Single stage stat on a two staage furnace, can I do that ?

[Shophound]

What it does is provide a gentle start. It puts some btu to the duct so that when the fan ramps up it's not blowing really cold air at max velocity.

But imagine my frustration spending hours trying to figure out how to manage staging so I could help improve comfort and energy consumption, and my disappointment when I finally understood what this thing is.

It's the epitome of "heat UP and shut off" design philosophy.

If the furnace is sized correctly for the structure, do you still see this control strategy as problematic?

[tedkidd]

As I watch my grossly oversized furnaces quickly satisfy my rental units and shut down even with a 2f deadband, I have a hard time imagining ever recommending single stage appliances. "correctly" sized means it's oversized all but a few hours of a few random days during a season.

So I'm not a good one to ask.

[Shophound]

As I watch my grossly oversized furnaces quickly satisfy my rental units and shut down even with a 2f deadband, I have a hard time imagining ever recommending single stage appliances. "correctly" sized means it's oversized all but a few hours of a few random days during a season.

So I'm not a good one to ask.

The way I see it, residential heat load calculations came of age in a time when single stage equipment, heating or cooling, was the norm. Sizing correctly for that 2% slice of the year does mean the equipment is oversized 98% of the time, provided the equipment can actually deliver the amount of BTUs a "design day" situation calls for. For gas/oil heating this is usually done in spades; for a/c and heat pumps, it can get very close, or even come up short.

[George2]

What it does is provide a gentle start. It puts some btu to the duct so that when the fan ramps up it's not blowing really cold air at max velocity.

But imagine my frustration spending hours trying to figure out how to manage staging so I could help improve comfort and energy consumption, and my disappointment when I finally understood what this thing is.

It's the epitome of "heat UP and shut off" design philosophy.

The older Goodman's 90+% (and maybe other brands), without the variable-speed blowers, did not have 2-stage blowers. They blew hard on "low stage". You get what you pay for, generally speaking.

[beenthere]

It is a "modified 2 stage" system. Low can run for the first 5 minutes, or 1-12 minutes at the BEGINNING of the cycle. To me, this is a soft starting single stage.

I guess technically it is a 2 stage, but it misses 90% of the benefit of 2 stage. Any furnace that can't run high first then low, in my view (energy and comfort), misses the point. Ideally a 2 stage can cycle back and forth, modulating in an attempt to match envelope losses.

(http://site.mypointnow.com/documents/25-k120602.pdf p12. Also, notice it doesn't even list a low output on spec sheet. )

The real problem comes in when the sales person who like to oversize doesn't understand how this turd operates. He figures he can size to low stage and have "high for fast recovery". Now you have a grossly oversized piece of equipment you can't even lock on low.

I Audited a house with one of these. 2200 sf, 120,000 btu. Client replaced a 30 year old furnace a year before I got there (same size of course). Bill analysis showed NO ENERGY SAVINGS and they have all kinds of comfort issue.


IMO, calling it 2 stage is like painting a dog turd gold and calling it a gold brick. Makes true 2 stage devices look bad by association.

A dog with3 legs is still a dog. A 2 stage furnace weather it can be controlled by the stat or not is still a 2 stage furnace.

Although I'm not a fan of the GMH. When sized correctly. And used with a stat that is CPH based. It runs in first stage a majority of the year.

There's lots of other 2 stage furnaces installed there controlled by the thermostat, but there over sized so they don't get long run times anyway.

[tedkidd]

Although I'm not a fan of the GMH. When sized correctly. And used with a stat that is CPH based. It runs in first stage a majority of the year.

CPH. That's a trick I never dug into. Im trying to get my head around it. (ObeewhanBT, how could you not have taught me about this before now!)

Set board for long first stage? Limit CPH to drive cycle time up? Sort of like setting larger deadband?

What other fun stuff do you use CPH for?

[beenthere]

CPH is in the thermostat. It digital mimic of a heat anticipator. Set to 3. It will attempt to do 10 minute on, 10 minutes off. On Honeywell thermostats CPH is only accurate(maintained) at the equipments 50% load. Above or below that point it my do more or less cycles.

On the GMH. If you set the stats CPH to 3 or 4, you won't get run times longer then 10 minutes, until its past the homes 60% heat loss mark. So it won't time into second stage, since it can be set to 12 minutes of first stage before it goes to second stage. Weather 3 or 4 CPH works better, is dependent on what the person/customer wants.

[Shophound]

CPH is in the thermostat. It digital mimic of a heat anticipator. Set to 3. It will attempt to do 10 minute on, 10 minutes off. On Honeywell thermostats CPH is only accurate(maintained) at the equipments 50% load. Above or below that point it my do more or less cycles.

