Furnace takes too long to raise temp


  #1  
Old 01-01-09, 03:03 PM
M
Member
Thread Starter
Join Date: Jan 2009
Location: san jose CA
Posts: 5
Upvotes: 0
Received 0 Upvotes on 0 Posts
Furnace takes too long to raise temp

RHEEM RGLC-07NC-GR , 1988, 75,000BTU/HR, 1/2 HP motor. To raise house temp from 67F to 74/75F (outside temp is 50F San Jose CA) takes CONTINUOUS running for 3 hours. Is this normal?
All ducting has been checked and is good. Furnace runs normally and gas jets are fired up for the whole 3 hours. Floor registers shows 110F. Unit is a combined Heating/AC downflow with a hallway ceiling return vent. Air flow seems normal at all registers. Turn off takes place normally once temp has finally been reached. House is 1750 sq ft, single story, wall and ceiling insulated, and has double glazing.
 
  #2  
Old 01-01-09, 03:19 PM
Grady's Avatar
Forum Topic Moderator
Join Date: Jan 2005
Location: Delaware, The First State
Posts: 12,682
Received 41 Upvotes on 39 Posts
Furnace run time

Check the return duct to make sure it has not come apart in the attic causing you to actually be heating fresh rather than recycled indoor air.
 
  #3  
Old 01-01-09, 03:55 PM
SeattlePioneer's Avatar
Banned. Rule And/Or Policy Violation
Join Date: Jul 2007
Location: Seattle, Wa
Posts: 4,469
Upvotes: 0
Received 1 Upvote on 1 Post
I'd have the furnace inspected for the correct gas input. The 110 degree dupply temperature is lower than should probably be the case. That could also be caused by excessive air flow caused by the 1/2 HP blower motor, which seems oversized for that size furnace.

The rating plate on the furnace should specify the acceptable temperature rise (air temperature out of the furnace - air supply going into the furnace) If the temperature rise is too great, you can cool off the heat exchanger and combustion gasses causing condansation problems where you don't want them, and early failure of the heat exchanger from corrosion from condensate.

I've actually seen a furnace that had the heat exchanger completely plugged with rust caused by being underfire by the wrong size burner orifices.
 
  #4  
Old 01-01-09, 08:31 PM
M
Member
Thread Starter
Join Date: Jan 2009
Location: san jose CA
Posts: 5
Upvotes: 0
Received 0 Upvotes on 0 Posts
Thanks. I will do some checking. What should the "temperature rise" be for this Rheem model? I cannot find it on the furnace plate. As you can see, I am getting about 40 degrees.
 
  #5  
Old 01-01-09, 10:46 PM
SeattlePioneer's Avatar
Banned. Rule And/Or Policy Violation
Join Date: Jul 2007
Location: Seattle, Wa
Posts: 4,469
Upvotes: 0
Received 1 Upvote on 1 Post
Look again. There should be a metal rating plate inside the burner cabinet with the make, model, serial number of the furnace BTU input, rated temperature rise and so on.

For my forced air gas furnace the listed temperature rise is for between 70-105 degrees F, which would be typical for a good many gas furnaces, although not all. The size of the blower motor recommended may also be listed on the rating plate.
 
  #6  
Old 01-02-09, 10:35 AM
M
Member
Thread Starter
Join Date: Jan 2009
Location: san jose CA
Posts: 5
Upvotes: 0
Received 0 Upvotes on 0 Posts
Yep, I had missed it. It says "Temperature rise 35 to 65degrees F". So, I take this to mean that my register air temp should be 67+35 to 67+65 which is 102F to 132F. Did I get that right? I am getting 108F to 117F (closest vent to the furnace). It would seem I am within spec then.
If so, I'm back to square one - why is the thing running full bore for up to 3 hours to get me from 67F to 74/75F? To confuse things further a heating guy has told me that this 75,000 BTU furnace is actually oversized for this 1750 sq. ft house!!!!!!
 
  #7  
Old 01-02-09, 11:24 AM
SeattlePioneer's Avatar
Banned. Rule And/Or Policy Violation
Join Date: Jul 2007
Location: Seattle, Wa
Posts: 4,469
Upvotes: 0
Received 1 Upvote on 1 Post
Yes, the furnace is within that operating spec.

Another thing to check is the BTU input, which can be measured using the gas meter. Usually there are two small test dials, typically labelled for one and two cubic feet.

That means an entire revolution of that dial indicates consumption of one or two cubic feet of gas.

