I can't say about your specific model, but my electric unit does sometime trigger when cold water runs on/off, such at the kitchen faucet or washing machine, I assume due to pressure changes within the lines. I can hear a relay snap on, then off a minute or so later when there's no actual hot water flow.

I have had this same issue with a Rheem tankless. I installed a check valve in the cold water line before the heater and it didn't solve the problem.

I suppose that it could be from air in the hot water line acting as a cushion, so when the cold water is opened, water briefly flows backwards through the heater - thereby causing it to start.

That's all I could think of.

Although it's a minor nuisance, I'm not sure it's a big deal.

Thanks to you both. At least I feel like my problem isn't unique! I agree it's not a huge deal but it is a pretty loud fan. We decided that if the plumber and the Takagi rep can't come up with a solution, because the closet area where the tankless is mounted is a deep closet, we may have a carpenter make some kind of a false wall that will help muffle the sound of the fan but is still able to be opened to access the unit for service. That way the person sleeping in the bedroom won't be disturbed by the fan starting up.

I read and reread your statement, do you have home runs with your water piping like a zern manifold. You have some cross bleeding in your water piping. That would be your final answer. You have to determain how water in cold is going through the water heater if the water heater only makes hot water. The fan comes on when there is .5 gpm going thru it. somewhere you have water going thrue your tm-32 to make it come on. That is the only way it ever goes on. Also sincce its in a closet do you have it directly vented outside, do you have servicing valves on your unit and make sure you have below.7 grains of hard water.