# Furnaces: How to Keep Them Going After a Power Outage



## wackymother

Okay, TUG geniuses. Think about this one. 

Our power went out during Sandy. Then it stayed out for most of two weeks. We could have stayed in the dark house, except that the temperature dropped to 37 degrees outside and god knows what inside the house. 

A little background: 
Our natural-gas hot water heater kept heating water throughout everything. 

Our natural-gas furnace is old enough that it has a pilot light, but it's less than ten years old.

I was asking everybody I know why the hot water stays on but the furnace does not. No one really seems to understand this 100 percent. Lots of people told me that it's because the thermostat uses electrical controls. So I wanted to swap out our "smart" thermostat for a "dumb" all-mechanical thermostat. 

Then other people told me that would not work because the furnace itself has electrical controls and safety valves that require electricity, and if you somehow sidestepped those electrical controls, the feed of gas would be uncontrolled and there would be a huge explosion. 

Mostly everyone agreed that the only way to keep a gas furnace going in a power outage is by using a generator to provide the power. A couple of people said that if you have an OLD furnace, it might have all-mechanical controls and you would keep going. Ours is not old enough. 

Why is that? Can anyone explain this? Are there such things as all-mechanical furnaces today?


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## AwayWeGo

*Let There Be Heat.*




wackymother said:


> Can anyone explain this?


During Hurricane Sandy, I came _this close_ to running our gas furnace on generator power -- had it all hooked up & ready to plug in. 

Power here was out 21 hours.  Our nephew brought over his portable generator. (His neighborhood in a nearby community did not lose power.)  Our house is right next to The Chief Of Staff's sister's house. (Nephew is The Chief Of Staff's sister's son.)  

Connected by long orange extension wires, the generator had enough power to run both sisters' refrigerators plus 1 small electric heater in 1 house.  (The sisters took turns with their heaters.) 

Late in the afternoon, as we faced the possibility of our 2nd heatless night, we got the idea of hooking up the gas furnaces to the generator.  Gas flames produce the heat, but electricity operates the thermostat & runs the blowers.  

To get'm to operate on generator power, the furnaces 1st had to be disconnected from the house wiring.  I got our furnace disconnected & all ready to plug into generator power.  Then I went to work on my sister-in-law's furnace.  When I had that one disconnected from the house wiring & all ready to connect to a temporary plug-in wire, the utility power came on again.  So I restored the original furnace wiring at both houses.  

I'm pretty sure both furnaces would have run OK on generator power, but I can't prove it because the outage ended before I could actually try it.  (So it goes.) 

I don't know of any gas-only heating systems.  They all have electric thermostats, relays, limit switches, actuators, blowers (or pumps), etc., that require utility power or back-up power.  

The Chief Of Staff says I should have stand-by wiring for the furnace all hooked up in advance & ready to connect to back-up power on a moment's notice.  I'm thinking about it.  But if a set-up like that is to be semi-permanent, I want it to be neat & tidy, unlike the rough & ready arrangement that I almost got to try right before the power came on after being out 21 hours.

-- Alan Cole, McLean (Fairfax County), Virginia, USA.​


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## csxjohn

AwayWeGo said:


> During Hurricane Sandy, I came _this close_ to running our gas furnace on generator power -- had it all hooked up & ready to plug in.
> 
> Power here was out 21 hours.  Our nephew brought over his portable generator. (His neighborhood in a nearby community did not lose power.)  Our house is right next to The Chief Of Staff's sister's house. (Nephew is The Chief Of Staff's sister's son.)
> 
> Connected by long orange extension wires, the generator had enough power to run both sisters' refrigerators plus 1 small electric heater in 1 house.  (The sisters took turns with their heaters.)
> 
> Late in the afternoon, as we faced the possibility of our 2nd heatless night, we got the idea of hooking up the gas furnaces to the generator.  Gas flames produce the heat, but electricity operates the thermostat & runs the blowers.
> 
> To get'm to operate on generator power, the furnaces 1st had to be disconnected from the house wiring.  I got our furnace disconnected & all ready to plug into generator power.  Then I went to work on my sister-in-law's furnace.  When I had that one disconnected from the house wiring & all ready to connect to a temporary plug-in wire, the utility power came on again.  So I restored the original furnace wiring at both houses.
> 
> I'm pretty sure both furnaces would have run OK on generator power, but I can't prove it because the outage ended before I could actually try it.  (So it goes.)
> 
> I don't know of any gas-only heating systems.  They all have electric thermostats, relays, limit switches, actuators, blowers (or pumps), etc., that require utility power or back-up power.
> 
> The Chief Of Staff says I should have stand-by wiring for the furnace all hooked up in advance & ready to connect to back-up power on a moment's notice.  I'm thinking about it.  But if a set-up like that is to be semi-permanent, I want it to be neat & tidy, unlike the rough & ready arrangement that I almost got to try right before the power came on after being out 21 hours.
> 
> -- Alan Cole, McLean (Fairfax County), Virginia, USA.​



