Go Back  DoItYourself.com Community Forums > Heating, Cooling, Air Conditioning, Ventilation and T-Stat Controls > Gas and Oil Home Heating Furnaces
Reload this Page >

New SLP98V installed with Honeywell TH8320U Thermostat - Proper CPH Setting?

New SLP98V installed with Honeywell TH8320U Thermostat - Proper CPH Setting?


  #1  
Old 02-15-12, 08:08 PM
N
Member
Thread Starter
Join Date: Feb 2012
Location: Canada
Posts: 4
Upvotes: 0
Received 0 Upvotes on 0 Posts
New SLP98V installed with Honeywell TH8320U Thermostat - Proper CPH Setting?

Hi everyone,

Just had an SLP98V (70,000 BTU) furnace installed. Loving it so far - big change from my massive Keeprite 100k BTU dinosaur.

It is setup for variable capacity with a 2-stage Honeywell TH8320U thermostat. CPH is currently set at "5". I'm wondering if the CPH should be set at "3" as per the Honeywell manual (for furnaces of 90% or higher efficiency).

I'm noticing that the furnace doesn't modulate down, yet will turn off and on the next cycle turn off at a different capacity. If I raise the CPH to 5 it seems like this will allow the furnace to "modulate" more effectively.

Does any of you have any idea?

Thanks
 
  #2  
Old 02-15-12, 08:26 PM
hvactechfw's Avatar
Member
Join Date: Jan 2011
Posts: 5,491
Upvotes: 0
Received 4 Upvotes on 4 Posts
It can be set to have variable capacity, 2 stage, or 3 stage operation with a 2 stage stat. Page 37 http://www.lennox.com/pdfs/installat...P98DFV_IOM.pdf
I would set it at 3 cph.

For proper full modulation you would need to use the lennox icomfort stat.
 

Last edited by hvactechfw; 02-15-12 at 08:42 PM.
  #3  
Old 02-16-12, 12:51 AM
N
Member
Thread Starter
Join Date: Feb 2012
Location: Canada
Posts: 4
Upvotes: 0
Received 0 Upvotes on 0 Posts
I was told you only need a 2 stage thermostat to take advantage of full modulating capabilities.
 
  #4  
Old 02-16-12, 05:17 AM
hvactechfw's Avatar
Member
Join Date: Jan 2011
Posts: 5,491
Upvotes: 0
Received 4 Upvotes on 4 Posts
It will modulate with a 2dedicated stage stat. It won't modulate the same as what it will with the communicating stat though.
 
  #5  
Old 02-16-12, 05:32 AM
N
Member
Thread Starter
Join Date: Feb 2012
Location: Canada
Posts: 4
Upvotes: 0
Received 0 Upvotes on 0 Posts
Not saying your wrong, but what is your source? Everything I have read seems to state otherwise.
 
  #6  
Old 02-16-12, 07:11 AM
hvactechfw's Avatar
Member
Join Date: Jan 2011
Posts: 5,491
Upvotes: 0
Received 4 Upvotes on 4 Posts
The communicating stat tells the furnace exactly what percentage of firing rate to run at. When using the non community.ovating stat the furnace fry's to learn how your home heats and adjusts the firing rate on a timer to learn.
 
  #7  
Old 02-16-12, 07:16 AM
N
Member
Thread Starter
Join Date: Feb 2012
Location: Canada
Posts: 4
Upvotes: 0
Received 0 Upvotes on 0 Posts
Checkout this link:

HVAC-Talk: Heating, Air & Refrigeration Discussion - View Single Post - Proper Thermostat Setup w/ Lennox SLP98V and Trane XR11

These guys are saying otherwise. Can't seem to get a straight answer from Lennox either.
 
  #8  
Old 02-16-12, 02:24 PM
hvactechfw's Avatar
Member
Join Date: Jan 2011
Posts: 5,491
Upvotes: 0
Received 4 Upvotes on 4 Posts
According to the install manual you are correct. I dont think that is a great way for the furnace to modulate.
On a call for heat, thermostat first−stage contacts close
sending a signal to the integrated control. The integrated
control runs a self-diagnostic program and
checks high temperature limit switches for normally
Page 49
closed contacts and pressure switches for normally
open contacts. The combustion air inducer is energized
at ignition speed, which is approximately the
same as the inducer speed at 70 percent firing rate.
2 − Once the control receives a signal that the low−fire
pressure switch has closed, the combustion air inducer
begins a 15−second pre−purge in low speed.
3 − After the pre−purge is complete, a 20−second initial ignitor
warm−up period begins. The combustion air inducer
continues to operate at the ignition speed.
4 − After the 20−second warm−up period has ended, the gas
valve is energized and ignition occurs. At the same time,
the control module begins an indoor blower 45−second
ON delay. When the delay ends, the indoor blower motor
is energized at a speed that matches the firing rate.
After the 10−second ignition stabilization delay expires,
the inducer speed is adjusted to the appropriate
target rate. If the furnace is operating in the initial heating
cycle after power−up, the initial firing rate will be
approximately 35 percent. The firing rate on subsequent
cycles will be automatically adjusted by the integrated
control based on thermostat cycles. The firing
rate will vary and will range from 35 percent to 90 percent.
The furnace will continue this operation as long
as the thermostat has a first−stage heating demand.
5 − If second−stage heat is required, the thermostat second−
stage heat contacts close and send a signal to the
integrated control. The integrated control either increases
the firing rate to 70 percent (if the current rate
is at or below 60 percent) or increases the firing rate
by 10 percent (if the current rate is above 60 percent).
If the call for heat continues 5 minutes beyond this initial
upstage, the rate will be increased by 10 percent
every 5 minutes until the call for heat is satisfied or the
furnace reaches 100 percent rate. As the firing rate increases,
the indoor blower motor is adjusted to a
speed which is appropriate for the target rate.
6 − If second−stage heat demand is satisfied, but first
stage is still present, the furnace will continue to operate
at the present firing rate until the heat cycle ends.
7 − When the demand for first− and second−stage heat is
satisfied, the gas valve is de−energized and the field−
selected indoor blower off delay begins. The combustion
air inducer begins a 20−second post−purge period.
8 − When the combustion air post−purge period is complete,
the inducer is de−energized. The indoor blower
is de−energized at the end of the off delay.
 
 

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