better match with the AC sizing as Ruud only offered 90k or 105k.
Heat cycles are running about 12 minutes long with about 8-9 minutes of actual burn time.
All supply ductwork is in conditioned space and all joints/seams are sealed. The system appears to be pretty well balanced as most rooms are normally within 1 degree of each other.
As I mentioned in previous post yesterday on a performance check, I may have him come out and check the system out but want to cover all the bases first.
thanks again for everyone's help.
01-27-2006, 02:09 PM
If you could have him give you the return air temperature and the plenum temperature, check the static pressure of both and then check the furnace blower specs it would be possible determine within smoe approximation how many BTU's are actually getting transferred into the air stream.
01-27-2006, 02:11 PM
We reduce if at all possible. Only time it isn't is retrofit.
But I see no effect on operation or cost to run the unit. I would guess they do that to throw the exhaust away from the house more.
His logic on 90 or 105 makes no sense. Both are same cabinet, same blower. Probably that the bigger one in stock. Still don't see that affecting the gas bill.
In another post, Dennis, you stated,"Our calculated heat load is 83k at 92% with a design temp of -14 and a TD of 84 deg. In Nov we used 21.7 MCF of natural gas. This was with 826 heating degree days in the month, not quite double the October HDD, but with almost 4 times as much gas usage."
21.7 mcf or 217 ccf is an awful lot of gas for only 826 hdd. How many sq ft is the house? I looked back at our highest usage with the old 80% we replaced last summer and it was only 172 ccf for 1189 hdd in 1/05 for our 1900 sq ft house.
Do you have any other gas appliances? Cooking, water heating with recirculation, hydronic floor heating, driveway melting, shop/garage heater, gas fireplace, etc.? They can all add to the usage. We use 16-20 ccf in the summer for water heating, so it's at least that in the winter.
Any fireplaces that might have a damper open? You said the house is tight, do you have a fresh air inlet or air exchange system? If so, maybe it's not working correctly.
The house is 2600 square feet with 2000 in the unfinished insullated basement. The water heater is gas and there is a direct vent gas fireplace but we only use it a couple times a month for an hour or two. Other than that all is electric.
There is no air exchanger.
01-27-2006, 09:32 PM
Calculations are done using "standard" usage and sometimes "ideal" conditions.
Do you have more than normal traffic in and out of the home? Someone taking long showers? I have kids that can run that ccf up real fast. Wife cooks a lot in the oven.
Other would be to make sure all ducting is sealed properly. You can get more infiltration in the home with the exhaust above the stove running.
I was at a home the other day looking to see why the heat pump was running alot- the damper was open on the fireplace (and no fire) and the exhuast over the stove had no flapper or "damper".
I'm almost out of ideas...but here's a couple more.
Manual J also deals with heat gain. Maybe you aren't getting as much solar gain as the MJ suggests. Do you have many south facing windows? Are they uncovered during the day? If so, are they regular or low-e?
Do you have any neighbors with similar sized houses with whom you could compare heating bills? If theirs are in the same ballpark, then there is probably not much you can do if the system checks out and the gas meter is not defective.
01-27-2006, 11:57 PM
you could also try clocking the gas meter with the furnace,and water heater both individually.
01-28-2006, 09:13 AM
which month had the lowest gas usage, May or June?
subtract that number, since it closely indicates the gas used for cooking, water heating, etc, from your last months gas --
now divide that number by the DegreeDays
now divide that number by the sq foot ( just for info )
now track these numbers
do you have storm windows & doors? they add insulation & protect the main window, cheaply!
have trees to act as wind breaks?
wasn't the basement insulated on the outside? shame, shame -- how much of this wall is exposed? was that taken into account?
read at BUILDINGSCIENCE.com
are the ducts insulated? are the plenums insulated?
it seems that you had an excellent contractor, in- as- much- that the rooms are holding within 1F of each other!
01-28-2006, 10:10 AM
A short cycling furnace is going to be less efficient because furnaces do not perform at peak efficiency until they have had time to warm up a bit. This is why it is usually better to slightly undersize a furnace to the design conditions then to slightly oversize it.
Since your design temperature is -15, and it is doubtfull that this is norm for long running temperatures but rather a once in a while "get down to" temperature, and you have the larger of the two choices of furnace sizes, if nothing else is going on it is most probable that your furnace is simply oversized.
Of course, it could also not be properly operating or you could have a leak in you gas line somewhere. You have to determine with very critical data details exactly what is going on in order to be able to track down if you even have a problem. It could be that the energy usage estimates are just based on a more frugal usage of the heating system then you prefer.
01-28-2006, 12:59 PM
What temps have you been having, and how much gas did you use.
It has been in the low 30's, we had 961 heating degree days this billing period and we used 25.8 MCF. Our gas bill was around $300.
01-29-2006, 01:38 AM
again, how much gas did you use in June?
dan sw fl
01-29-2006, 07:28 AM
Originally posted by djj
... low 30's
961 heating degree days this billing period
Usage & HDD's for each month last winter ?
I am sorry I thought I had mentioned in an earlier post, this is a new construction in Iowa and we just moved into the house in September. The only previous bill I have a copy of(other than the ones posted) is for October where the gas bill was about $70 and we used 5.5 mcf of gas.
01-29-2006, 08:48 PM
Is it possible that they estimated your bill.
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