Thermostat CPH Settings

 
Thread Tools Search this Thread Display Modes
Old 02-24-2005, 08:54 AM   #1
Registered User
 
ArgMeMatey's Avatar
 
Join Date: Feb 2005
Location: Milwaukee WI
Posts: 7

Thermostat CPH Settings


From another thread ...
Quote:
Originally Posted by HVAC Doc
As any manufacture will tell you, when using a properly sized 2 stage furnace operates at a lower capacity most days except in extremes when high fire is needed. Just because it is in low stage does not mean it "increases or doubles" its run time. You are only using the capacity needed to heat your home at that given time.
I have a related question - I changed my programmable stats from 6 cph to 3 cph. If I understand this, 6 CPH means a maximum run time of ten minutes, then purge and restart and 3 CPH means twenty minutes. What is the theory behind this cycling? How should it be applied in a zoned system?

Background:
The contractor set the MABS2EZ zone controller to go to Stage 2 after ten minutes, and the T8602D stats at 6 CPH. So the furnace would very frequently go to stage 2, then the stat would reach the cycle timer limit and shut it down. So the only time high fire would run long was when there were overlapping calls from both zones.

Maybe that was by design but all that switching to high fire for 30 seconds then going to purge just didn't sound right. Since our zones have staggered start times there is only a bit of overlap: Downstairs weekdays we go from 60F to 68F between 7 AM and 8:30 AM then 5 PM to 10 PM. Upstairs we go from 60F to 68F from 6 AM to 8 AM then 7 PM to 10 PM. Unless it's in the tens or below zero the furnace only runs during those hours and generally it takes an hour to heat up either zone.
ArgMeMatey is offline  

Warning: The topics covered on this site include activities in which there exists the potential for serious injury or death. ContractorTalk.com DOES NOT guarantee the accuracy or completeness of any information contained on this site. Always use proper safety precaution and reference reliable outside sources before attempting any construction or remodeling task!

   
 

Old 02-24-2005, 10:33 PM   #2
The Doctor is In!
 
HVAC Doc's Avatar
 
Trade: HVAC owner
Join Date: Apr 2004
Location: Kansas
Posts: 227

Re: Thermostat CPH Settings


2 stage units on a zoned system can be tricky for that very reason. If you have an overlap, your run time or "demand" increases since the call for heat still exists for which ever zone is needing heat. There is no "set" rule for CPH with a zoned system since each person and home is different as to how they will set their temps. or how the contractor has set them up. For example, we most generally install zone sensors in the upstairs bedrooms with each being a zone rather an an actual T/stat in a hallway or master bedroom. In cases we do install an actual t/stat, we use a base of 4 CPH and then make adjustments as needed from there.

HVAC Doc is offline  
Old 02-25-2005, 07:43 AM   #3
Registered User
 
ArgMeMatey's Avatar
 
Join Date: Feb 2005
Location: Milwaukee WI
Posts: 7
Question

Re: Thermostat CPH Settings


Quote:
Originally Posted by HVAC Doc
2 stage units on a zoned system can be tricky for that very reason. If you have an overlap, your run time or "demand" increases since the call for heat still exists for which ever zone is needing heat. There is no "set" rule for CPH with a zoned system since each person and home is different as to how they will set their temps. or how the contractor has set them up.
Thanks for your response. I told the contractor I wanted less heat for a longer period but reserve capacity for the coldest days. We have two zones because the upstairs is remodeled and built to current insulation standards. The downstairs is mostly built to 1926 standards.

But anyway how does this sound for my case:

1. This CPH thing would go away.

2. A zone controller would go to Stage 2 (high) in two circumstances:
-Simultaneous calls for heat from both zones.
-Call for heat from either zone longer than 30 minutes.

3. If there are overlapping calls, at the end of the overlap the zone controller drops back to Stage 1 (low) to serve the zone still calling for heat.

Follow-up questions:
It looks like the closest the stats can get is 1 CPH; I assume that means it will shut down every 60 minutes. Why does this stat require cycling instead of just running until the set point is reached?

And I don't think my zone controller can do #2 and #3. Is there one out there that can?
ArgMeMatey is offline  
Old 02-25-2005, 11:53 AM   #4
The Doctor is In!
 
HVAC Doc's Avatar
 
Trade: HVAC owner
Join Date: Apr 2004
Location: Kansas
Posts: 227

Re: Thermostat CPH Settings


Answers under your follow ups:
Quote:
Originally Posted by ArgMeMatey
But anyway how does this sound for my case:

1. This CPH thing would go away.

2. A zone controller would go to Stage 2 (high) in two circumstances:
-Simultaneous calls for heat from both zones.
-Call for heat from either zone longer than 30 minutes.

3. If there are overlapping calls, at the end of the overlap the zone controller drops back to Stage 1 (low) to serve the zone still calling for heat.

Follow-up questions:
It looks like the closest the stats can get is 1 CPH; I assume that means it will shut down every 60 minutes. Why does this stat require cycling instead of just running until the set point is reached?

Believe it or not, even the old Mercury bulb stats had a "CPH". It was what we referred to as the heat anticipator where the milliamps in a heating circuit had to be measured and adjusted. The more complex the stat have become with things they can control (or be added to a system and controlled) the need for more customizable settings. You can vary every thing from temp. swing (how hot or cold you want it to get before kicking the system on) to the CPH to benefit things such as HRV/ERV's, UV lights, and aircleaners.

And I don't think my zone controller can do #2 and #3. Is there one out there that can?

I can't ever say that I have seen one that integrated as to be able to overide either a control board setting in the furnace or T/stat program. Because what you are asking is that the furnace control board "forgets" that it had the initial call for heat and extended run time to cycle itself BACK down to 1st stage thinking it had a "new" call. At times it is complicated enough to get a T/stat and control board to cooperate none the less adding a third control that makes all 3 operate with the same thinking. It would be nice I admit, but haven't seen such a beast.
HVAC Doc is offline  
Old 02-28-2005, 12:56 PM   #5
Registered User
 
ArgMeMatey's Avatar
 
Join Date: Feb 2005
Location: Milwaukee WI
Posts: 7

Re: Thermostat CPH Settings


Quote:
Originally Posted by HVAC Doc
Answers under your follow ups:
Thanks again HVAC Doc. I learned a lot about Heat Anticipators. Great idea, but it sounds like it's strictly for economy purposes and not a safety issue. Is that correct?

If I am right, since most of the runtime is bringing the temp up 8 degrees (60F to 68F), isn't it more efficient to just keep the burner on (1 CPH) instead of cycling three times (3 CPH) to reach the set point?

Otherwise the runtime, I would guess, is rarely over 20 minutes anyway.

Regarding the Stage 1-Stage 2 issue: The Trane Installer's Manual says that if the furnace has the W1 and W2 contacts wired to a 2-stage thermostat, it will drop back to Stage 1 after the Stage 2 call is satisfied. Right now the W1-W2 contacts in the furnace are jumpered.

The idea is to go to Stage 2 whenever both zones are calling for heat, and also after 10-30 minutes calling from one zone only.

So I took a look at the MABS EZ-2 book and there is an "E" (Emergency Heat) contact that is energized when a thermostat "L" contact has 24V present. Would this work:

1. Wire two relays, with each coil powered by the W1 contact in each zone
2. Wire M1 (24V) contact in series with both relay load contacts to either zone's "L" contact.
3. Wire MABS EZ E contact to W2 in furnace.
4. Wire MABS EZ W2 contact to W2 in furnace.
5. Then furnace will go to Stage 2 when either both zones are running or when W2 timer is exceeded.

Do I need more relays to isolate E from W2? Do I need to get a life? Thanks.
ArgMeMatey is offline  
Old 03-02-2005, 07:23 PM   #6
The Doctor is In!
 
HVAC Doc's Avatar
 
Trade: HVAC owner
Join Date: Apr 2004
Location: Kansas
Posts: 227

Re: Thermostat CPH Settings


If I am right, since most of the runtime is bringing the temp up 8 degrees (60F to 68F), isn't it more efficient to just keep the burner on (1 CPH) instead of cycling three times (3 CPH) to reach the set point?

The CPH doesn't mean it will cycle 3 times just to reach the set point. What it does is keep the temperature from going too high (before cooling) or too low (for heating) before kicking on the thermostat. In other words, at 1 CPH, you could theroretically drop 5+ degrees before the unit finally kicks on in heating. That will make for an uncomfortable and stuffy home.

So I took a look at the MABS EZ-2 book and there is an "E" (Emergency Heat) contact that is energized when a thermostat "L" contact has 24V present. Would this work:

1. Wire two relays, with each coil powered by the W1 contact in each zone
2. Wire M1 (24V) contact in series with both relay load contacts to either zone's "L" contact.
3. Wire MABS EZ E contact to W2 in furnace.
4. Wire MABS EZ W2 contact to W2 in furnace.
5. Then furnace will go to Stage 2 when either both zones are running or when W2 timer is exceeded.

Do I need more relays to isolate E from W2? Do I need to get a life? Thanks

Now this is the hard part. I drew out a rough sketch of what I "think" you have and what you are proposing. The thing is, if W1 and W2 are being used in your thermostat, I would not think "E" would be energized since that is primarily for heat pumps (outdoor unit is locked out due to extreme drop in temps and unable to keep up) or the stat is set manually to the Emergency heat mode. With the W1 and W2 contacts though wired together in the unit, you are defeating any stage calling by the stat since W1 is automatically calling W1/W2. So the only thing running it is time factor, not multiple staging (W1 furnace to W1 stat and W2 furnace to W2 stat, no jumper) .

HVAC Doc is offline  


Thread Tools Search this Thread
Search this Thread:

Advanced Search
Display Modes Rate This Thread
Rate This Thread:

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
Who runs the Thermostat wire? DupuisConst Electrical 51 08-19-2007 10:36 AM
Wireless thermostat Speedy Petey Electrical 5 02-22-2007 03:30 PM
Recommend single pole double throw thermostat mtplus Electrical 0 12-31-2006 07:42 AM
Feed back Please ,water heater Thermostat tzzzz216 Plumbing 4 11-11-2006 01:42 PM
New Thermostat for American Standard Supmac HVAC 2 12-05-2005 06:16 PM

Join Now... It's Fast and FREE!

I am a professional contractor
I am a DIY Homeowner
Drywall Talk is for
PROFESSIONAL CONTRACTORS ONLY!

At DrywallTalk.com we cater exlusivly to professional contractors who make their living as a contractor. Knowing that many homeowners and DIYers are looking for a community to call home, we've created www.DIYChatroom.com DIY Chatroom is full of helpful advices and perfect for DIY homeowners.

Redirecing in 10 seconds
No Thanks
terms of service

Already Have an Account?