Gas furnace--electrical thermostat?

[foolsguinea]

I was wondering whether to put this in GQ or the Pit. I want an actual answer, so it's in GQ.

Is there some actual engineering advantage to building natural-gas-powered furnaces that require electrical power as well? That means if either utility is cut off, the furnace doesn't work.

I almost understand the electric fan, sort of. But apparently the designers decided, "Well, the fan is already electric, we'll make the thermostat electric too, & then if there's no power, well, they can sit in the cold."

Much of my family is dislocated due to lost power after the ice storm in the Central US. We can stand losing the lights & the refrigerator. We have been trying to cope with flooded basements due to having no power to the sump pumps. But having no heat is really trying.

I wish I had a Franklin stove.

[raindog]

Originally Posted by foolsguinea

I was wondering whether to put this in GQ or the Pit. I want an actual answer, so it's in GQ.

Is there some actual engineering advantage to building natural-gas-powered furnaces that require electrical power as well? That means if either utility is cut off, the furnace doesn't work.

I almost understand the electric fan, sort of. But apparently the designers decided, "Well, the fan is already electric, we'll make the thermostat electric too, & then if there's no power, well, they can sit in the cold."

Much of my family is dislocated due to lost power after the ice storm in the Central US. We can stand losing the lights & the refrigerator. We have been trying to cope with flooded basements due to having no power to the sump pumps. But having no heat is really trying.

I wish I had a Franklin stove.

Many years ago furnaces were simply cavernous barrels of sorts where coal initially and later natural gas were burned. There was nothing complicated about them and there was restriction to the heat exchanger. The result was that much of the heat went through the chimney. The heat exchangers were made of plate steel. They are called gravity furnaces. (no blowers, although they did use low voltage electricity)

Modern furnaces require blowers. If there was no blower the furnace would overheat. The construction of the heat exchanger (both in material selection and efficiency) requires a blower.

You best approach is to install a backup generator for critical applications---like HVAC, refrigeration and limited lighting.

[Joey P]

Originally Posted by foolsguinea

I almost understand the electric fan, sort of. But apparently the designers decided, "Well, the fan is already electric, we'll make the thermostat electric too, & then if there's no power, well, they can sit in the cold."

What would you suggest replacing the fan with that doesn't use electricity?

[Q.E.D.]

Originally Posted by Joey P

What would you suggest replacing the fan with that doesn't use electricity?

A natural-gas-powered engine. Duh.

[Joey P]

Originally Posted by Q.E.D.

A natural-gas-powered engine. Duh.

Make sure it has a pull start.

[foolsguinea]

I would settle for a sort of hybrid power setup with battery backup.

[Q.E.D.]

Originally Posted by foolsguinea

I would settle for a sort of hybrid power setup with battery backup.

You could do it like this guy did. If it were me, I'd use two or three 9 V batteries in parallel to supply 18 or 27 V.

[Q.E.D.]

Might help if I included a link, huh?

[ZenBeam]

How much power would it take to run a furnace blower? Could you hook a small generator to an exercise bicycle and hope to run it, while warming yourself up in the process?

OK, the voltage fluctuations would probably be bad for the blower, but what about an AC inverter that plugs into a car's cigarette lighter? Could 140 watts run a furnace?

ETA: Here's one that does 750 Watts, but you' probably have to connect it directly to the battery.

[Q.E.D.]

Originally Posted by ZenBeam

How much power would it take to run a furnace blower?

Just an educated guess here, but I'd say in the neighborhood of 50-100 watts.

[raindog]

Originally Posted by Q.E.D.

Might help if I included a link, huh?

!?!?

I'd be careful with this guy and his DIY brainchild. Without a blower, a modern furnace would quickly overheat and burn down a house if not for some "high limit" safeties.

By his own words, "With no fan running, the furnace furnished a surprising amount of heat to the upstairs. After an extended period of time, the over-temp (blower failure safety switch) switch kicked "out" and shut off the gas flow until a cool down period..." he ran the furnace with no blower and relied on the safety to keep him and his home safe.

That is a dangerous way to live. (or die) His drawing showed some additional fans mounted at the vents, controlled by batteries. I highly doubt that these fans could move the required amount of air needed to operate this furnace safely.

I work on these things for a living and understand them viscerally and wouldn't do this in my own home.

[raindog]

Originally Posted by Q.E.D.

Just an educated guess here, but I'd say in the neighborhood of 50-100 watts.

Most furnaces use A/C motors and have HP from 1/6 to 3/4. The most common are 1/3 and 1/2 HP.

IIRC......theres 746 watts per HP. So at 1/3 we're about 225 watts. Is that right?

At 1/2 HP we'd be at 375 watts.

IRL, I often see amp draws between 4 and 8 at 120V. 8 amps at 120V would be closer to 950 watts I believe.

