I don't know about the UPS, but the boiler may be a seperate issue. I'm guessing you have a circulator pump. The boiler's controller checks to make sure the pump is turning. Pumps are very hard to start from generators.

Written on the pump is a LRA ( locked rotor amps) number. Your pump may need all those amps to start.

With nothing else plugged in, I don't think your gen can supply more than 45.8A momentarily. If the pump is wired for 120V, the gen may max out at 13A, momentarily.

Call the gen's tech support to find out for sure.

46A*120V â 5520Watt, which is well below the generator's "spike" capacity of 6500Watt. But, if that advertized capacity is the total one (spread across all outlets), then that would explain it, indeed... Wow, I never thought, a single device in my home could require so much power, even if momentarily.

Calling the manufacturer is out of the question... You should've seen the manual they supplied :)

And using a UPS is out of the question, I guess, because those usually provide no more than 1000VA -- there are bigger ones, but they cost in the thousands...

Thank you for the explanation.

If you look at the control board of the boiler, it may give you some indication as to why it declined to run. If this is a later unit (i.e., one in the past twenty years or so), it probably needs to have a fan and certain sensors that are prerequisites to it kicking the gas on.

Given the UPS griping about things, I would put check your generator output for voltage and frequency and assure it is working properly.

How is the generator connected to the furnace/ UPS?

The UPS may be seeing ground and neutral ties at the panel AND the generator.

Generators set up for portable use normally have ground and neutral bonded in the generator.

This bond must be removed if you are feeding the generator into a main panel since it Raddy has a ground to neutral bond

" Another "data-point" is that a UPS (APC's BackUPS) turns on the red "Building Wiring Fault" light, when plugged into the generator. It still charges itself fine and remains "Online", but the light stays on. The light does disappear, when the UPS is plugged into the regular wall outlet. "

So what is the UPS plugged into when on generator power? That wiring fault light generally indicates an open ground or a reversed hot/neutral.

Are you using a transfer switch or running extension cords?

Connect your Back-UPS to your PC (it should have a USB port) and check out the logs. These devices log overvoltage, undervoltage, everything...

Also do the diesel generators output an energy pattern that mimics standard house power? Like the difference between a cheap UPS and an APC "Smart-UPS"... cheaper UPS's and DC to AC inverters will put out stair-stepped alternating current... that is, a flat voltage high, then low then high, then low... whereas real power is a gradual slope from high to low and forms a Sine-wave.

Just wondering how pure of power the generator puts out... If its pattern is slightly different from normal AC then maybe the UPS is picking up on this. It'll probly still work.

I'd think your boiler doesn't draw 46 Amps, unless it's on a 50-Amp breaker. Is it? If it's not, and it's on a 20-amp breaker, then you know it's not drawing more than that or you'd be tripping the breaker when it starts.

These units normally use 2-pole self-excited alternators. The output is sinusoidal (for the most part). The motor runs at 3600rpms +/- some amount so that the frequency stays within 60Hz +/- 0.5 Hz. But of course that can vary depending on how good the speed control is operating at the time.

The rating may be for 5000w, but that is divided across the two poles of the unit. Hence any one 120v plug can only supply 2500w or 20 amps, not 40amps. The 240v connector supplies from both poles, or 240v/20amps 5000w.

I'd be looking at a hot/neutral reversal or that extra ground problem. As a test, plug the UPS into the outlets on the generator and see if you get the same problem. If not, then check your wiring from end to end.

"I'd think your boiler doesn't draw 46 Amps, unless it's on a 50-Amp breaker. Is it? If it's not, and it's on a 20-amp breaker, then you know it's not drawing more than that or you'd be tripping the breaker when it starts. "

Breakers for residential use are all thermal-magnetic.

They will sustain 200% of their rating for a few seconds to allow for motor starting loads (just like the old 'slo-blow' fuses).

A pump starting up on a 20 amp circuit could easily pull enough to NOT trip a breaker but cause problems with a marginal generator.

And even if the generator can deliver sufficient ams continuous, suddenly switching a large load can make it bog down for a second which can also cause problems for the motor that is trying to start that caused the load to begin with