# Generator Powering Furnace - Clean Power needed?



## somedumbguy (Sep 17, 2020)

I'm looking for some advice on powering my furnace via my generator. I have an older, inexpensive generator that has served me well through several long power outages over the years. I understand that such generators can produce "dirty" power well above the 3-5% distortion provided by utility power and that this distortion can cause "sensitive electronics" to work badly or even be damaged. But I didn't have any problems that I am aware of using the generator on the fridge, TV, computer, STB, router, etc. for four days during an outage this summer. As winter approaches I realize the most expensive gizmo in the house is actually my relatively new high efficiency furnace which has never been run on a generator. I would like to test the furnace on my generator; some info I have seen says these furnaces actually test the power and automatically shut down if the power is not clean enough. Anyone have experience with that? 

If the furnace _doesn't _protect itself, my test could turn out to be an expensive mistake. But buying a new inverter generator could be just as expensive and I'd always wonder if it was really needed. That has lead me to consider powering the furnace via a small "pure sine wave" inverter powered by a car battery that is connected to a charger/maintainer powered by the dirty generator. 

Anyone else thought through this situation to a reasonably economical conclusion? I'm running in circles at this point.


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## drmerdp (Apr 10, 2018)

From my limited experience with high efficiency furnaces 90% AFU and greater... the furnace will fail to run if the electrical source is heavily distorted.

A quick test will either confirm it works or does not. The caveat is that dirty power can cause long term degradation to the device. If your generator was built in the last 10 years and has an AVR, chances are, you’ll be just fine.


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## tabora (Sep 6, 2018)

My 2017 Powermate runs both my ancient (1990s) and modern (2018) oil burners just fine. The sine wave is a little bit dirty, but not bad. It will not run any of my three APC UPS units, though...


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## somedumbguy (Sep 17, 2020)

Thanks drmerdp. Looks like the furnace has separate error codes for low voltage, high voltage, reversed polarity, no ground, poor ground (warning only), and frequency out of range. (It doesn't say what the range is, unfortunately). Given that, and the experiences shared here I feel safe enough to run a test. (I'll have to wait a week till we get some consistent cool weather so I can leave it on for a few hours at a bearable temp).


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## somedumbguy (Sep 17, 2020)

Thanks tabora. That's a nice meter! I thought about renting something like that but it didn't look too easy locally. I was surprised about your comment about the UPSs not running on the generator. I've never owned a UPS. I guess they need clean power to charge their battery? Or maybe they cannot charge their battery and run off battery at the same time? That makes me skeptical of my car battery plus inverter jury-rig idea.


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## somedumbguy (Sep 17, 2020)

I'm trying to get an idea of how much clean power would be needed just to run just my furnace if I find out my existing generator isn't clean enough. My furnace label says it has a 1/2 HP motor with a Full Load Amperage (FLA) of 7.7. The furnace label also says "Max Amps - 12"; does that mean the furnace draws a max of 12 amps when the fan motor is starting up? I saw this generator sizing example at a web site; it implies (I think) that a generator supporting just a 1/2 HP motor would require a generator capable of 3150 watt surge (2350 plus 800). That's a little over 26 amps. But I saw another web site, Can A Portable Generator Run A Furnace? - Portable Generator Reviews that says: "To help make your generator purchase easier for your furnace, we have taken the guesswork out and laid out 5 top rated generators that can easily power any household furnace. " But alll five are inverter gens providing 2200 peak watts or less, not 3150 like the example above implies.

So I'm still confused. Anyone have insight on what is needed just for a furnace based on personal experience?


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## speedy2019 (Jan 29, 2019)

I have 2 UPS's a "1800watt generator friendly UPS" that runs no problem with my 2800wat gen and a normal "650watt UPS" that wont run on my gen.


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## iowagold (Jan 22, 2020)

somedumbguy said:


> I'm trying to get an idea of how much clean power would be needed just to run just my furnace


a meter setup for the breaker panel is always a good idea.
click here for the generator connection page
the meters will show the load on grid as well as on generator.
nice to see when planning your loads.


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## speedy2019 (Jan 29, 2019)

somedumbguy said:


> Thanks tabora. That's a nice meter! I thought about renting something like that but it didn't look too easy locally. I was surprised about your comment about the UPSs not running on the generator. I've never owned a UPS. I guess they need clean power to charge their battery? Or maybe they cannot charge their battery and run off battery at the same time? That makes me skeptical of my car battery plus inverter jury-rig idea.


