Is My New Furnace Short Cycling??

[C-Rex]

I had a new system installed in January. I heard the furnace coming back on shortly after it had just run, so I took some measurements. It is typical for the thermostat to call for heat for about 2 minutes, then stop calling for 4 minutes. This is repeated over and over. Outside temp, about 20F. Here is more specific info:

Furnace: Amana 70,000 btu, 95%, dual stage, variable speed (AMVC950704CX)
(AC: Amana 3 ton, also new)
Stat: Honeywell VisionPRO IAQ
House: 1500 sq ft on one level. Raised ranch with walk-out basement. No second story. Half of basement is heated, but not well.

I tested the heat exchanger temp rise with a two thermocouple Fluke 52 (I don't think the installer took the time - all the blower speed dip switches are in the factory default settings, including 1429 CFM for the 3 ton AC). Per Amana's instructions I measured the temp rise after 10 minutes of continuous running on high heat - 37.4 degrees F. Amana specs: 30 to 60. I believe this eliminates over temp as the short-cycle cause.

I have a second Fluke at the thermostat, with the thermocouple probe right under, but no touching, the stat. The Fluke only varies about .4 F during these cycles. Perhaps the stat has to tight of a deadband. Honeywell tells me it is 2F. Or perhaps the stat is seeing a larger temp swing for some reason.

The stat is in the hallway, no returns or registers. There is an attic fan grill in the hallway ceiling, which is covered for the winter - but not infiltration free. There is also a hole in the wall were the wires feed into the stat. Perhaps hot and cold air enter the stat from behind.

But regardless, the heat rise data, which I also did on low fire, indicates that the furnace is capable of raising the temp at the thermostat about .4 degrees per minute according to the Fluke. So it would take this furnace only about 5 minutes to raise the temp 2 degrees. Is the furnace way to big? BY the way, the heat rise data also taught me that the furnace does not produce any heat for about the first 1 minutes of a heat cycle. However, it does continue to heat the house after the thermostat quits calling for heat.

None of the three installers that quoted did a manual J, and all recommended the same size furnace. I downloaded the manual J software and did it myself, being careful about all the parameters I entered. I ran several scenarios, varying the tightness from average to best and the attic insulation from R19 to R38. For my current situation (average and R19), heat loss was 63,xxx BTU. For best case (where I hope to get to), it was 49,xxx.

I have attached my temp short cycle and temp rise data.

Any thoughts on what it might be and what I should expect from the installer? How would you approach this?

Attached is the data in two pdf files: "Temp Rise" and "Furnace Cycling".

Thanks a lot - sorry for the darned long question.

[Hoosierland]

Not a pro here but.
May be able to lower CPH(cycle per hour) setting on thermostat. Sounds like it may be set high(9?). If so a lower number(3-5) may give longer run times.

[beenthere]

As above. Looks and sounds like you CPH is set to 9.

Set it to 3.

[C-Rex]

First - thanks so much for the reply.

The four CPH setting are as follows:

220: stage 1 compressor: 6 CPH
230: stage 2 compressor: 6 CPH
240: stage 1 heat: 10 CPH
250: stage 2 heat: 10 CPH

Since the default is "E", either the system set these values or the installer set them.

Honeywell recommends 3 CPH for 90%+ efficient furnaces. Their default for the AC is also 3, but they don't give guidelines based on SEER or other ratings.

Are the CPH settings maximums, in other words the thermostat can do whatever it calculates so long as it does not exceed 10 CPH? Or does the setting force the thermostat to cycle the furnace 10 times per hour?

Is lowering the CPH sort of equivalent to increasing the dead-band?

Any reason to think the furnaces is too large at this point?

How often should a furnace cycle if it is sized correctly?

Again - thank you

[beenthere]

No wonder its short cycling. Your settings are completely screwed up.

They should all be 3.

How often varies with indoor and outdoor temp. Set them all to 3 and leave it like that for a week. You'll like the improvement.

[C-Rex]

Unrelated question. I cannot download attachments on this site. Who do I communicate with to clear this up?

[beenthere]

Unrelated question. I cannot download attachments on this site. Who do I communicate with to clear this up?

The problem is probably on your end. Do you get an error message.