The only question I have about your plan is this.  Is the generator big enough to power the electric motor on the fan?  They draw a lot of amps when they start.  Check the specs on the furnace then the power output of your generator.


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## csxjohn

wackymother said:


> Okay, TUG geniuses. Think about this one.
> 
> Our power went out during Sandy. Then it stayed out for most of two weeks. We could have stayed in the dark house, except that the temperature dropped to 37 degrees outside and god knows what inside the house.
> 
> A little background:
> Our natural-gas hot water heater kept heating water throughout everything.
> 
> Our natural-gas furnace is old enough that it has a pilot light, but it's less than ten years old.
> 
> I was asking everybody I know why the hot water stays on but the furnace does not. No one really seems to understand this 100 percent. Lots of people told me that it's because the thermostat uses electrical controls. So I wanted to swap out our "smart" thermostat for a "dumb" all-mechanical thermostat.
> 
> Then other people told me that would not work because the furnace itself has electrical controls and safety valves that require electricity, and if you somehow sidestepped those electrical controls, the feed of gas would be uncontrolled and there would be a huge explosion.
> 
> Mostly everyone agreed that the only way to keep a gas furnace going in a power outage is by using a generator to provide the power. A couple of people said that if you have an OLD furnace, it might have all-mechanical controls and you would keep going. Ours is not old enough.
> 
> Why is that? Can anyone explain this? Are there such things as all-mechanical furnaces today?



Do you know if the pilot stayed lit the entire time?

One thing I know for sure about your system is that low voltage runs to and from the thermostat, even the oldest ones you can find.

Without this electricity, the thermostat cannot call for heat from your furnace and the gas valve can't open. Even an all mechanical thermostat uses electricity to tell the furnace to supply heat.  When the spring moved enough to get the mercury switch to complete the circuit, the furnace would come on.

In the days before electronic controls on furnaces you still had the need for electricity to open the gas valve when the thermostat called for heat.

(I had a paragraph here telling how the old systems could be bypassed to let the burners come on for a short time but someone might try it at home so I changed my mind about it.)




The generator is a much better solution.


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## Sandy VDH

I am assuming you have radiators, as obviously forced air heating would not work as you need power to run the blower. 

On my old old radiator heater it did not require power to light it as it had an always on pilot light, but when it died and I had to get a new one it has an electronic ignition.

Is there some sort of circulation pump that requires power?  Hot water heater stays in the tank and is drawn out.  Radiation requires circulation of the water. 

Not sure, not a HACV specialist.


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## T_R_Oglodyte

Gas furnaces - except perhaps for ancient ones - also have flame sensors that check whether the gas actually ignited.  If the sensor doesn't detect ignition it immediately shuts down the furnace (because discharging gas without ignition would be a huge hazard).  

Without electricity, that sensor isn't going to function.  And if the sensor doesn't function, the furnace should shut down.


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## AwayWeGo

*Maybe Next Time.*




csxjohn said:


> The only question I have about your plan is this.  Is the generator big enough to power the electric motor on the fan?  They draw a lot of amps when they start.  Check the specs on the furnace then the power output of your generator.


We figured it would be OK unless both furnace blowers started up at the same time. 

I am not disappointed that we didn't get to try it -- much prefer having the regular power on once again. 

