Question: Three Stage Vs Modulating

[beenthere]

He's correct. On mine when we get say -15˚, the furnace will run non stop in 1st stage, and the t-stat will cycle into 2nd stage as needed.

A couple of years ago, we had 3-4 days of -20˚, and the furnace never shut down. (Had t-stat on Hold)

Most thermostats can be set to statisfy in second stage instead of cycling from first ot second back to first again.

[mayguy]

sorry I guess im going to 2 diff posts again... So that being said me having a 2 stage furnace is a waste it will always run in low stage unless its after a set back. this is crap man they never told me this and I am not happy to say the least !

No, it's not a waste.. It's COMFORT, and quiet! I'd rather have a furnace that can run quiet.. (Mine is about 8' away from me in the family room down here)

When it get at design temp, then 2nd stage is going to run more often.

[mayguy]

Most thermostats can be set to statisfy in second stage instead of cycling from first ot second back to first again.

Correct, but I'd rather have it cycle back and forth.

[beenthere]

Correct, but I'd rather have it cycle back and forth.

Yes, and for you, you want a high comfort level.

cvcman, is looking more for efficiency, and may not have been aware from your post that its possible to have them satisfy in second stage if so desired.

[RoBoTeq]

Hi:

Just this month I had a 100Kbtu 80% replaced with a 75Kbtu Rheem mod, model RGJD. What I am reading here is somewhat of a concern. I picked the unit for both efficiency and comfort. Here is what I've observed so far:

The unit is very comfortable, and most of the time you can hardly hear it. I haven't had it long enough, and the temp is mild enough so far, that I don't have a handle on gas usage.

I programmed my home automation controller to read the thermostat PWM signal. When bringing up the house in the morning, the T-stat starts at 100% and ramps down to 40% before shutoff. When cycling to maintain temp, it starts at 60% and ramps down to 40% before shutoff.

Now, my question for Captain CO and RoBoTeq: Do you believe the mod or staged furnaces are less efficient on low-fire due to the furnace itself, or conduction loss in the ductwork because the furnace spends more time on? As an engineer, I'm interested in as much detail as you are comfortable providing.

Thanks, Larry

The efficiency loss is due to the thermal transfer of heat through the metal heat exchanger. When a heat exchanger is designed to transfer 95% of the heat produced through a certain amount of heat exchanger wall area at a certain temperature, then reducing the amount of Btu's of heat in that same space simply will not transfer the same percentage of heat through the walls of the heat exchanger. However; this reduction in heat transfer under properly operating conditions is no where near as low as Jim is inferring.

Single staged furnaces would simply short cycle more often which also decreases the rated efficiency of the furnace. Furnace efficiencies are rated at their peak performance in test labs under ideal conditions. Since all furnaces are rated this way, all furnaces are rated fairly. This does not mean that your furnace is going to always operate at peak efficiency in your home. Efficiency ratings is just a number determined during optimum conditions. When was the last time a vehicle you owned got anywhere near the gas mileage claimed by the manufacturer?

The Rheem/Ruud modulating furnace has been my personal favorite furnace for years. Now that other brands have come out with modulating furnaces, I may have other favorites to choose from. As with all staged furnaces, the Rheem modulating furnace is designed to operate as long as possible under the least amount of required heat to do the job keeping the home comfortable.

Because staged furnaces operate longer cycles, they do not waste fuel during as many warm up periods. All furnaces are very inneficient during the initial warm up of the heat exchanger. By keeping the heat exchanger warm longer, staged furnaces make up the difference in efficiency loss from low fire by not having to continually warm up the heat exchanger over and over again.

So, while low fire on staged furnaces may by itself be tested to be less efficient (50% is an extreme where something other then the furnace design was at fault), the efficiency loss on a properly sized furnace is still no greater then the efficiency loss on a single stage furnace cycling more often.

You guys really need to stop letting the Captain of Doom and Gloom scare you

[RoBoTeq]

if these companys selling these units do not tell their customers that these are 50% efficient running at low stage I would think they can be held liable.

Mine always fires and runs at low UNLESS it has to raise the temp more han 2 degs which the only time it does this is after a setback of 5 degs at night. Then it goes to high.

