I have a 3800 watt generator with two 120 volt 15 amp outlets and a 120/240 volt 20 amp twist lock outlet. Can I use this 120/240 volt outlet somehow with my 120 volt oil/hot water heater with a 100 feet of 12 gauge wire running from the garage?
mike snipped-for-privacy@hotmail.com
how many watts is the heater? sounds ok, provided its not too many watts.
what other loads do you have?
ideally intall a transfer switch
Its a smaller furnace with one circulation pump so im not sure how many watts it is. It does not say on the blower and there will not be any other loads. I transfer switch install costs more then the generator so im going the cheapo way. How do I connect the 120/240 twist lock to the 120v furnace?
how many watts is the heater? sounds ok, provided its not too many watts.
what other loads do you have?
ideally intall a transfer switch
First, let's start with some proper terminology. Your house has oil-fired hot water heat, right? What you have is not an "oil/hot water heater", nor is it a "furnace". What you have is a boiler. Furnaces heat air. Boilers heat water. (If you ever have to call for service, you want to make sure you use the right terms. Boilers and their associated equipment are a bit more complicated than furnaces or water heaters, and you don't want the service company to send out a technician who knows furnaces backward and forward, but hasn't ever seen a boiler.)
Now that we've got that out of the way, you want to know how to connect the
120/240 circuit to your 120V boiler (and presumably to your 120V circulating pump as well).In order to do this properly and safely, you need to have a 4-pin twist lock receptacle (two hots, neutral, and ground). If you have only 3 pins (two hots, ground, and no neutral) then you can't use that outlet. Remainder of my post assumes you have 4 pins.
You'll need to make up your own extension cord for this, using a compatible twist-lock plug purchased at a hardware or electrical supply store. The other end, you can hardwire directly to the boiler controls -- but disconnect the boiler from the house wiring system first, so you're not backfeeding the breaker panel. Use an extension cord with three conductors, black, white, and green. Connect as follows:
- green wire to bare wire(s) at boiler, to green screw on plug
- white wire to white wire(s) at boiler, to silver-colored screw on plug
- black wire to black wire(s) at boiler, to *either* of the two brass screws on the plug (the two brass screws on a 240V plug correspond to the two hot legs; it doesn't matter which one you use)
According to miker :
That furnace doesn't use 240V does it? If it doesn't, you don't use the 120/240V twistlock for it. You use one of the 120V receptacles.
Presuming that the furnace runs on < 12A (a single 120V 15A circuit under normal conditions), you may be okay. However, the blower startup may noticably load the generator causing significant light dimming. In fact, it may stall/brownout the generator or even damage the furnace.
Remember that motors (blower and pump) have significant start up current surge. Meanwhile, the max continuous amps you can draw from the generator is 15A (3800W is 15A at 240V, OR 15A on _each_ of the two 120V legs). You generally prefer to have the generator's surge rating to be double or triple the continuous draw of the motor. Which, if the blower is 10A, means you need 20A or 30A surge. You probably have 20A. Barely. Not 30A surge.
We don't know the amp draw requirements of the furnace blower or pump. It's going to be very close.
According to Doug Miller :
Most generators have 4 pin plugs now. They're intended to drive
240V/120V circuits - up to and including transfer switches.A modern standard oil furnace unit will have all of its internals on a single 120V circuit. In order to make use of the second leg of a 120V/240V circuit, you'd have to rewire the guts of the furnace. And probably unrewire them when you took it off the generator.
Not a good idea.
I noticed the two 120V plugs both have small reset buttons on them. I think they are called GFI buttons. I was worried that when the blower kicks in it could trip. The guy that sold me the generator powered the same furnace that I have so I should be ok. I guess I'll find out soon.
I would use a clamp on ampmeter to check current draw, then add 30% for motor start up.
is the furnace/ boiler on its own breaker exclusively? what amp is that breaker?
I would wire the unit to plug. then plug unit in line for normal operation or generator for emergencies.
give some thought to extension cords, they can be trip hazards and having a door open in freezing weather with risk of exhaust gas getting back into home.
better to install a outdoor outlet designed for generators anti shock design
You heat water, then you have a boiler not a furnace, and a pump not a blower. The pump may take only 150w and the Boiler only 20-50. Use a clamp on amp meter to know draw and see pump surge which may be 6x run power. You will waste alot of money using a gen to only generate a few hundred watts to heat your home which will need alot of run time. Many cheap gen motors only last 3-350 hours that might only be a few months for you. Also check output of the gen at idle and under load and recheck it every few days, as you could ruin your boiler as well as the gen. City power will be alot cheaper.
e:
i think OP is concerned with emergency heat.
if the heating demands are that low a inverter may be all thats needed, for keeping things from freezing powered by vehicle
There is an on/off box above the furnace with a 20 amp fuse in it. It looks like the blower and pump run to a small box on the furnace with just one power cable coming from it. This one cable goes up to the on/off box with the 20 amp fuse.