That's cool to know about the 50% being indicative of the system's run state. My Honeywell VisionPro was cycling the a/c this afternoon at 10 minutes on, 10 minutes off, and it was nearly 90 degrees outside. Meaning on a 90 degree day with my new cool roof, my home's heat gain is ~50% of what the system was originally sized for? Hmm....

[beenthere]

That's cool to know about the 50% being indicative of the system's run state. My Honeywell VisionPro was cycling the a/c this afternoon at 10 minutes on, 10 minutes off, and it was nearly 90 degrees outside. Meaning on a 90 degree day with my new cool roof, my home's heat gain is ~50% of what the system was originally sized for? Hmm....

With the amount of improvements you've made to home in the last few years. it wouldn't surprise me if you reduced it load that your system is over sized close to 50%.

[54regcab]

With the amount of improvements you've made to home in the last few years. it wouldn't surprise me if you reduced it load that your system is over sized close to 50%.

That's why I always say install the smaller system then fix the house to get the temperatures where you want them. It's easier to put off fixing the house over high bills than if it's hot/cold inside.

[tedkidd]

CPH is in the thermostat. It digital mimic of a heat anticipator. Set to 3. It will attempt to do 10 minute on, 10 minutes off. On Honeywell thermostats CPH is only accurate(maintained) at the equipments 50% load. Above or below that point it my do more or less cycles.

On the GMH. If you set the stats CPH to 3 or 4, you won't get run times longer then 10 minutes, until its past the homes 60% heat loss mark. So it won't time into second stage, since it can be set to 12 minutes of first stage before it goes to second stage. Weather 3 or 4 CPH works better, is dependent on what the person/customer wants.

That might help them. Thanks!

That's why I always say install the smaller system then fix the house to get the temperatures where you want them. It's easier to put off fixing the house over high bills than if it's hot/cold inside.

I say the first part too.
Gonna steal that second bit.

[Shophound]

That's why I always say install the smaller system then fix the house to get the temperatures where you want them. It's easier to put off fixing the house over high bills than if it's hot/cold inside.

In my mind that would mean all planned house fixes should be known in advance, and calculated to deliver a figure for targeting the reduced capacity.

Yes, I'm going at it back-azzwards. Interesting to note, however, is that the house fixes to date have not rendered the HVAC incapable of creating a comfortable environment, year round. It is oversized, but we're not cool but clammy in hot, humid weather, nor chilly and nostril-cracking dry in cold weather. Being it's not a comfort issue, it's an efficiency issue. At some point the HVAC will be redone to fit the envelope mods. In the meantime I look forward to next summer to log and observe the difference our new cool roof makes on the cooling load.

[beenthere]

That's why I always say install the smaller system then fix the house to get the temperatures where you want them. It's easier to put off fixing the house over high bills than if it's hot/cold inside.

Would also be an easy way to get in a law suit. Can't force someone to make improvements to their home's envelope.

[beenthere]

In my mind that would mean all planned house fixes should be known in advance, and calculated to deliver a figure for targeting the reduced capacity.

Yes, I'm going at it back-azzwards. Interesting to note, however, is that the house fixes to date have not rendered the HVAC incapable of creating a comfortable environment, year round. It is oversized, but we're not cool but clammy in hot, humid weather, nor chilly and nostril-cracking dry in cold weather. Being it's not a comfort issue, it's an efficiency issue. At some point the HVAC will be redone to fit the envelope mods. In the meantime I look forward to next summer to log and observe the difference our new cool roof makes on the cooling load.

If the blower is slowed enough, it can keep the RH down.

[tedkidd]

Would also be an easy way to get in a law suit. Can't force someone to make improvements to their home's envelope.

Truth of the matter is it won't unfold as being uncomfortable. In my experience "undersizing" IMPROVES comfort, balance, AND energy bills. But it sounds like a good way to orient a ho perspective.

Sort of like the idea - no matter how many times I cut it, it's still to damn short.


No matter how much I improve the home, my equipment is still exhibiting problems associated with oversized equipment. You can always cut it shorter, cutting it longer is a challenge.

[shankle]

Actually there is a toggle switch on the board in my Goodman that lets me set it to run as 2-stage (high heat) all the time, no delays, no single stage only, so I could just run it that way. I don't believe there is any way to wire up this furnace to run with a two stage stat even if I wanted to.
It does not have a variable fan, one speed only, unless you move the jumper on the board to a different 'speed'

[beenthere]

Let it run as a 2 stage.