Turn any other gas consuming appliances down or off so the burners don't come on. Turn the furnace thermostat way up so it will stay on. Measure the number of seconds it takes for one dial (2' dial preferred) to complete one full revolution.

Post which dial you are using and the seconds and I'll compute the actual BTU input to your furnace. Also post the rated BTU input again from the rating plate on the furnace.
 
  #8  
Old 01-02-09, 12:30 PM
M
Member
Thread Starter
Join Date: Jan 2009
Location: san jose CA
Posts: 5
Upvotes: 0
Received 0 Upvotes on 0 Posts
With water heater turned off (only other gas consumer) and furnace running, the 2ft dial does one complete rev in 122 secs. The furnace plate says 75,000 BTU per hour.
 
  #9  
Old 01-02-09, 01:58 PM
SeattlePioneer's Avatar
Banned. Rule And/Or Policy Violation
Join Date: Jul 2007
Location: Seattle, Wa
Posts: 4,469
Upvotes: 0
Received 1 Upvote on 1 Post
Calculatin BTU Input

there are 3600 seconds in an hour. Since it takes 122 seconds to consume 2 cubic feet, 1 cubit foot is consumed in 61 seconds.

Therefore, 3600/61 or 59 cubic feet of gas are consumed per hour. A typical figure for the BTU input of natural gas is 1000 BTU per cubic foot, which gives a BTU input of 59000 BTU.

This suggests that the BTU input of your furnace is significantly below what it should be.


Frankly, I don't want to coach you in setting the BTU input to your furnace. That should be done by a competent repairman who will also check out the heat rise once it's completed, inspect the heat exchangers and otherwise make sure the furnace is working properly.

But the low input suggests this is a likely part of the problem.
 
  #10  
Old 01-02-09, 05:25 PM
M
Member
Join Date: Dec 2004
Location: San Jose,Ca
Posts: 1,277
Upvotes: 0
Received 3 Upvotes on 3 Posts
Call PG&E and have them clock the input. There may be spider nest(s) blocking gas flow or some other reason which may be simple to figure out.
 
  #11  
Old 01-02-09, 06:07 PM
M
Member
Thread Starter
Join Date: Jan 2009
Location: san jose CA
Posts: 5
Upvotes: 0
Received 0 Upvotes on 0 Posts
Thanks so much. I suspect the 20% low BTU input should account for the problem. If it boosts the temp rise by the same 20% then my registers should go up to about 125F which I suspect will do the trick. I will get an HVAC guy to fix and service it. I will post again later when hopefully BTUs have been restored.
 
  #12  
Old 01-25-09, 11:34 AM
L
Member
Join Date: Jan 2009
Posts: 8
Upvotes: 0
Received 0 Upvotes on 0 Posts
Question

I'm having the same problem with my Rheem 90 Comfort modulating furnace. I measured the input rate and it's only 43000 BTU instead of 60000 BTU. (when running 100%)
What could cause the problem ?
Hot water tank and fireplace both work fine.
 
  #13  
Old 01-25-09, 12:24 PM
Grady's Avatar
Forum Topic Moderator
Join Date: Jan 2005
Location: Delaware, The First State
Posts: 12,682
Received 41 Upvotes on 39 Posts
Low BTU input

How are you determining the actual BTU input? As previously stated, your problem could be a partially clogged orifice. The orifice could also be the wrong size.
 
  #14  
Old 01-25-09, 12:24 PM
SeattlePioneer's Avatar
Banned. Rule And/Or Policy Violation
Join Date: Jul 2007
Location: Seattle, Wa
Posts: 4,469
Upvotes: 0
Received 1 Upvote on 1 Post
A modulating furnace is DESIGNED to change the BTU input based on the outside temperature or the amount of heat needed to keep the house warm.

So the furnace may be operating properly with the BTU input you found. Or perhaps not.


Frankly, I'm not an Xpert on how those furnaces control the input to the furnace. Perhaps someone else will be able to offer some useful comments.
 
  #15  
Old 01-25-09, 12:54 PM
L
Member
Join Date: Jan 2009
Posts: 8
Upvotes: 0
Received 0 Upvotes on 0 Posts
Thanks for the prompt reply guys !
The furnace was working fine since 1999 when I installed it.
( My friend did the gas line part for me)
I have service manual and I have followed the start up procedure.
The furnace starts calibrating cycle upon first power up ( 5min) and than
if there is demand more than +4C it will go 100% .
I verified this using ampere meter ( 180mA = 100% power)
Once the house will reach the desired temperature it works fine as usual
but it is having hard time to get from 16C to 20C ( takes about 3.5hrs now)

I measured gas consumption on the meter to calculate the input rate.