It sounds like the way to go is to cut the cable and insert a 20 amp male/female standard 120v plug. When the power goes out, unplug the furnace from the AC and plug it into the generator.
Its not the most elegant solution but I'll only need it once a year or so. Im in Canada so it will keep the pipes from freezing up in winter.
i think OP is concerned with emergency heat.
if the heating demands are that low a inverter may be all thats needed, for keeping things from freezing powered by vehicle
Fine -- if he said that he has a *new* generator, I missed it. I figured it was possible that he didn't, and wanted to address that.
That's fine. I told him how to wire a single 120V circuit from a 240V four-wire source. Where's the problem?
Of course not. But nobody ever suggested using both legs, either. He wanted to know how he could run his 120V boiler off of a 240V circuit -- and I told him how, using *half* of that circuit. (Maybe you ought to read my post again, a bit more closely on the second time around?)
Very likely they're actually circuit breakers.
What blower? You said you have hot water heat.
Doubtful.
boiler
He doesn't *have* to use one of the 120V receptacles. It's perfectly possible, and safe, to derive a 120V circuit from the 120/240 receptacle, which is what he asked about, and what I told him how to do.
If he has hot water heat, he doesn't have a furnace blower. These dire predictions just aren't even remotely close to reality for a hot-water system.
Significant in comparison to their running current, yes. Significant in comparison to a 15A breaker, a circulator pump is not even close. Circulator pumps in residential hydronic systems use fractional-HP motors, typically 1/6 HP or less.
But he doesn't have a blower. He has a circulator. And it isn't going to draw anywhere near 10A running -- probably won't even get close to 10A on startup surge. My hydronic system is running *four* circulators, plus all the circulator switching controls, plus the boiler controls and ignition, plus the boiler fan-forced flue, all on a single 15A 120V circuit. We've been in this house eight years. Never tripped that breaker, not even when switching the system on for the first time each fall when the boiler and _all_four_ circulators kick on _simultaneously_.
This just isn't an issue with residential hot water systems.
No, it won't. He's perfectly fine.
When I said blower I meant the parts that blows the flame into the furnace. Its an oil hot water furnace.
According to miker :
Does the circuit feeding the fuse box have a breaker or fuse on it at the panel? Is it 15A or 20A? If the smallest breaker/fuse on the circuit is 20A, you're going to have a problem.
The generator is only rated for 15A continuous at 120V. And you're not supposed to run 20A thru a ordinary 120V receptacle outlet. If it needs >15A, on AC power, you will be overstressing the outlet. When on generator, you'll be overstressing the generator (and there's no head room for surge) too.
Check the amp ratings _on_ the furnace.
It's not strictly legal, but in case of emergencies...
Use heavy duty (eg: SO or SOW) flexible stranded cord at least 12ga for the cable from the furnace and a heavy-duty plug and receptacle. Eg: Hubbel "spec grade" devices. It really should be a locking receptacle, but that won't match the 120V outlets on the generator.
You could get it to use _half_ of the 240V/120V receptacle, so you can use a matching locking receptacle. But that will have a dead leg on it, unless you turn this cable into a "Y" and feed other things from it.
We saw a lot worse during the ice storm.
According to Doug Miller :
Methinks I shoulda. Now that my suggestion has come around to the same thing... ;-)
In industrial situations, Canadian codes aren't very anal about making the outlet match the circuit (in voltage) as long as (a) the outlet has sufficient amps, and (b) that pattern is used _only_ for that voltage within a building. Or at least it used to. May not anymore.
But I think he needs more current than the generator can deliver.
Using the 120 regular plug will work but you need to know what it draws, you keep mentioning blower. I imagine your 3800w unit is rated near 2-2500 continous so it should do fine,, is this a 350 $ chinese generator.
No, it's not. It's an oil-fired *boiler*. You do not have a furnace.
Like I said... the distinction is important whenever you're calling for service (or posting repair questions on Usenet). Boilers and their associated equipment are very different from, and more complicated than, furnaces. The world is full of (apparently) competent furnace techs who don't know their arses from their elbows when it comes to boilers. And if you need to call a local HVAC outfit for service on your system, you'd better tell them you have a boiler, so they send out a *boiler* tech.
Correction: he *might* have a problem, *if* the system draws more than 15A. Considering that it's a hydronic system, though, this is vanishingly unlikely: they just don't use all that much power.
He doesn't *have* a furnace. He has a boiler. Hydronic systems don't have anywhere near the power requirements of forced-air furnaces. It just doesn't take much power at all to keep the water circulating.
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