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Ah… if you are checking at the service entrance panel…. more than a volt or two difference between the two legs and the neutral is a problem. Ditto for the busses in the switchgear. But if you have a heavy load on one leg but not the other, you might see as much as a five volt difference out in the far reaches at a remote panel, such as a barn a couple of hundred feet away…

A heavy load on one side may drop the voltage at the load a few volts, as well — but it shouldn't change the voltage balance at the panel.

I believe that @pecmsg wants to be sure you are measuring the 18 VAC. at the proper terminals. LIKE THIS:

On the left diagram you should be checking the transformer R and C terminals, with the thermostat calling for heat (solid leads) The thermostat should be checked at the thermostat location with the thermostat not calling for heat to get the voltage reading using the R and W terminals at the thermostat.

Some transformers on some steam boilers have up to 5 terminals. You always want to check at R and C

Also did you check the incoming voltage to the transformer? Right diagram if that transformer gets less than 120 VAC coming in then it will hhave less than 24 VAC coming out.

I would go for it.

bad Neutral like this?

yup