[C-Rex]

Downloads are now working!!

Back to a few last short-cycle questions:

Is a 10 CPH setting a maximum (in other words the thermostat can do whatever it calculates so long as it does not exceed 10 CPH)? Or does the CPH setting force the thermostat to cycle the furnace 10 times per hour?

Is lowering the CPH sort of equivalent to increasing the dead-band?

Any reason to think the furnaces is too large at this point?

How often should a furnace cycle if it is sized correctly?

[beenthere]

Temp always over rides CPH. So no, its not a max rate. It is and it isn't like like increasing dead bnd, you won't notice any real difference in room/house temp.

No way to tell if its oversized at this point. The number of cycles will vary as the outdoor temp gets colder or warmer. CPH is only accurate at 50% load.

[tedkidd]

How big was your old furnace? When it really got cold, did it run continuously? If not, how many minutes per hour? These are some questions I like to ask homeowners before audit. 70 "sounds" pretty big for a 1500 sf ranch, but only an audit can dope that out.

New equipment is a fairly large investment that sets you on a pretty irrevocable path. It's really unfortunate that more homeowners don't understand this. The added step of figuring out what's going on with there houses and reviewing improvement options before locking in a furnace can save a lot of anguish, a lot of energy, and mean a significantly more comfortable home. Even if the audit DOESN'T change your strategy, you know the path you are following is sound instead of hoping.

The issue I have with goodman is sizing and range. Size matching between the 70 and the 45 often means the 70 wins out. The 70 only drops to 45, so if your load is 50 that furnaces range makes you oversized all but 5% of the time... On low stage.

The 45 only drops to 30, also not much range when compared to a competitor that ranges from 60 to 24.

I suspect your basement and attic penetrations have not been air sealed, and the only thing above wall top plates it filterglass insulation. Fiberglass makes a good air filter because air can move through it with so little restriction.

You might want to putty the hole in that center wall. Typically those walls are leaky between basement and attic, so air from the basement (and under some environmental conditions attic) is moving through that wall and out holes. Fiberglass makes a good air filter because air can move through it with so little restriction.

[C-Rex]

I appreciate and agree with your advice about a home energy audit. In fact, I had that done about 4 weeks prior to the AC install. The guy did a blower door test, although did not use smoke to find leaks - took us to here he knew they typically are. After the fact I realized that we have some unique ones he never found. He also measured CFM at all registers and returns, ESP, did some IR photo imaging. Charged us $600.00 as the beginning of a program where he writes a report, then as we make improvements we qualify for some rebates, including the $600 eval fee. He was also supposed to supply manual J results.

Turns out the guy is hopelessly behind on the paperwork. After two months and several calls, I realize that I probably will never get results. Meanwhile I am not making any improvements because I will not get credit for them until he documents the original condition.

I did download and do the manual J myself.

The old furnace was a 27 year old Carrier, 95,000 BTU in.

[beenthere]

Have the CPH settings been changed to 3 yet.

[C-Rex]

No - the owner of the HVAC company is coming over Friday. I feel that he should be able to see the system just like his crew left it, without my fingerprints on it. After his visit the CPH will be set to three, either by him or later by me.

There are several issues with the system, the AC pad, the filter turning off, the transformer hanging by its wires, install workmanship, the short cycling and none of the blower speed settings being adjusted from the factory default levels. For instance, the Amana furnace install manual recommends cooling CFM to be between 350 and 450 CFM per ton. The default settings are higher than that for my 3 ton, but they were not changed. Amana also requires that the heating CM by adjusted according to temp rise measurements, which I doubt was done.

If you see any install issues in the pics and pdf I posted last night, please let me know. I need to address them with the installer.

Post: "Installer Evaluation"

Thanks

[b26440510]

You may also want to call in another HVAC company and have them go over the installation. Perhaps contact your mfg's local distributor and ask for references of companies that install your make/model of system.

The time to make the needed changes is as soon as possible.

[tedkidd]

Just as a rough comparison, I have a client replaced an 87,000 btu furnace with a 40,000 btu Carrier in a 20 year old 1700 sf mobile home... On piers. Long, quiet run times. They report their master bedroom and bath are warm for the first time in 20 years.