I have only had it 3 months so not an real cold weather yet but im thinking of calling the company Monday now that I hear this stuff. I went from a 105k btu oil furnace to a 60k btu 2 stage and it sounds like I got screwed !

I cant imange even in the winter this will fire at hi stage when it only has to go up a deg or 2. The temp rise is right wher it should be both hi and low fire

Oh for crying out loud; let go of the Captain's 50% scare tactics! If all you are doing is looking for reasons to sue, then get Captain CO to go to court with you. Your furnace has been rated to a specific efficiency and under the conditions of the ratings it will perform those efficiencies. Since all furnaces are rated under the same regulations, your furnace is exactly what the manufacturer claims it to be under the conditions of the rating testing.

Yes! Jim is not lying. I have the utmost respect for Jim's knowledge and abilities. However, that guy on those Ginsu commercials is also not lying when he slices through rotten tomatoes and whipped cream with that knife that you haven't got a shot of being as good with when you get it home.

[hvaclover]

Gas pressure, and blower CFM are what you have to adjust.(there are a few other things, but not in this open forum).

Keep in mind, that the longer the flue and vent pipe are, the less air the induce can draw in, that effects combustion efficiency. So you adjust the gas pressure to compensate for the flue and vent pipe.

Also. You need to adjust the CFM for temp rise. Too much air, and you cool down the HX too much to get good heat transfer. Not enough, and you get a real hot HX, but put more heat out the flue then needed.

The install manual might say x.x" manifold pressure for first stage. But that is based on xxxx BTU per CF of gas. And many areas have more, or less BTU content then the manufacturer uses for their test. So you may have to adjust it up or down a couple tenth of an inch to get proper rise.


AFUE, and SSE are not the same thing.

Ok thanks for being helpful. Dang Some guys act like combustion science is some arcane art that you need to join a secret society to learn.

Well its good to know that I probably got a pretty hadle on the fundamentals so if I sign up for a course it won't be so tough for an older guy.

[beenthere]

Ok thanks for being helpful. Dang Some guys act like combustion science is some arcane art that you need to join a secret society to learn.

LOL...

I know. Its strange how they will tell you how to check and adjust the charge of an A/C or HP, but ask about combustion. And its clam up time.

I can understand they might feel loyalty to Jim, and want him to make money.
But, then you would think they would not answer questions about anything else either, since there are schools and seminars for everything else also.


Some day, I may join that secret society. LOL

[hvaclover]

A member of my RSES chapter has subed for Jim davis (or so he says. I guess they are friends) and I know that guy is a combustion expert.

But we seem to have a controversy going now ,that caveman guy or what ever his name is, thinks he got boned and I think a lot of guys are starting to wonder what really is true or hype.

[hvaclover]

And a joint would be good about now (check the spelling in the las line of your reply).

[beenthere]

There is a slight drop in steady state combustion efficiency in first stage.
A CA can only tell you combustion efficiency, not total heat transfer efficiency.

Total heat transfer efficiency is effected by everything hooked to the system that effects air flow. Including return air temp.

A furnace is more efficient with a return air temp of 50°F, then it is with a return air temp of 85°F.

I tore out a 90% single stage, and installed a York 95% mod. It uses less gas for the year then the single stage did(which did supprise me).

It does seem to use a little more on some of the milder months(but the daughter was
home from college those months).
Other then when it was commisioned, its never been at full modulation(we're having warm winters for the last 3 years now).
And the gas usage is being compared to the first warm winter, which was the last winter the single stage was used.

I don't think the question should be how efficienct is a 90% furnace, But more, how efficient will the install situation/set up allow it to be.

A 80% with proper duct and register throw will use less gas the a 90% riding the high limit because of undersized duct work.

[beenthere]

And a joint would be good about now (check the spelling in the las line of your reply).

ROFL.

I fixed it.

[cvcman]

beenthere, I tapped EVERY joint on my duct work both hot and cold with metal duct tape. They are in my basement so maybe it didnt need to be done but it shouldn't do any harm right ? I was only going to do the cold air so it didnt suck any air from the basement but I did the hot also.

These ducts seem big, house built in 1955 all galv metal duct . Thx again for the help and the Valium