Do a load calc.
You may have no choice but to put in the proper size furnace or deal with the condensation.
Dear Friends,
I have 10 yrs old, Amana 80% efficiency gas furnace and it vents out through b-vent flue pipe. The b-vent pipe leaks in attic and basement due to condensation.
I hired a HVAC pro to diagnose the problems. He used an analyser and found that the flue gas temperature is only around 225F and this low temperature results in condensation in the b-vent pipe. He mentioned that it should be around 300F for the proper venting of flue gases without condensation.
He found that the furnace is also short cycling. He adjusted the gas supply which helped to minimize the short cycling to some extent. But, it didn't increase the flue gas termperature.
He spent more time trying to get the flue temperature high. But, he was not successful.
He finally recommended changing the furnace unit which I don't like to do.
Please, help me with my situation now. Did he miss something in his diagnosis? How to increse the flue gas temperature? Any help is appreciated. Thanks a lot.
Sanrishi.
[Edited by sanrishi on 02-15-2005 at 02:28 PM]
Do a load calc.
You may have no choice but to put in the proper size furnace or deal with the condensation.
How tall are you Private???!!!!
Hi BoltonNC,
Thanks for your reply. What is load calc? My home is approx. 1500 sqft and we don't have any problems with the heating. We get sufficeint heat with this furnace. The only problem is condensation water leaks in attic and basement.
Was the reason for the furnace short cycling ever determined?
Did this condensation start after the short cycling problem appeared?
Did this contractor measure the total external static pressure of the fan to determine how much air the furnace is moving?
Did he measure the temperature rise of the furnace?
If a furnace is moving too much air this will also cause the flue gas temperature to be low.
Extreme care needs to be taken when adjusting gas pressure to increase the firing rate of the furnace.
There is a fine line between a properly fired furnace & an overfired furnace.
There are readings such as carbon monoxide,oxygen & draft that also need to be measured to insure the safety of the appliance.
Sorry for the questions,just trying to get a place to start.
He means you have too much heat availble and no place to use it. Your condensation problems will likely continue and you will sooner or later be replacing it anyway as that furnace doesnt like water.
It may also be underfired by monkeying with the gas pressure but he sounds like he knew what he was talking about.
Good luck, no quick fix here.
Was the vent system inspected for blockage. The inducer pressure switches on some of these furnaces are so outrageously high that they will continue to run with a partially blocked vent. This slows down the flow of vent gases through the furnace removing more heat and reducing stack temperature and causing condensation.
Ok,
The gas pressure was adjusted to solve the short cycle problem. Is it still short cycling ? because that in itself can cause the flue gases to condense if the vent is running through could areas in the house or adjacent to an outside wall. If I hear that the gas was adjusted to compensate for short cycling I'm thnkng he turned the pressure down not up.If he turned it down this will probably compound the moisture, I think he should of got a temp rise and clocked your meter to see what the actual input was to the furnace it sounds like your furnace is probably oversized and underfired or dealing with some extremely cold air in the basement . What was the BTU rating of the furnace and what region are you in? and what exactly is condensating is it the outside of the pipe or in the pipe?
What temp do you keep your home set at?
I am thinking you are moving too much air across the heat exchanger!
Quality and Value Service and Repair
Thank you all for your help. Some of the answers that I know. Please, help me.
Freezeking2000:
We set the temp at 66. My home is 1440 sqft.
fat eddy:
You are right. He turned down the pressue and it really helped to fix the shortcycling. Now, it is definitely running longer than it used to be. But, I am still collecting the same amount of water in my attic. So, turning down the gas pressure didn't increase or decrease the amount of water that I collect in my attic.
My furnace is 67000 BTU, 80% effecient, serving 1440 sqft. I am in eastern Massachusetts.
It is condensing inside the pipe. I have not seen it outside.
My basement is not very cold. It is always around 55F. But. My attic really gets very cold. It is closer to outside temperature.
MechAcc:
He partially inspected the vent system and found no blockage. His main concern was the low flue gas temperature.
docholiday:
My furnace is 67000 BTU, 80% effecient, serving 1440 sqft. I am in eastern Massachusetts. Is it oversized?
davidr:
I think that gas pressure was causing the short cycling. He turned down the gas pressure and now it is running longer than it used to be. It also helped to reduce the initial CO level to within 400PPM( before he adjusted the gas pressure it was around 600PPM)
I am not sure he measured the total external static pressure of the fan. But, he gave me the final stats of the analyser. It reads as follows,
Efficiency-------- 81.7%
Excess Air-------- 162.4%
Primary Temp------ 51.0 F
Stack Temp-------- 214 F
O2---------------- 13.5%
CO2--------------- 4.2%
CO---------------- 24ppm
CO AIR FREE------- 68ppm
Draft------------- 0.17 WC
He also measuerd the differential temp which is in the proper range(40F) as described in the furnace manual.
Does the readings are okay? Is the flue gas temp too low? Should it always be around 300F?
I hope that I answered all your questions. Please, help me.
Thanks again for all your support. This forum is amazing!!