Did a fair bit of air and duct sealing (3000 cfm50 to 2050), improved return, nothing else. Added no insulation. Cut gas use from 1000 to 600 therms a year.

[C-Rex]

I called Honeywell yesterday to get their explanation of the effect of CPH settings. They said that the furnace will cycle the number of times per hour set by the CPH, unless temperature requires it to cycle more. So CPH is the minimum number of times the furnace will cycle per hour.

For instance, suppose CPH is set at 6. If a home is very tight, the furnace will cycle 6 times per hour even though the temperature is not driving a call for heat. If a home is really drafty, the furnace will cycle 6 times per hour unless the temperature requires it to cycle more.

Does this sound right to you?

[beenthere]

I called Honeywell yesterday to get their explanation of the effect of CPH settings. They said that the furnace will cycle the number of times per hour set by the CPH, unless temperature requires it to cycle more. So CPH is the minimum number of times the furnace will cycle per hour.

For instance, suppose CPH is set at 6. If a home is very tight, the furnace will cycle 6 times per hour even though the temperature is not driving a call for heat. If a home is really drafty, the furnace will cycle 6 times per hour unless the temperature requires it to cycle more.

Does this sound right to you?

You got a bunch of BS from who ever you talked to. When its 60 degrees outside, your furnace is NOT going to cycle 6 times an hour if you have it set to 6 CPH. It WILL NOT run unless the temp in the house drops and requires a heat cycle.

I have several Honeywell thermostats that I have tested in my place(from a 3000 series to a IAQ). NONE of them even cycled 3 times an hour when it was 60 outside and my set temp was 72. Once every hour or so is all they did.

CPH is NOT a Minimum or a Maximum number of times to run an hour.

[garya505]

I called Honeywell yesterday to get their explanation of the effect of CPH settings. They said that the furnace will cycle the number of times per hour set by the CPH, unless temperature requires it to cycle more. So CPH is the minimum number of times the furnace will cycle per hour.

For instance, suppose CPH is set at 6. If a home is very tight, the furnace will cycle 6 times per hour even though the temperature is not driving a call for heat. If a home is really drafty, the furnace will cycle 6 times per hour unless the temperature requires it to cycle more.

Does this sound right to you?

No. They didn't explain it right, or you misunderstood. Listen to beenthere. He knows his stuff (well, almost all of the time).

The stat will do the CPH setting if it can. If it needs longer furnace run times to meet demand, it will run longer cycles (not more cycles). If the cycles get too long, it will do less CPH. Comprende?

I have a Prestige IAQ which works almost the same as a VisionPro. I have it set to 3 CPH, which it will do almost all of the time (though the run time may vary with outside temperature). When it gets real cold, it will run 2 CPH. If I set it to run on 1st stage only, when it's really cold it will do 1 CPH.

[garya505]

Sorry BT, simultaneous post! Anyway, I think that explains it.

[tinner73]

3 tons...for 1500 sq'???:stinks:

[C-Rex]

The owner of the company stopped by Friday, 2/18, and saw the job for the first time. My approach was to be pleasant and ask questions about the areas of concern. I wanted to see his reaction to several situations regarding the install without a big emotional backdrop. Since this thread as specific to short-cycling, I will limit this thread comments to that. For other topics, see my "Installer Evaluation" post.

I told him I was concerned about the furnace cycling 10 times per hour on a 20 degree day. His response was that old thermostats controlled to a 4 to 6 degree temp swing. The new thermostat controls to 1 degree, so the furnace's behavior is to be expected. So I brought up the CPH setting, since the furnace is cycling 10 times per hour and that happens to be the CPH setting. His response was "Is the furnace keeping your house warm?" Me: "Yes". He: "Then it must be set OK". He did not even look at the thermostat. So this is hand I have been dealt (or dealt to myself - and paid for, to boot).

Since the visit I have set the CPH to 3.

1) Does it take the thermostat a while to adjust, or will the full effect of this change occur immediately.

2) With the CPH at 3, what would be the evidence for the furnace being too large versus about the right size? Same for the AC.

Once warm weather comes, I wonder what is in store when the AC is fired up for the first time!!!