-- Alan Cole, McLean (Fairfax County), Virginia, USA.​


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## wackymother

T_R_Oglodyte said:


> Gas furnaces - except perhaps for ancient ones - also have flame sensors that check whether the gas actually ignited.  If the sensor doesn't detect ignition it immediately shuts down the furnace (because discharging gas without ignition would be a huge hazard).
> 
> Without electricity, that sensor isn't going to function.  And if the sensor doesn't function, the furnace should shut down.



Yes, that was what my engineering consultant said. My super-handyman consultant agreed with him, but didn't explain it as clearly. My architect consultant thinks a really old furnace with a mechanical thermostat would have worked. We do have an old house, but not an old furnace--he thought our house was so old it might have a really old furnace. 

How about oil furnaces? My parents had one and I don't remember if it worked in a power outage...but my impression is that it did.


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## T_R_Oglodyte

wackymother said:


> Yes, that was what my engineering consultant said. My super-handyman consultant agreed with him, but didn't explain it as clearly. My architect consultant thinks a really old furnace with a mechanical thermostat would have worked. We do have an old house, but not an old furnace--he thought our house was so old it might have a really old furnace.
> 
> How about oil furnaces? My parents had one and I don't remember if it worked in a power outage...but my impression is that it did.



edited - incorrect information removed.


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## wackymother

T_R_Oglodyte said:


> It's pretty easy to check if you have one.  If it's there it will be a metal rod that sits right in the middle of the combustion, just downstream from the gas nozzles.  Google furnace flame sensor for some pictures if thst helps.  But when you know what one is, what it does, and where it should be, it's pretty hard to miss it if it's there.
> 
> And if it's not there you should get a new furnace.  Not having one is a tremendous safety hazard.



I must have one. If I didn't, wouldn't the house have exploded by now? 

BTW, a few years back our thermostat broke and the house got really, really warm and toasty. It took us days to realize that it was about 80 degrees indoors, and another couple of days to get the thermostat replaced. (And the plumber didn't seem too fussed about getting over to replace it.) And the house didn't explode. What kept it from exploding?


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## csxjohn

wackymother said:


> Yes, that was what my engineering consultant said. My super-handyman consultant agreed with him, but didn't explain it as clearly. My architect consultant thinks a really old furnace with a mechanical thermostat would have worked. We do have an old house, but not an old furnace--he thought our house was so old it might have a really old furnace.
> 
> How about oil furnaces? My parents had one and I don't remember if it worked in a power outage...but my impression is that it did.



The old mechanical thermostats did not work on magic, they worked on electricity.

The furnace (old or new) has a transformer that converts 120 volts to 12 volts.  This is the current that runs to the thermostat.  The old mechanical thermostats have a mercury switch that will complete the electrical circuit to tell the furnace to open the gas valve.  Without electricity, the thermostat can't tell the furnace when to open the gas valve and when to close it.

The type thermostat and furnace you have did not cause this.  The lack of electricity did.


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## csxjohn

T_R_Oglodyte said:


> It's pretty easy to check if you have one.  If it's there it will be a metal rod that sits right in the middle of the combustion, just downstream from the gas nozzles.  Google furnace flame sensor for some pictures if thst helps.  But when you know what one is, what it does, and where it should be, it's pretty hard to miss it if it's there.
> 
> And if it's not there you should get a new furnace.  Not having one is a tremendous safety hazard.



This is not true.  

The older furnaces that don't have a flame sensor have a thermal couple that shuts down the gas if there is no flame.

Older gas stoves did not have this but hot water heaters and older furnaces do.

If your gas supply is temporally interrupted and then goes back on, your pilot light will go out.  When the pilot light goes out the thermal couple cools down and shuts off the gas supply.  When the gas supply is back on, your hot water tank or older furnace will not be spewing out gas.

The newer furnaces don't have a continuously lit pilot light.  My pilot goes on when the thermostat call for gas.  The gas valve then opens and the flame sensor tells the gas valve whether to stay on or off.

So don't run out and spend a lot of money for a new furnace just because you don't have a flame sensor.


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## wackymother

csxjohn said:


> The old mechanical thermostats did not work on magic, they worked on electricity.
> 
> The furnace (old or new) has a transformer that converts 120 volts to 12 volts.  This is the current that runs to the thermostat.  The old mechanical thermostats have a mercury switch that will complete the electrical circuit to tell the furnace to open the gas valve.  Without electricity, the thermostat can't tell the furnace when to open the gas valve and when to close it.
> 
> The type thermostat and furnace you have did not cause this.  The lack of electricity did.



