Short cycling/once
Results 1 to 7 of 7
  1. #1
    Join Date
    Nov 2003
    Posts
    38
    2 year old thermopride furnace. Is it normal to take 3 minutes or so to warm up before blower comes on? Its warms up the house fine then goes off but blower remains on to blow out rest of warm air. It goes off then a couple minutes later or so the blower will kick on briefly then stop. I know its called short cycling and is it normal. It does it only once. It use to do it 8-10 times then we extended the fan off setting to a lower fig so it would remain on longer( 92). Also tech tightened the pulley to med high so it blows better but it started to short cycle and sides of unit itself seem to suck in very quickly as blower goes on then its fine.

  2. #2
    Join Date
    Jan 2003
    Location
    Kansas City
    Posts
    1,482
    I am not familiar with Thermopride, but it sounds like you have a temperature actuated fan switch. If this is the case, you may have an airflow restriction that causes the furnace to operate very close to its high temperature limit. When the blower shuts off, there may be enough residual heat left in the unit to bring the fan back on for a short time. This is just a guess at this point.

  3. #3
    Join Date
    Apr 2004
    Posts
    7,680
    It doesnt even have to be a temperature actuated fan switch. The residual heat in the HX can actually trip the limit bringing the fan on also if airflow performance is low.

  4. #4
    Join Date
    Apr 2004
    Posts
    201
    It is normal for the burner to run about 3 minutes before the blower comes on in a Thermopride. They do have the L4064 type fan/limit in them. As for the extra cycle, I wouldn't worry too much about it if it only cycles once. In the early season when the outdoor ambient isn't severely cold it's not unusual. It probably happens in the spring too. You might want to have someone check total static pressure of the duct system though, esp if you have central air. Could be undersized ducts. Or not. If the sides of the unit are sucking in though that sounds like high static to me. Does it whistle?
    ...SHEEESH!!!

  5. #5
    Join Date
    Nov 2003
    Posts
    38
    No it does not whistle. It only sucks in one side when the blower comes on then its fine. I just researched merv ratings and filters. I'm going to take the merv 8 precisionare out and put one that has better air flow(lower merv) to see. What is interesting the thermopride comes with a 20X25X1 Brillo type filter. Which def lets alot of air through, find that interesting, May go back to that. Have a hvac man coming on mon to check it.

  6. #6
    Join Date
    Nov 2003
    Posts
    38

    Filter related

    Well, i did put in one of the blue filters lower rated ones and it seems the air restriction is less and the side of the unit sucks in much less and i did not get a shotr cycle, So it definitely has to do with restricted air flow when a higher merv rating filter (8) is put in. As said it was a pleated precisionaire. So with this knowledge i assume i cannot use higher merv filters or the system needs more air.But i wonder if that would really help. Tech is coming mon, want to give him all the info i can. With Higher pressure drop more air it takes to move thru the filter. So do i need more air or learn to live with it and not use the higher merv filters, which is a shame since this unit is only two years old.Even though a previous tech tightened the pulley since its belt driven blower to med high , is when the sucking in got worse as more air comes in faster. But i feel no matter what its set at this should not be happening. Oh well i'll see what i come up with on monday

  7. #7
    Join Date
    Feb 2003
    Location
    Huntsville,AL
    Posts
    4,125
    change to a 5" thick filter! -- at least on trial basis -- maybe the overall size would have to be increased

    best would be to do | have a load calc to show what the duct sizing should be --
    harvest rainwater,make SHADE,R75/50/30= roof/wall/floor, use HVAC mastic,caulk all wall seams!

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •  
Comfortech Show Promo Image

Related Forums

Plumbing Talks | Contractor Magazine
Forums | Electrical Construction & Maintenance (EC&M) Magazine
Comfortech365 Virtual Event