lack of velocity

[NWChicago]

Definitely get someone else to check it out, that guy is an idiot. 67* is NOT too cold of a return temp for the furnace! IIRC, the manual states it is good to 50* RA. Temp rise should be 30-60*, preferably in the middle-to-high side of that range. My first thought is low gas pressure, either supply or manifold. (Or both!) Make sure whoever comes out has experience with gas furnaces. The first guy obviously didn't have a clue!

You definately have a problem not being able to have your home get above 67 degrees.

GG has some very good points about having another tech come out. The gas pressure should be checked at the inlet and the discharge sides of the gas valve. In addition to that, you possible could be cycling on the limit, meaning that your flames are on again and off again. They should be steady until the call for heat has been satisfied.

Let's be gentle with the tech that was out there....we all were new or learing at some point. The problem with this one is that the tech should have called for help from the shop.

[ZZZRSC]

First, clock the gas meter to verify the correct firing rate as on the furnace specs. Second, check the temperature rise across the heat exchangers as called for in the furnace spec's for that blower speed. I would check the gas pressure during this time also. The first test is to see if you are actually getting the amount of btu's the unit was designed for. The second test is to verify if you are getting the correct of airflow through the furnace as called for. If both these values are correct then the furnace is producing the correct amount of heat. Then make sure the siuation isn't changing, such as the flame cycling on limits, or the blower cycling off, or even the stat could be cycling it off mistakenly. If you have the stat set on 72 (for example) the unit shouldn't shut down until it is 72 or maybe 71 at the stat location. If this unit had a variable speed indoor blower, then I would have recommended monitoring the speed, also.
If nothing is found there, then I would start checking for duct problems. If it worked O.K. for 3 years then I don't see sizing or design problems as very likely.

[milender]

Ok Thanks to all!

Going to get referrals for a qualified HVAC tech.

[milender]

That's the way it moves, thanks

[milender]

Update: Had carrier dealer out, bad secondary heat exchanger and questionable primary HE. Both under warranty. Parts ordered, hope to finialize this problem this week. Thanks to all.

[nubbin]

if air flow is reduced/low, overheating can occur and shorten the heat exchanger life.
older variable speed could not handle high static pressures.
have your dealer check temperature rise and for duct restrictions.
fixing just the symptoms can cause you to repeat this again every few years.

[geodude]

Check your delta t at furnace.

[ontariogastech]

Just to let you know. The temp. rise or the change in temp. from the cold air return to the supply duct ( after the heat exchanger led me to believe that it was a secondary exchanger problem. Make sure the tech. checks after they clean/fix the problem that the temp. rise (delta t) is adequate.

[Joe Mascitti]

Is there a 'rule of thumb' what delta t should be?

[ontariogastech]

The rule of thumb is "check the installation info manual". Temp. rise varies on installation. Gas input (Btu/hr) and motor size. Some HE's are as low as 35 F and as high as 75 F. Have a Tech check the installation.

[geodude]

It is also stamped on the data plate in the furnace.