[Edited by sanrishi on 02-16-2005 at 11:58 AM]
No, those numbers are not ok.
Oxygen number is huge. Should be down somewhere around 6% to 9%. Stack temp is low. Underfiring is contributing to the high O2 number, but other factors can figure in there - like the vent configuration. If the furnace is sized right, it should be fired right. We still don't know what the temp rise is. Another thought on short cycling may be the anticipator or cycle rate setting on the stat.
the high o2 number could also be due to a cracked heat exchanger i haven't seen many gas units run a 6% o2 most of the time it is between 8-11%. what size is the flue pipe, could be oversized. I would change the furnace to a direct vent 90% save money on gas a do away with the chimney. How old is the heater anyway??Originally posted by 7X
No, those numbers are not ok.
Oxygen number is huge. Should be down somewhere around 6% to 9%. Stack temp is low. Underfiring is contributing to the high O2 number, but other factors can figure in there - like the vent configuration. If the furnace is sized right, it should be fired right. We still don't know what the temp rise is. Another thought on short cycling may be the anticipator or cycle rate setting on the stat.
7X:
what do you mean by temp rise? Please, help me. I have programmable honeywell thermostat. I looked at the user manual and it seems that it doesn't have manual cycle rate setting.
pipedope:
The flue pipe is 5inch b-vent. The furnace is 10 yrs old. I bought this home on last August. I am not sure how long this problem is going on.
what causes high oxygen number?
thank you.
That's caused by underfiring a furnace. It's the reason your fuel bills are going to go up.
Temp rise is measured between the supply duct temperature and the return duct temperature, near the furnace. If it's not right, the duct size doesn't matcgh the furnace size someplace.
This discussion should have happened BEFORE a furnace size was selected. WAaay back when...
Noel
Thanks for all the info, It is a lot easier to troubeshoot a problem with the right info, although I do not understand some of the info that you have stated, I'm hearing that you having a condensing problem in the attic space and that it is also on the inside of the vent and not the outside, how are you confirming that it is condensing on the inside of the flue running through the attic, how can you tell ? if it is running I'm sure you are not looking into the vent and if it is not running and you look in and see moisture then the pipe is getting very cold very fast after shut down, are you sure that this is a double wall pipe ? and is the water heater flue common to this vent maybe we are looking at the wrong appliance ? also are there any bathroom, kitchen or laundry exhausts vented through the attic space or into them. As for the numbers you are reading they are not the best but they are also not the problem seem to be a little short on combustion air. Did you get a CO reading in your attic because in many cases that I have seen where a furnace was emitting fumes with CO into a living space the spaces also had a high content of moisture in the air, comes from burning gas I would also like to know if your house is in a negative pressure go crack your door open slightly hold a kleenex there and see if it wants to drift in or out of the house. Thanks
[QUOTE]Originally posted by pipedope
The O2 number would have to increase after the blower comes on to indicate a breech in the heat exchanger. Unfortunately we don't have access to those readings.
the high o2 number could also be due to a cracked heat exchanger i haven't seen many gas units run a 6% o2 most of the time it is between 8-11%.
Pipedope on a furnace that is running 8%-11% O2 it is probably a tad bit underfired.
Eddy,how did you determine he was short on combustion air from the readings that were given?Originally posted by fat eddy
As for the numbers you are reading they are not the best but they are also not the problem seem to be a little short on combustion air.
Thank you all for your help.
fat eddy:
It is double wall b-vent pipe and it leaks in joints in the attic. I have also checked with kleenex and the drift is inbound to to the home.
davidr:
what causes the under firing? How to decrease the O2 level?
I am not sure whether I am still having short cycling. When I monitered it, the fire stays on for only 3 minutes and the blower continues to run for 2 minutes after the fire. so the total cycle is 5 minutes. Is this normal?
Originally posted by sanrishi
davidr:
what causes the under firing? How to decrease the O2 level?
Not enough gas. Increase firing rate. Add gas
I am not sure whether I am still having short cycling. When I monitered it, the fire stays on for only 3 minutes and the blower continues to run for 2 minutes after the fire. so the total cycle is 5 minutes. Is this normal?
Did the tech check the heat anticipator to see if it was set properly on your thermostat if it is not digital.
If it is digital it may not be configured properly.
It could also be that the thermostat is being satisfied,any supply outlets close to the thermostat?
davidr:
my thermostat is digital programmable honeywell thermostat.
Should I configure the digital thermostat?
An electrical supply outlet is within 2 feet range. Can it cause anything?
thank you.
considering that no wind is blowing it in it would sound as though your home is running in a negative pressure, as for the leaks in the attic I'm assuming that you have condesation leaking through a joint and running down the outside of the pipe, this is not typical of the configuration of a b-vent although not waterproof it is usually a pretty tight connection. both these problems need to be properly addressed a house should never be in a negative pressure and your attic is probably freezing because it is, If you have a fireplace check to see if the damper is shut, then turn off any equipment you may have running that is exhausting air from your house. Check to see if you can find any soure for combustion air entering the furnace room.