CPH for 90% forced hot air?

ChrisJ

I know my steam system likes 1 CPH but I'm curious what I should set my dads VP 8110 to for forced hot air.



The stat is set to 5 CPH right now which is the default. System is a 90% AFUE direct vented forced hot air furnace I think made by LuxAir and its in a very well insulated house built by my dad in 07. Outside walls are 2x6, top of the line Anderson windows and so on.



The manual to the tstat says to set it to 3 CPH for a 90% furnace but that seems suspicious to me.



If it matters, they often see temps near 0F and below here.



Thanks for your time.

Tim McElwain

Three cycles per hour

is the recommended cycle for high efficiency equipment.



It typically goes this way:



1 CPH Steam

3 CPH High & Mid Efficiency

6 CPH Conventional older equipment

9 CPH electric strip heat

ChrisJ

3CPH

Hi Tim,

Thank you for responding.



Unfortunatly I could not get my dad to change it.  He claims when its -10F he needs those extra cycles.

I'm kind of curious why 6CPH for older equipment yet 3CPH for newer?  Do newer forced hot air furnaces need the extra run time in order to achieve something special?  Or is it simply harder on them to cycle more often?



I also made a mistake, apparently its rated 92% AFUE not 90%.  I'm assuming set to 5CPH under most conditions it probably never gets remotely close to this.  Unfortunatly my dads response is "I don't care".

Tim McElwain

When the high end furnaces and boilers first

came out they were set to cycle at the same as conventional equipment( 6 CPH). In particular those in above 84% AFUE category started to have high bill complaints and short cycling issues. It was determined I believe by Honeywell to go to a 3 CPH range and things seemed to steady out. The majority of these were condensing units and in many cases modulating. Cycles per hour is sort of a thing of the past now with outdoor reset and variable speed blowers and circulators.



No system should cycle on limit (except for steam on the pressuretrol) with a stable range of 20 to 30 degree Delta "T" or on a furnace 45 to 75 degree temperature rise then pump flow rates and in the case of blowers cubic feet per minute (CFM) and the cycle rate of the blower usually with most today 30 to 60 seconds on time and a range of 90 to 140 seconds off time.