Is this common problem to get dirt in the orifice ?

Here is link to the install manual if you willing to help me out
http://www.allreds.net/WS2/docs/rhe/rgfd_manual.pdf
 
  #16  
Old 01-25-09, 01:48 PM
Grady's Avatar
Forum Topic Moderator
Join Date: Jan 2005
Location: Delaware, The First State
Posts: 12,682
Received 41 Upvotes on 39 Posts
Input rate

Since this is a multi-burner appliance, it's unlikely the problem is a fouled orifice but it can't hurt to remove the orifices & blow them out. I suspect a gas pressure problem. It is not unusual for gas pressures to drop if you are experiencing unusually cold weather.
 
  #17  
Old 01-25-09, 02:14 PM
L
Member
Join Date: Jan 2009
Posts: 8
Upvotes: 0
Received 0 Upvotes on 0 Posts
It is very cold this year (-25C at night) but this is not the first year
I presume that there would be more people with the same problem - no ?
Is it possible that the gas valve is partially clogged ?
There is screw on the gas valve that will increase the pressure
perhaps I can use it to do minor adjustment.
Thanks
 
  #18  
Old 01-25-09, 02:18 PM
Grady's Avatar
Forum Topic Moderator
Join Date: Jan 2005
Location: Delaware, The First State
Posts: 12,682
Received 41 Upvotes on 39 Posts
Gas Pressure

It is quite possible others are having the same problem. DO NOT try to adjust pressure without a gauge.
 
  #19  
Old 01-25-09, 02:32 PM
L
Member
Join Date: Jan 2009
Posts: 8
Upvotes: 0
Received 0 Upvotes on 0 Posts
OK, I won't touch it then ...
They describe in the manual that it should be used for small adjustments only.
 
  #20  
Old 01-25-09, 03:55 PM
SeattlePioneer's Avatar
Banned. Rule And/Or Policy Violation
Join Date: Jul 2007
Location: Seattle, Wa
Posts: 4,469
Upvotes: 0
Received 1 Upvote on 1 Post
Is it actually starting and operating at the 100% input level according to the elctrical specifications of the furnace? Is it at that time while it's at 100% input that you are measuring the 43,000 BTU input level?

I agree that it would be a mistake to horse around with the gas valve pressure. I would presume this involves some feature of this complex furnace rather than some change in the gas pressure, unless you have reason to be suspicious of low gas pressure to your house, such as poor operation of other gas appliances.

The 43,000 BTU input you are measuring is about 72% of the rated input. While the furnace can be fully modulating, it has three possible default inputs of 100% 65% and 40%. None of these inputs is at all close to the input you are measuring.
 
  #21  
Old 01-25-09, 06:28 PM
L
Member
Join Date: Jan 2009
Posts: 8
Upvotes: 0
Received 0 Upvotes on 0 Posts
The furnace actually modulates from 40% in 5% increments up to 100%.
When first connected to power it starts calibration at 75% and after 5 min it resumes regular operation.
When there wasn't power failure or intentional disconnect it skips calibration. I made sure that the gas valve was at 180mA = 100% while measuring the consumption.
I didn't notice anything unusual with the fireplace ( it's set usually to minimum) or the HWT
 
  #22  
Old 01-25-09, 06:37 PM
SeattlePioneer's Avatar
Banned. Rule And/Or Policy Violation
Join Date: Jul 2007
Location: Seattle, Wa
Posts: 4,469
Upvotes: 0
Received 1 Upvote on 1 Post
Originally Posted by lifeisfuneh
I made sure that the gas valve was at 180mA = 100% while measuring the consumption.
Where is this method of determing the input being attempted described in the furnace instructions?

I'd be looking for ways to test and check to see if the furnace is switching or attempting to switch to the appropriate input, or whether it is stuck on this 72% input no matter what.

For example, there are several ways to switch to the 40% input level and to the 100% level --- I'd be inclined to try those other methods and see if that produces a change up or down.

Do you have the fully modulating thermostat described in the instructions installed?
 
  #23  
Old 01-25-09, 06:59 PM
L
Member
Join Date: Jan 2009
Posts: 8
Upvotes: 0
Received 0 Upvotes on 0 Posts
I have the installation manual and there is table with mA ratings according to output.
(Page 69 Table 17 in the PDF doc) I can also hear the changes.
 
