Thermostat CPH Settings

ArgMeMatey

From another thread ...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.

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. 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.

ArgMeMatey

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?

HVAC Doc

Answers under your follow ups:

ArgMeMatey

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.

HVAC Doc

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) .