Sounds as if the generator is not grounded to the building system-- as it is required to be. So the grounds you have been using are not grounds from the power source-- the generator.
Gotta go, my power is out and am on UPS.

Sounds as if the generator is not grounded to the building system-- as it is required to be.

The generator has a special contact for the grounding. I attached a wire to this contact and inserted the other end of it into the wall socket's ground hole. Is not that a way to attach a device to the building's ground?

Thanks for advice!

You might have to ask the boiler manufacturer what it means.

Is the boiler itself installed correctly? Did the manufacture provide any instructions on bonding of the boiler?

No, that is NOT. You must round a wire of suitable size from the generator ground terminal to the buildings grounding system.

The other problem is many of these cheap computer UPS's make absolutely crappy AC power. It matters not with computers because the first thing a computer power supply does (after going through a fuse that never blows) is run through a diode (which does blow to protect the fuse).

I'm missing the issue here. When does it report "poor ground." Is that all the time the UPS is in the circuit, just when it is solely running off the battery, or when the generator is running?

No, that is NOT. You must round a wire of suitable size from the generator ground terminal to the buildings grounding system.
Is the ground-contact of every wall-socket not connected to the building's grounding system already? I would not plan to keep the generator grounded this way, but for testing it should be sufficient.

Also, what's a "suitable size"? I hope, you don't mean to say, I must have gauge 10 all over...
The other problem is many of these cheap computer UPS's make absolutely crappy AC power.
Have you read my posting? The UPS I'm using is not "cheap" -- it is an online unit, which means, its inverter is running at all times. The incoming current is used only to charge the battery -- the load does not see it.
When does it report "poor ground." Is that all the time the UPS is in the circuit, just when it is solely running off the battery, or when the generator is running?
The "poor ground" is reported, when the power is provided by generator -- via the online UPS. Without the UPS (straight generator) the error is something like "Wiring fault". Plugged into the wall socket directly the boiler works perfectly fine (as long as the utility is delivering, of course).

Now, I notice as well, that even the UPS itself is not happy with the generator-provided power: "Site wiring fault". It works anyway, bless its heart, unlike the boiler, generating pure 120V@60Hz on its output.

So, what's wrong with my grounding -- and why does connecting to the wall-socket's ground not work?

I did read your post and since you decided to get snarky in your reply, it's not worth my blood pressure to explain to you the tenets of grounding and bonding with it's relationship to the answer I already gave.

Someone else will have to sign up for the continued abuse.
You seem to have picked the right user name.

Does your generator have a bonded neutral or not?

I would check to see if there is a voltage difference between the ground and neutral output of the inverter. If the neutral is floating on the output of the inverter that might cause the boiler issues.

Does your generator have a bonded neutral or not?

I don't know... The manual, that came with it, is poorly translated and very short in any case. It is a Diesel generator made by Powerland, model PDU6500E - the only Diesel model available for immediate shipping (on Amazon) during Katrina last year.

Now, whether it is supposed to be bonded or not, let's assume, that it is not. Can this be rectified (no pun intended) somehow? I expected the online UPS to help, but it did not - not fully anyway...

I don't suppose, I can just short the two prongs together, can I? Thank you!

Generator grounding is complex and depends on the situation. It is always best to consult an electrician . If a generator is sold for job site usage then by OSHA rules the neutral must be bonded to the ground and frame of the generator . However a backup generator in most cases should not bind the neutral to the ground. (Floating neutral).That is only allowed once and in the us and canada that is in the main entry panel. If it is bonded in both places you introduce the possibility of a current on the grounding system. What you do depends on how you connect to your panel. (assuming a legal tranfer switch or interlock with backfeed). If the neutral is bonded in the generator then you should use a transfer switch which breaks both the hot and neutral wires . That way the bonding happens once at the generator. The generator should be connected with a 4 wire cable to the transfer switch. If the generator has a floating neutral then if local code allows you can use an interlock and backfeed breaker in your main panel (you don't need to switch the neutral) or a simpler transfer switch which only switches the hots. If you need to ground the generator you need to use at least the same gauge wire (10 gauge for 30 AMP) as the wire supplying the voltage. It must be attached to the building ground, typically a Water pipe or ground rod at or near the main panel. If you are illegally back feeding through a dryer or stove outlet, then you are own your own. You won't likely ever get the bonding/grounding to work properly. All this being said mine was wired by an electrician and my, UPS complains all the time. The reason is the voltage variance and the fact most portable generators do not generate a perfect sine wave ( aka dirty power) and the UPS detect this as faulty wiring. I also have a clothes washer which generator causes the electronics to go haywire. Everything else works fine.

What happens is you run a extension cord from one of the outlets on the generator directly to your UPS? If it doesn't work properly then dirty power is the likely issue.

the advice of a qualified electrician , local authority and/or local or national codes supersede any opinion or advice I have given here.

What happens is you run a extension cord from one of the outlets on the generator directly to your UPS? If it doesn't work properly then dirty power is the likely issue.

The UPS complains about "building wiring", but works anyway -- there is current on the output and most devices "just work" (except the boiler). According to the UPS documentation, the error indicates either no ground, or "floating neutral", so, yes, that is, most likely, the problem.

How can the generator's neutral be best bound to the generator's ground in the absence of the transfer switch -- when the devices are plugged into the generator directly via extension cords (a la work-site)?

(Yes, the generator itself is already grounded.)

Thank you!