  #24  
Old 01-25-09, 09:03 PM
SeattlePioneer's Avatar
Banned. Rule And/Or Policy Violation
Join Date: Jul 2007
Location: Seattle, Wa
Posts: 4,469
Upvotes: 0
Received 1 Upvote on 1 Post
All right, ---I'm presuming you are measuring 180 milliamps AC current?

I'd use the methods available to switch to the 40% and 60% input level to see if the furnace changes to those inputs properly.

If it's switching inputs, then I'd pull out the burner orifices to make sure they are clean and are #50 drill size as specified in the manual. You'd then need a good manometer capable of measuring the burner manifold pressures specified in the range of 2"WC - 3.5" WC as specified in table 17, page 69.


I'd also call your gas utility and get the figure they use for an average year around BTU value per cubic foot of gas and the specifi gravity of the gas they provide.

In the "setting the imput" section the manufacturer says they baser the orifice size and gas pressure based on a BTU value of 1075 and a specific gravity of .6, which is about as hot a natural gas as you'll find. That means that your furnace is probably underfire by perhaps 4% or so based on the heating value of the gas you have.

Also, at what altitude above sea level are you? That might make a difference too. Once you have that information, the distributor or manufacturer ought to be able to give you the correct burner orifice size and burner manifold pressure at which to set your furnace.

Those things aren't going to cause a burner input rate of 72% though. You still have to identify what's going on to cause that. Checking for properly sized (#50 drill size) burner orifices, and then checking the burner manifold pressure will tell you what's going on.
 
  #25  
Old 01-26-09, 02:45 AM
L
Member
Join Date: Jan 2009
Posts: 8
Upvotes: 0
Received 0 Upvotes on 0 Posts
Yes, 180mA DC current.
The furnace was working fine for the past 9 years so I presume the gas company can't just change BTU of the gas without notification.
There is four burners, does this mean that the furnace has 4 orifices ? Flame on all of the looks exactly the same.
 
  #26  
Old 01-26-09, 09:26 AM
SeattlePioneer's Avatar
Banned. Rule And/Or Policy Violation
Join Date: Jul 2007
Location: Seattle, Wa
Posts: 4,469
Upvotes: 0
Received 1 Upvote on 1 Post
As I said, variations in the heating value of the gas aren't going to account for the 72% firing rate. But setting the equipment to the correct pressure is what is recommended by the manufacturer. I guess it depends on whether you want to do a quality job on installing the equipment or not.

It's unlikely that the furnace has been working in an optimum fashion, although it does sound like it's been working. Chronically underfiring a gas furnace can result in excessive rusting of the heat exchangers if they run too cool and cause condensation in the primary heat exchangers, for example.

And yes, each burner typically has an orifice, a brass fitting with a hole in it that should just pass a #50 drill bit. They may have the #50 (or some other number) stamped on them as well. And while furnace burners don't typically get plugged with crud, they can be. And doing a quality job of repairing a furnace means checking such things dutifully.

You didn't report your elevation, which can also affect furnace input.
 
  #27  
Old 01-26-09, 10:32 AM
L
Member
Join Date: Jan 2009
Posts: 8
Upvotes: 0
Received 0 Upvotes on 0 Posts
Hi,
I'm sorry I didn't mean ignore your questions, I was rushing to work in the morning
W are only 80m above sea level ( I wish it would be close to the sea as well ).
I presume that if the exchanger was corroded the exhaust gas would be very hot bu this is not the case the PVC 2" PVC pipe is only warm to touch right at the furnace.
I guess I will have to check the input and manifold pressures.
 
  #28  
Old 01-26-09, 10:44 AM
SeattlePioneer's Avatar
Banned. Rule And/Or Policy Violation
Join Date: Jul 2007
Location: Seattle, Wa
Posts: 4,469
Upvotes: 0
Received 1 Upvote on 1 Post
Were you to wait long enough, a corroded heat exchanger might well rust out. Having the correct BTU input might well eliminate that risk.

I'm simply suggesting that if you paid a contractor to install or repair your furnace, you would expect them to do a competent job. I'm simply suggesting that you hold yourself to the same standard, for the same reason --- because it maximizes the probability of having a safe, efficient, reliable and comfortable experience from your heating equipment.

Just getting things to work for a few years until the problems caused by negligence crop up isn't quite the same thing.

I'm not trying to beat up on you, but to inform you of what some of the weaknesses of DIY work can be. Or work done by incompetent contractors, of whom there are unfortunately many.
 
 

Thread Tools
Search this Thread
 
Ask a Question
Question Title:
Description:
Your question will be posted in: