Furnace short cycling when only one zone used

TalShiar

Hi, I have a Comfortmaker C9UHX, 100k, BTU 5 ton (C9UHX100L20A2). It is installed with 2 zones and appears to operate fine if 2 zones are calling for heat. Most of the time only one zone needs heat. Every since it was installed when only one zone needs heat it will blow for a couple of minutes, turn off for 5-10 minutes and come back on. This takes a very long time to heat the house and I believe it is not operating properly. Yes, I called the installer back out after the install but they would never come out to fix it. I called someone else out but they were not sure what was wrong.

I have a degree in electrical engineering and built the house my self so I confident doing any repairs. I am hoping you guys can help.
THe error code is 3 short flashes. I believe this is the correct manual

Is it over heating and shutting down? Can I reduce the burner power when only using one zone?
Is it another issue?
I am happy to do some testing on my end

Jamie Hall

I couldn't find a list of the error codes in that manual. However, my guess would be that when only one zone is open the furnace isn't getting enough air and overheating. These things are designed for and require a certain minimum air flow through the heat exchanger!

Unless the burner is specifically designed to be two stage or modulating, it probably is not a good idea to try and down fire it.

TalShiar

So they installed the wrong furnace, I am not able to correct it and have to buy a new one?

ratio

It's possible that the furnace is too large. To know for sure, we'd need to know the heat loss of your house - how fast heat leaves the house. That determines how fast we have to be able to put heat into the house. There are online tools that will do a reasonably accurate heat loss calculation.

Did the old system work right? Was it zoned as well? What size was the old system? Can you tell if a bypass damper is installed - a "zone" that just ends in an open pipe, or connects back into the other big furnace duct.

TalShiar

It was new construction so there was not a furnace before.
It has one intake, goes to the furnace, then to a condenser coil for an A/C which has not been installed yet. Then to a large box, that box has 2 outlets with electric dampers on them depending on which zone is calling for heat.
It is all horizontally installed in the crawl space.

ratio

What are the measurements of the intake grill? Are the two outlets round? Measure them too, diameter or height & width depending.

It's not conclusive, but how many square feet is the house?

Also, do you know the brand & model number of the zone panel or controller?

Abracadabra

Is there a bypass damper?

TalShiar

Hi All,

I apologize for the delay, I had to head out of town for work for a week.
The interior diameter of the intake pipe is 20".
The house is about 3600 sqft with an open floor plan
I am not sure about the diameter of the outlets since I do not know how much insulation is between the outer diameter and inner. Photo attached.
The zone controller is a honeywell HZ311 and has a remote sensor in the distribution chamber at the end.

I am guessing by bypass damper you mean an outlet that feeds back to the inlet to take up excess airflow? I was able to talk to an MFG rep and they said that sometimes that is built in but it can also cause the components to overheat since you are putting hot air back in.

ratio

Do all of the zone dampers open & close correctly? They look like "ARD"s, they should be open normally & power close when the system tells them to. There is a long screw between the actuator and the duct - you should see it in two different positions depending on wheter the corresponding zone light is red or green. How many are on each zone

Is the flash code in the zone controller or the furnace? The next time it stops blowing, look and see if one of the leds on the zone controller is flashing. You can find the manual here.

And that is the bypass damper - a quick check should show it closed when both zones are calling, and partially closed when only one zone is calling. I might adjust it to completely shut when the larger zone (add up the diameters of all the dampers on the zones) is calling to see what happens, but watch the temperature of the air leaving the unit. There should be a maximum discharge air shown on the nameplate of the furnace that can't be exceeded.

John Mills_5

That's the problem with zoning. Trying to stuff 100% of the furnace capacity into 50% of the ductwork.

Not much can be done. A good installer would have put in a 2 stage furnace and set up the zone panel to not bring on high unless enough zones are calling to avoid overheating.

The bypass damper can actually make things worse dumping super hot air into the return. May have to put a minimum position (so damper won't fully close) on each damper to bleed some air into non calling zones. That's what the high end zone systems do. They have modulating dampers and open non calling zones a bit if temp gets too hot or cold.

ratio

My Old Guy says that zone systems work best when all the zones have about the same heat loss & about the same load...

unclejohn

Most two stage furnaces will run on low fire for 10 or 15 mins and then switch to hi fire. Even if you have two separate tstats that are two stage stats if the 2nd stage calls you will over heat the furnace and it will shut off on limit. Until you can get someone out there to set it up right. [ Lock out 2nd stage when only one zone calls maybe ] manually open one damper and that should keep the furnace from shutting down.

TalShiar

Thanks for all the tips!
I will look into setting a close limit on the zones. I will have to find a manual and see how that is done.

Also, the floor grates may be creating excess back pressure. I will try removing all the grates to see if the system runs better. I may have to get some higher flow grates.