If my math is right I think most blower motors will be between 250 and 400 watts, with some larger motors approaching 1000 watts.

(correct my math if it's wrong)

[raindog]

Originally Posted by the raindog

!?!?

I'd be careful with this guy and his DIY brainchild. Without a blower, a modern furnace would quickly overheat and burn down a house if not for some "high limit" safeties.

By his own words, "With no fan running, the furnace furnished a surprising amount of heat to the upstairs. After an extended period of time, the over-temp (blower failure safety switch) switch kicked "out" and shut off the gas flow until a cool down period..." he ran the furnace with no blower and relied on the safety to keep him and his home safe.

That is a dangerous way to live. (or die) His drawing showed some additional fans mounted at the vents, controlled by batteries. I highly doubt that these fans could move the required amount of air needed to operate this furnace safely.

I work on these things for a living and understand them viscerally and wouldn't do this in my own home.

One thing to note:

He mentioned ".....After an extended period of time, the over-temp (blower failure safety switch) switch kicked "out" and shut off the gas flow until a cool down period........" (highlighting mine)

Almost all furnaces made in the last 25 years would reach high limit conditions in less than 3 minutes---some of them sooner. They would "kick out" the gas valve until the furnace cooled down----2-5 minutes or so.

The safeties would close and the gas valve will reopen and in 2-3 minutes the furnace will overheat and the safeties will open again

Repeat

Repeat

The furnace and surrounding area will be very hot. So please note that it is not an extended period of time before the furnace is in an unsafe condition.

It's more like 2-3 minutes.

[gotpasswords]

As stated above, it's entirely possible to have a furnace that needs no utility power to run. Either a gravity furnace, or a typical wall furnace uses what's called a "millivolt" gas valve rather than the 24 volts AC used by standard furnaces. The thermocouple that sits in the pilot flame generates a tiny bit of electricity, and that's enough to trigger a millivolt valve when switched on by the thermostat.

The problem with gravity or wall furnaces is heat distribution.

Wall furnaces just dump heat out in front of them and depend on ambient air circulation to move the heat. IME, this is efective for only about 20 feet - anything further out will be cold. Gravity furnaces use convection and big ductwork - the warm air rises and displaces the cold air, which sinks back down to the basement. These work perfectly well, but suffer from being horrible space hogs - the stereotypical old furnace filling up the basement with huge round ducts running off at angles is a gravity furnace. The ducts have to be big to offer smooth flow to the gently circulating air - any restriction will just stop the air from moving.

If you want to use a blower to more effectively distribute the heat, you might as well use it to control the gas valve. As the raindog said, a furnace will only be able to run a few minutes before the stack limit switch kicks out and kills power to the gas valve if there's not positive airflow to pull heat out of the heat exchanger. The guy in Q.E.D.'s link is an idiot for intentionally keeping a furnace running at overload. At the very least, he's stripping years of lifespan out of it for stressing it so badly - the stack limit is an emergency cutout that will normally never engage in a furnace's life, not a normal operational control. Eventually, he can expect the heat exchanger to crack from the abuse, allowing deadly carbon monoxide into the home.

[foolsguinea]

OK, the one that weirds me out is under my grandmother's house. It's a giant old thing modified from coal-burning, & I thought it was fanless & needed no electricity. But it conked out, just like the rest of ours in the area that lost power. I was hoping we could crash there in a pinch; instead Grandma's been moved three times (her oldest daughter's house got hit a day later).

[raindog]

Originally Posted by gotpasswords

As stated above, it's entirely possible to have a furnace that needs no utility power to run. Either a gravity furnace, or a typical wall furnace uses what's called a "millivolt" gas valve rather than the 24 volts AC used by standard furnaces. The thermocouple that sits in the pilot flame generates a tiny bit of electricity, and that's enough to trigger a millivolt valve when switched on by the thermostat.

The problem with gravity or wall furnaces is heat distribution.

Wall furnaces just dump heat out in front of them and depend on ambient air circulation to move the heat. IME, this is efective for only about 20 feet - anything further out will be cold. Gravity furnaces use convection and big ductwork - the warm air rises and displaces the cold air, which sinks back down to the basement. These work perfectly well, but suffer from being horrible space hogs - the stereotypical old furnace filling up the basement with huge round ducts running off at angles is a gravity furnace. The ducts have to be big to offer smooth flow to the gently circulating air - any restriction will just stop the air from moving.

If you want to use a blower to more effectively distribute the heat, you might as well use it to control the gas valve. As the raindog said, a furnace will only be able to run a few minutes before the stack limit switch kicks out and kills power to the gas valve if there's not positive airflow to pull heat out of the heat exchanger. The guy in Q.E.D.'s link is an idiot for intentionally keeping a furnace running at overload. At the very least, he's stripping years of lifespan out of it for stressing it so badly - the stack limit is an emergency cutout that will normally never engage in a furnace's life, not a normal operational control. Eventually, he can expect the heat exchanger to crack from the abuse, allowing deadly carbon monoxide into the home.

