Pressure Problem?

[donstump1]

New construction-pipes are not crossed-triple checked.

Pressure entering the home is 45 lbs. Two story home.

Tankless water heater is 28 feet above entry point.

My friend, a GC, told me last night about the problems he is having with this tankless water heater. When the main floor toilet is flushed the tankless water heater comes on. His plumbing sub says the water pressure is low and that he needs to add a pump and pressure tank. I thought the water heater needed to sense flow to kick on. Why would a dip in pressure cause the water heater to start? I told him I would post here to get some better ideas. Thanks in advance for all of your help.

Don

[jadnashua]

There are at least a couple of ways to detect flow: put some optical device (doesn't work with clear water very well) observing the supply, have an in-line vane that connects to a switch, have an in-line turbine or something that spins (may have advantages since you could detect the flow rate), or a pressure transducer. Pretty much all tankless systems have a flow restrictor in them, and as a result, during flow, the pressure will drop some on the output. This may be the type that that tankless uses. Reading the manual may indicate how they dectect flow...sure sounds like a pressure sensor/switch.

[donstump1]

Thanks for the response Jim.

Are you saying that when the toilet flushes in another part of the home, the pressure sensor on the hot side of the water heater will react to it, then start the water heater? Why would the water heater care about any fluctuations in incoming pressure?

Still scratching my head.

Don

[Nicholas_ii]

I don't follow why the hot water should turn on when the toilet flushes. Are you connecting your toilets to the hot water line? I wouldn't think flow on the cold water line would activate the tankless.

[jadnashua]

Both the hot and cold are interconnected, so when the cold water pressure drops, so will the hot. The tankless appears to think there is flow IN it because the pressure drops. Normally, this would only happen when you were flowing hot water, but if the inlet pressure drops quickly it could look like hot water flowing.

It would take a really sensitive switch to do that, and a less than ideal layout. Assuming you have some drains on the tankless so you can delime it, you may have an easy access point where you can put a pressure gauge...I'd install one temporarily and see what's really happening with the pressure. Then, talk the tankless manufacturer.

[mpaia]

I'm actually the person with the problem.

There is no expansion tank at all.

We've been living in the house for a few months but recently installed the new toilet (we moved in before the house was entirely finished)...since we've been using the plumbing system with no issues I can't imagine that air in the lines is an issue.

The water heater comes on as soon as the water flow to the toilet tank stops...not when we flush. We speculated that the problem might be the increase in pressure when the water stops is causing pressure at the water heater inlet and causing the water heater to start. Based on that we thought some kind of water hammer prevention might help but we haven't tried installing anything yet. We have turned the water flow to the toilet down to about half so that the tank fills much more slowly thinking that might mitigate the change in pressure when the tank stops filling but that didn't seem to have any effect on the situation.

We have contacted the manufacturer who thinks we may need to increase the overall pressure of the plumbing system and suggests adding a pressure tank and pump. They say they've seen this before and that was determined to be the cause but they can't guarantee it will help in our case. Before we spend a hunk of money on a pressure tank we'd like to see if there is another possible solution.

Thanks for your thoughts,

Michelle

[SewerRatz]

I am curious what brand and model tankless do you have?

[Gene Haynes]

Just wondering <> if the tankless turns on after the toilet stops filling ... is it possible that the new toilet cuts off water abruptly ... and the toilet line does not have an air chamber?

If the experts think this is the cause, then adding air chamber behind the toilet would be cheaper than a pressure tank.

[Scott D. Plumber]

When the toilet flushes there is a surge in the pipe for a split second where the water "bumps" backwards...and then back forwards. It is only a tiny bit of water but it is enough to set off the firing sequence on the heater. Its not firing, only the fan is coming on and then running as post purge per the programming.

It won't hurt anything but if it bugs you, put a check valve in the cold line to the wate heater and problem should stop. It's not a pressure issue, its a flow issue in the piping that causes the surge. Again, no big deal adn the heater is not firing off. It thinks about it for a split second, sees the "flow" has stopped adn runs the fan in "post Purge" mode for a short time.

Hope that helps.