Air Blower

02-05-2012, 01:34 PM

Hi there, I would appreciate any feedback or guidance.
Background:
Just had a new Luxaire high efficiency (95.5%) propane gas furnace installed in a 17 year old two story home. (old ductwork with no dampers).
Replaced a 17 year old oil fired Summeraire furnace with a side vent blower exhaust that failed and could not be replaced. Decided it was time to upgrade to propane and high efficiency.

Problems/issues:
The new furnace appears to have two issues, neither of which seem to be viewed as a problem by the installer. He says it is operating 'normal.'
1. The two speed DC motor appears to be constantly searching for the correct speed. It switches back forth rapidly between speeds, sometimes every second for up to a minute before settling on low speed. This constant cycling of blower speed was continuous throughout a full heating cycle until I replaced the paper air filter with a high quality hydro-static filter. I presume this introduced a little more back pressure on the cold air return and this seems to have helped settle the cycling down during the heating phase of the start-stop cycle. The installer indicated that the constant speed cycling of the 2-speed motor is normal???? He says that Luxaire has stated the same.
Anyone have any advice?
As an aside, the installer had the furnace specially equipped with a 1200 scfm blower instead of the normal 1000 scfm that normall comes with this model.
It is definitely more noisy (air and duct noise) than the old furnace. And when it cycles between low and high speed it produces a whistle sound.

2. The furnace also seems to turn on-off more frequently than it should.
Less than a half degree celcius change on the thermostat and the furnace restarts after it shuts down. At minus 10 degrees celcius outside temp the furnace is only off for less than 5 minutes. Is this a termostat problem or a furnace controller problem/adjustment? The thermostat is a fully programmable digital unit and is less than five years old.

thanks again for any help or advice.
Steve

02-05-2012, 02:18 PM

1 is a problem regardless of what he says.

2 can be fixed by adjusting the spread of the thermostat or possibly CPH (cycles per hour).

02-05-2012, 03:55 PM

The blower 'hunting' is not normal for an installed unit. My first question is the normal one, did anyone do a full room-by-room load analysis and subsequent duct analysis? You have stated that the new propane furnace replaced an old oil furnace. That statement could lead me to think the duct system may be large and if so, not enough static pressure on the variable speed/ECM motor. They'll hunt all over the place without sufficient static pressure on them. Now having said that, you've also stated that the new furnace is noisier than the old and that would indicate a duct system that is too small for the new blower, thus high static pressure. Taking the two into consideration, I'd say the blower motor or drive in all likelihood has a problem and that the blower is over sized for the existing duct system. If the blower was "specially equipped" with a larger than normal blower, I would assume the installing company had done an accurate load analysis and determined that the cooling system blower needed to be 3.0-tons and that the blower normally installed in the furnace only handles up to 2.5-tons of cooling.

As for the cycling, it's more difficult to tell. How near to design temperature is -10°C? The closer the OAT (outdoor ambient temperature) gets to design, the longer are the heat cycles and the shorter are the dwell (no heat) times. At design temperature the furnace never turns off and below design temperature the furnace cannot keep the t-stat at set-point. As the OAT increases outdoors, the run times become less and the dwell times become longer. So trying to gauge the proper run times against dwell times can be a career opportunity for someone. Or and accurate load analysis could tell you how many Btu's your home needs PER HOUR and the size of your furnace in Btu's PER HOUR will allow anyone to calculate how many minutes of an hour the furnace needs to be running in order to supply the right Btu's to the home to maintain set-point. e.g. House needs 45,000 Btu's per hour at some OAT, furnace delivers 75,000 Btu's per hour, furnace needs to be running .6 hours to put 45,000 Btu's into the home. .6 x 60 minutes = 36 minutes of run time per hour at that OAT, resulting in a dwell time of 24-minutes. If the same home at some lower OAT needs 65,000 Btu's/hr. then the furnace run time is increased to 52-minutes per hour, thus shrinking the dwell time to only 8-minutes. Hope this all helps.

02-05-2012, 04:42 PM

Thank you for your relpies to my query. (both BaldLoonie and Skippedover)

I suspected that the 'blower hunting' was not normal. You have confirmed this for me. The installer was pretty forcefull in stating to me that it was completely normal. I doubted this however, and this was further reinforced when I changed the filter and immediately noticed a change in the blower behaviour.
During installation, the installer did mention that he thought the ductwork was undersized by todays building code requirements. So when he mentioned he oversized the blower, I thought this was counter intuitive. Not being HVAC trained I did not challenge the concept.
The other puzzling thing is that he suggested installing filters in the floor vents to me as he was leaving.
To your point about diagnostics; I do not believe he conducted a full load analysis of the house. I did not see any evidence of this. He based the replacement furnace specs on the pre-existing furnace and the sq footage of the house and the climate zone. Obviously I am getting more concerned with his knowledge.
Where should I focus him in dealing with this issue? Suggestions?
You referenced static pressure. How is this determined or controlled?
(BTW, The furnace is not used for cooling (AC), only heating.) thanks again.

Re the starting and stopping, honestly you lost me in the technical stuff.
I looked through the digital settings on the control face of the thermostat and could not see any adjustment for temp spread. I also opened the thermostat to see if there is a spread adjustment, a bias dial or lever, but could not see anything inside that provides adjustment capability. Maybe I will purchase and install a new thermostat.

Thankyou to both of you for your assistance,
Steve

02-05-2012, 06:09 PM

Don't know he name.
It is at my cottage and I will have to check next weekend.

If I purchase a new one I will make sure it is good quality.
thanks again
Steve

02-05-2012, 07:52 PM

Luxaire doesn't make any gas furnaces that have a 1000CFM blower. The smallest is 1200CFM. So he fed you a line on that also.

Sounds like over sized furnace, on an under sized duct system.

02-05-2012, 11:51 PM

Hi Beenthere, and others who have commented:

If you are correct about it being an oversized furnace and undersized ductwork, where would you suggest I go from here?
Have the new furnace replaced?
Modify the ductwork?
Adjust or replace the ECM? (assuming it can be adjusted)
something else..?

The furnace installation was done in a rather remote area. Not many qualified contractors to choose from in the area. The house is a storey and a half wood frame construction, appx 1850 sq ft. Waterfront with north exposure.
North end of Zone B in Canada for Energy star ratings.

Unfortunately, I am learning on the fly here....
thanks again,
Steve