Agreed 100%

As you know, safeties are not designed as operational controls. What happens when the safety opens and closes 50 times? 100 times? Eventually it fails because it was used as a thermostat!

And when it fails it [may likely] burn down the house and kill it's occupants.

To add to the carbon monoxide issue, most furnaces made in the last 10-15 years use a 120V draft inducing fan to draw flue gasses through the heat exchanger.

If he bypasses the pressure switches to power the gas valve directly you won't have to wait for the heat exchanger to fail to kill everyone in the house.

[raindog]

Originally Posted by foolsguinea

OK, the one that weirds me out is under my grandmother's house. It's a giant old thing modified from coal-burning, & I thought it was fanless & needed no electricity. But it conked out, just like the rest of ours in the area that lost power. I was hoping we could crash there in a pinch; instead Grandma's been moved three times (her oldest daughter's house got hit a day later).

As gotpasswords mentioned, it was a gravity furnace that used a millivolt system.

No electricity needed!

[foolsguinea]

No electricity needed!

Well, yes, that's what we all thought. But the thermostat on that one still appears to be electrical. Grr Arrgh.

[Rick]

Originally Posted by Q.E.D.

You could do it like this guy did. If it were me, I'd use two or three 9 V batteries in parallel to supply 18 or 27 V.

hey don't sweat the overlimit switch, I'm still trying to wrap my mind around 9 Volt batteries in parallel providing more than 9 Volts.

[Q.E.D.]

Originally Posted by Rick

hey don't sweat the overlimit switch, I'm still trying to wrap my mind around 9 Volt batteries in parallel providing more than 9 Volts.

D'oh!

Yeah, I meant series, of course.

[Rick]

Originally Posted by foolsguinea

Well, yes, that's what we all thought. But the thermostat on that one still appears to be electrical. Grr Arrgh.

Yes it is a millivolt thermostat. The electricity to run it is generated by the thermocouple in the pilot flame. (Like gotpasswords said)

[ZenBeam]

Yes it is a millivolt thermostat. The electricity to run it is generated by the thermocouple in the pilot flame. (Like gotpasswords said)

So why doesn't the furnace work when the power is out?

[raindog]

Originally Posted by ZenBeam

So why doesn't the furnace work when the power is out?

It does work when the electricity is out.

[ZenBeam]

It does work when the electricity is out.

That's not what foolsguinea said (bolding mine):

OK, the one that weirds me out is under my grandmother's house. It's a giant old thing modified from coal-burning, & I thought it was fanless & needed no electricity. But it conked out, just like the rest of ours in the area that lost power.

[gotpasswords]

Without seeing the furnace in question,I can only assume that when the coal-fired gravity furnace was converted to gas, they kept the existing electric-powered controls - just a WAG, but the coal furnace probably had an electric-powered stoker to feed in the coal. Electricity was already there, so they didn't change it.

Could they have done a full conversion to gas and a millivolt control system? Certainly. It just sounds like they didn't.

[Kevbo]

IME, millivolt thermostat loops can be a bit fussy (less than reliable). While it is bad to have a furnace that doesn't work due to a power outage, it isn't much better to have one that quits for no apparent reason at all.

We have two wall furnaces with millivolt controls. Having two of them provides some redundancy, which is needed. In the last five years I have gone through three generators* and two thermostats to keep them working. If there were only one of them, I would probably have replaced it by now.


*Millivolt thermostats use special high output thermocouples which are known as generators to distinguish them from the smaller thermocouples used as flame sensors on 24V controlled furnaces.

[Solfy]

I had an ancient millivolt system in my last house. One cold winter's evening I was wallpapering the dining room. I cut the power to that room to work around the electrical outlets. It turned out that the furnace fan was on the same circuit (weird wiring, one of the many treats of old houses!). The furnace kicked on, then kicked off a minute later. There was a burning smell.
Turns out the current generated by the pilot light was enough to give the "turn on" signal to the furnace, but without electricity to the fan, the heat wasn't distributed properly. It fried a different thermocouple in the furnace (hence the burning smell). The overheat sensor kicked the furnace off. (fortunately) When we realized what happened and turned the power back on, the darned thing still wasn't working right (due to the other fried thermocouple). Of course this happened on a Sunday evening, which meant expensive service call by furnace repair man to get new thermocouple. Which didn't work right, which meant a repeat service call the next day.

Further research taught me that such systems are no longer kosher - there should have been a mechanism in place (other than the overheat sensor) that kept the burners from lighting without a fan to disperse the heat. (the very same feature which disgruntles the OP) We are lucky we only fried a thermocouple and not the whole house.