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Tankless Water Heater Fan Starts w/COLD Water

Posted by farmersdaughter_sf (My Page) on
Thu, Jul 29, 10 at 20:47

We just had a Takagi T-M32 tankless water heater installed in a down to the studs remodel of our house. The house is two stories, 2800 square feet, with 2 full bathrooms (only one tub though) and a half bath, 2 kitchen sinks and 2 laundry rooms (the second laundry room is for an in-law area). Our plumber recommended the T-M32 which is a commercial level heater because of the possibility that both families will be using a fair amount of hot water at the same time. The problem is that the fan on the tankless water heater fires up when some of the faucets in the house, but not all of them, call for COLD water. It also happens when a toilet is flushed. The gas/ignitor of the heater doesn't fire up (just the fan), in preparation for the next step which would be the gas firing up to heat the water. I don't think our plumber knows what he is doing but he thinks it is an issue with water pressure (?). Does anyone have any ideas why this might be happening. The fan is VERY loud as the tankless heater is installed in a bedroom closet, so it will be very disturbing to the person sleeping in that bedroom if the fan kicks on every time a toilet is flushed or someone turns on the cold water. I understand this will happen when the hot water is turned on, but this does not seem right to me that the fan should kick on when cold water is being requested from a plumbing fixture.

Follow-Up Postings:

RE: Tankless Water Heater Fan Starts w/COLD Water

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.

RE: Tankless Water Heater Fan Starts w/COLD Water

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.

RE: Tankless Water Heater Fan Starts w/COLD Water

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.

RE: Tankless Water Heater Fan Starts w/COLD Water

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.

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