The whole idea of a UPS is,,, A UPS monitors the incomming power, so any jerky, ups/downs in power, the UPS flicks over to battery power untill the power stablizes.(I wouldn't totally trust my in a bad thunderstorm though). The problem is the power from a generator is not stable enough for a UPS, and thats why you can buy "generator friendly UPS's" now......... I have a big 1800watt/2000va "generator friendly" online UPS, and that runs fine with my generator, the only time when it flicks to battery mode is when the power increases or decreases on the generator due to devices turning on and off.

The UPS is idel for me, as I run all of my lounge devices from it, TV computer, lights, sound system. So when I have a powercut everything stays on, and it will keep everything powered for 1-2hrs. So I just get the generator going and plug the UPS into the generator before the batteries die,, for continuous constant power.

This is the specs of my UPS.. VFI 2000 TG
Its as large as a small pc tower/case, with it looking like a beast inside and its stupidly tidy aswel. look.......


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## speedy2019 (Jan 29, 2019)

duplicated post


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## iowagold (Jan 22, 2020)

speed if you have an avr ups it does not switch over, it is live all of the time.
buck, boost style so the power is the same 100% of the time.

back up ups is the style you have.
that is the switch over style.

the better high dollar apc units are avr.
lol you know when you buy one..
they run over $1000.00 usd.
the last big unit I bought new was $7000.00 usd
and they come with insurance for $100k for any attached gear.

apc is a good company
lots of back up ups models to choose from.
small office units are less than $300.00
and average units are $1500.00 to $3000.00
and yes some have live avr!


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## speedy2019 (Jan 29, 2019)

iowagold said:


> speed if you have an avr ups it does not switch over, it is live all of the time.
> buck, boost style so the power is the same 100% of the time.
> 
> back up ups is the style you have.
> ...


It's a proper online ups, but I use it as a "line-inactive" UPS" "ECO" mode just because it uses less power.. So yeah if I used my UPS properly, it would be using battery mode 24/7 for maximum protection, without the mains power going through my devices at all, so basically meaning 100% smooth constant power without any movement in the volts ect. But using the UPS in "line-inactive" or "ECO" mode it only switches over to battery mode when volts goes under or over by 2%, and yeah it uses less mains power, but the problem with that, is the 3- 5ms delay switch over from mains to battery backup, that 3-5ms might be a destructive surge.

My UPS costed about $1000.00 and a APC brand of the same spec would be about $4000.00+ and thats the thing, your paying more for the name then the actual UPS.....


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## somedumbguy (Sep 17, 2020)

I wanted to find out how much clean power (watts/starting watts) I would actually need for my furnace in an attempt to minimize the overall cost of the cleanup via inverter generator or some other means with batteries and inverters etc. I sent email to my furnace manufacturer requesting wattage needs for the specific model and serial number of my furnace. They did not provide me the wattage info (sigh) but they did provide some other good info I've copied below (without the usual long paragraph on following electrical codes and warranty jeopardy, etc.)

"The unit voltage requirements when used with a generator are:
• 120 volts + or – 10% (108 volts to 132 volts)
• 60 Hz + or – 5% (57 Hz to 63 Hz)
"The ground sense function of the integrated controls is a safety feature and the product will not operate on permanent or temporary power without a proper ground. It is also important to remember, that any product using flame rectification for sensing flame signal, will have problems without a proper ground.
"It is recommended to use a generator with a wave form distortion of less than 5% Total Harmonic Distortion (THD). Generators that produce a wave form distortion greater than 8% THD, should not be used.

I never heard of "flame rectification" before. I watched a couple videos on youtube. Basically the old thermocouples that sensed the presence of a flame have been replaced by this technology which measures current traveling through flame to "ground" which is really the metal part of the flame tube. So, one more reason to have a "proper ground". But I'm not sure what this means, exactly, in practice. From the videos it seems like this "rectification" stuff can be a problem even on regular grid power if the the metal parts that are "ground" to the rectification current are not the same as the electrical supply ground because of bad bonding. But maybe this is like those issues with GFIs if there are too many ground paths??

Anyhow, I think this means an inverter gen is required and connection must be very mindful of grounding and bonding if other stuff like separate inverters, UPS, etc. are in the path.


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## iowagold (Jan 22, 2020)

well you are back to a basic interlock system for the generator to the main breaker panel.
chassis un bonded at the gen set for the generator inlet.

yea most of the new water heaters and furnace units have way too much brain for their own good!

or if you want expensive and fancy go with an off grid inverter unit with multi power inputs for charging the battery array.
you can use the grid power as one of the ways to charge with a proper converter.

I would just go with an honda eu7000is gen set and tri fuel.
and a basic interlock.
click here for the honda eu7000is pages


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