If the remote temp sensor in the distribution box is shutting the system off; are there settings for that limit or do they have a set limit? What should the temp limit for exhaust output be?

ratio

The manual for that zone controller is here. It does not appear to be very customizable.

Does the blower in the furnace actually turn off? The short cycles might be caused by the thermostat making many frequent heat calls instead of several longer ones. What kind of thermostats are installed?

tim smith

Furnace should have been a 2 stage thermostat with a 2 stage zone control. Maybe it is a 2 stage furnace and they put in a single stage zoning control? You will also really have to watch out when you add a/c as you will freeze the he#! out of the coil and in turn destroy your compressor. Need to get the zoning issue corrected before adding a/c besides to stop the supposed limiting.

Abracadabra

tim smith said:

Furnace should have been a 2 stage thermostat with a 2 stage zone control. Maybe it is a 2 stage furnace and they put in a single stage zoning control? You will also really have to watch out when you add a/c as you will freeze the he#! out of the coil and in turn destroy your compressor. Need to get the zoning issue corrected before adding a/c besides to stop the supposed limiting.

Always add a freeze protection thermostat on the coil when doing zoning with a/c. jmho. a low ambient head pressure control wouldn't hurt either.

TalShiar said:

If the remote temp sensor in the distribution box is shutting the system off; are there settings for that limit or do they have a set limit? What should the temp limit for exhaust output be?

I don't believe the 311 has a settable DAT shutoff. The 432, which I have installed several times, does have a settable high and low limit.

TalShiar

ratio said:

The manual for that zone controller is here. It does not appear to be very customizable.

Does the blower in the furnace actually turn off? The short cycles might be caused by the thermostat making many frequent heat calls instead of several longer ones. What kind of thermostats are installed?

Yes, the furnace fully turns off. The thermostats are Honeywell Visionpro 8000 TH8320U1008

ratio

Wow, that's a lot of thermostat. What is the system type set to, & the cycles per hour heating? That would be options 170 & 240. Is the furnace condensing (plastic vent pipe vs metal)? The system type should be set to 1 (1 heat 1 cool conventional) & cycles per hour should be about 3 if it's s condensing furnace.

TalShiar

Yes, it is a condensing unit using both PVC intake and exhaust. I will check the setting on the thermostat

ProblemSolver

TalShiar, you are not going to be able to fix this yourself, it's too messed up. My suggestion is - call the "customer service" number off your zoning system. Ask them for the number of a local supply house. Call the supply house and ask them for a reference to several contractors that purchase zone systems from them. Call the contractor out to give you a bid to fix things, and make sure they explain themselves.

TalShiar

I finally got the manufacture to send someone out yesterday. He found a few issues and apologized for the original installer's poor job. But since it has been in for 5 years I am SOL on any warranty:

1) The heater was not properly supported.
I am adding new uni-strut supports

2) The zone temperature sensor was in direct line with the output rather than being installed on one of the zone branches.
I am moving the sensor

3) The 5-ton 100k BTU was on the high side if all the vents were open. It was too much for Z1 (main floor) and way too much for Z2 (upper floor). When just one zone was open the furnace was tripping and being shut down.


He gave 2 options:
1) To fully replace the furnace with a variable output furnace and new thermostats

2) Go with the system as it is with a few changes: 2 noted above. I have an unconditioned entryway. I will be installing a barometric damper and a couple of new vents to that entry way to try to take up the excess heat output from the heater. I will also be installing new vent grates which should reduce the back pressure on the system.

He also said that he would have used 60k or 80k BTU 2 stage system, it would have been sufficient. So this leads me to a question. He said the 2 stage system reduces the gas PSI to achieve a lower heat output when called for. Can I put in a flow restricter on the burner to reduce the heat output? Yes, it will still be moving the 5 tons of air may take a little longer to heat the house but it would further prevent the short cycling that is happening now.

ratio

On 2) above, do you mean the discharge air sensor that wires back to the zone panel? Make sure that it always gets moving air across it in its new position, regardless of the state of the zone, i.e. before the damper.

Regarding down firing the burner, the manufacturer of the furnace may be able to make a recommendation, but this isn't something that a homeowner can do. It can end up in a very dangerous situation, as well as significant liability if anything goes wrong.

SWEI

TalShiar said:

He also said that he would have used 60k or 80k BTU 2 stage system, it would have been sufficient.

My guess would be a 60k, but the heat loss & heat gain calcs would say for sure. At a minimum, I would have spec'd a two stage furnace with a variable speed pressure-dependent ECM blower. Full modulation would be even better.

He said the 2 stage system reduces the gas PSI to achieve a lower heat output when called for. Can I put in a flow restricter on the burner to reduce the heat output? Yes, it will still be moving the 5 tons of air may take a little longer to heat the house but it would further prevent the short cycling that is happening now.

In short, no. Furnaces are not all that expensive to buy, but the installation costs can add up quickly.