So does the heating system use a LOT of electricity? Alan, above, and my handyman agree that you can use a generator to provide the power for your heating system during a power outage. Would there be any other way? It needs more power than batteries can provide, right?


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## T_R_Oglodyte

csxjohn said:


> This is not true.
> 
> The older furnaces that don't have a flame sensor have a thermal couple that shuts down the gas if there is no flame.
> 
> Older gas stoves did not have this but hot water heaters and older furnaces do.
> 
> If your gas supply is temporally interrupted and then goes back on, your pilot light will go out.  When the pilot light goes out the thermal couple cools down and shuts off the gas supply.  When the gas supply is back on, your hot water tank or older furnace will not be spewing out gas.
> 
> The newer furnaces don't have a continuously lit pilot light.  My pilot goes on when the thermostat call for gas.  The gas valve then opens and the flame sensor tells the gas valve whether to stay on or off.
> 
> So don't run out and spend a lot of money for a new furnace just because you don't have a flame sensor.



Good correction.  You are exactly correct.  What is a hazard is for there to not be a means of detecting ignition.  It doesn't have to be a flame sensor.  And virtually any gas appliance, no matter how old, will have one.

I edited out my previous response to minimize the chances of incorrect information being picked up and propagated.


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## csxjohn

wackymother said:


> So does the heating system use a LOT of electricity? Alan, above, and my handyman agree that you can use a generator to provide the power for your heating system during a power outage. Would there be any other way? It needs more power than batteries can provide, right?



Your system uses more electricity than batteries can supply.

Before the modern electronic controls you could get the gas valve to open with a 12 volt battery source and give you some emergency heat but the blower would not operate so you would have to disconnect the battery after a few minutes to let the plenum cool down, then start it up again.

In today's age, a generator is the only way I know of to provide the power you would need.

As mentioned above, you would have to do some re-wiring because your furnace does not plug into an outlet but is hard wired, usually with a switch at the furnace.

Many people get emergency generators installed with battery start ups.  For as seldom as the power goes out around here, I just use a small generator for the fridge and freezer.  Also the kegerator when it's in use.

I have a vent less gas log heater in the basement to provide me with heat if I were to lose power in the winter.  I also have a wood burning stove that I use but it requires power to run the blowers.  I can use it without power but have to partially disassemble the sheet metal case around it and watch not to let it get too hot.  Easier to use the gas log contraption.


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## wackymother

csxjohn said:


> I have a vent less gas log heater in the basement to provide me with heat if I were to lose power in the winter.



Yes, that's a great suggestion. I think that's what I'm going to get--we can always sleep in front of the fireplace even if the whole rest of the house is cold. I have to get some battery-operated CO detectors, too. 

Thanks to everyone for the help! I'm still not 100 percent on understanding, but I get the bottom line: every type of gas furnace uses electricity, and it's too much electricity to power with anything less than a generator. 

Next problem: how to keep the sump pump powered. We didn't need it this time, but there's going to be a next time....


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## T_R_Oglodyte

csxjohn said:


> I have a vent less gas log heater in the basement to provide me with heat if I were to lose power in the winter.  I also have a wood burning stove that I use but it requires power to run the blowers.  I can use it without power but have to partially disassemble the sheet metal case around it and watch not to let it get too hot.  Easier to use the gas log contraption.



You're like me, but a bit more sophisticated.  I have the generator for the refrigerator and a few other essentials.  I run the generator intermittently during a power outage.  Then we have a wood-burning stove that I can use to heat one large room in the house. We can use the stove for cooking and to heat some water.  We can also get some heat from the heatilator on the fireplace in the living room, but that isn't nearly as efficient as the wood stove.  

If we lived in a more frigid climate, I would take some added precautions.  But what I described above will get us through winter time power outages.  We're not totally comfortable, but we can get by for an extended time as long as I make sure all of my gasoline cans are full.


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## Liz Wolf-Spada

When our power went out for 3 days in the winter about 20 years ago, we had a gas wall heater in one room that kept working, so we kind of lived in there. Now, with forced air gas furnace, forget it. It takes electricity to run the fan.
Liz


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