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tankless water heater flushing question

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[OP]
Jr. Member
Sep 12, 2013
158 posts
92 upvotes
Markham

tankless water heater flushing question

Hi guys,

I just tried to flush my tankless water heater.

I turned off the water inlet water supply valve (the green one) and also the blue one to my water heater, then I opened the flushing valve to let the water drain, but the water does not stop draining UNLESS I turned the main water supply off. Is this normal or both of the valves is broken?

I also cannot locate the "filter" that I was suppose to clean before flushing my tankless, is it normal for my tankless to not have that filter?

Thank you so much!
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13 replies
Deal Addict
User avatar
Nov 30, 2005
1849 posts
1434 upvotes
Ottawa, ON
Need to hook the hose also to hot water outlet service valve too. Make sure to cut off the valve on that side as well.

Take a picture of the other side of the unit so we can see more of it. Also the make and model could help.
[OP]
Jr. Member
Sep 12, 2013
158 posts
92 upvotes
Markham
Yes, i cut off both valves.

Even with all the valves cut, when i unscrew the flushing valves, water keeps pouring out nonstop.

The only way is to turn off the main water supply.

Does that mean the cold water inlet shutoff valve to the water heater is malfunctioning?
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Sr. Member
Jan 19, 2013
677 posts
592 upvotes
Ottawa
I think there’s water feeding in thru your re circulation line.
Shut the valve off for that and see what happens
Deal Addict
May 23, 2009
3346 posts
1940 upvotes
Mississauga
Yeah the water is probably coming from your recirculating line. The red 1/2” pex coming out of the tankless.

The shut off valve for that third line is not visible.
[OP]
Jr. Member
Sep 12, 2013
158 posts
92 upvotes
Markham
1chinaman wrote: I think there’s water feeding in thru your re circulation line.
Shut the valve off for that and see what happens
Theres no valve for the recirc line.

If i already shut off both the valve of the water inlet to my water heater, water should be stopped right? It is non-stop pouring out from my flush valve. That means both of the ball valve at the water inlet failed?
Deal Addict
Nov 4, 2007
1729 posts
1062 upvotes
GTA
You should also shut off the hot (Red) valve. This will isolate the Tankless from the house. The water is probably being drained from all the pipes in the house.
Sr. Member
Jan 19, 2013
677 posts
592 upvotes
Ottawa
redflagjj wrote: Theres no valve for the recirc line.

If i already shut off both the valve of the water inlet to my water heater, water should be stopped right? It is non-stop pouring out from my flush valve. That means both of the ball valve at the water inlet failed?
No if water is backfeeding into the re circ line from cold water line somewhere in the house.
Ball valves are fine.
[OP]
Jr. Member
Sep 12, 2013
158 posts
92 upvotes
Markham
kasm wrote: You should also shut off the hot (Red) valve. This will isolate the Tankless from the house. The water is probably being drained from all the pipes in the house.
Yea, i shut both off.

Thats what i thought too , but i drained 6 5L bucket full of water and it is still flowing
[OP]
Jr. Member
Sep 12, 2013
158 posts
92 upvotes
Markham
1chinaman wrote: No if water is backfeeding into the re circ line from cold water line somewhere in the house.
Ball valves are fine.
i drained 6 5L bucket full of water and it is still flowing
Sr. Member
Jan 19, 2013
677 posts
592 upvotes
Ottawa
redflagjj wrote: i drained 6 5L bucket full of water and it is still flowing
Yes. If it’s backfeeding, it’s never going to stop flowing until you turn the main shutoff off or stop the back feeding wherever it is.

Just flush the tankless with the main water off for now?
Install a shutoff on the recirc line in the future
[OP]
Jr. Member
Sep 12, 2013
158 posts
92 upvotes
Markham
1chinaman wrote: Yes. If it’s backfeeding, it’s never going to stop flowing until you turn the main shutoff off or stop the back feeding wherever it is.

Just flush the tankless with the main water off for now?
Install a shutoff on the recirc line in the future

O! Lol thanks!
I already flushed it with the main water off.

I was just worried that my 2 shutoff valves failed and have to be replaced
[OP]
Jr. Member
Sep 12, 2013
158 posts
92 upvotes
Markham
1chinaman wrote: Yes. If it’s backfeeding, it’s never going to stop flowing until you turn the main shutoff off or stop the back feeding wherever it is.

Just flush the tankless with the main water off for now?
Install a shutoff on the recirc line in the future
Hi, sorry, one more question.

Do you know if an expansion tank is required for my tankless water heater with a check valve and a recirc pump? because I don't have one installed, and I "think" the pressure is causing a little bit of problem for my reverse osmosis and hot water is "kind of" backflowing into the cold water inlet. I am also worried that it will damage the various plumbing parts in my house.
Sr. Member
Jan 19, 2013
677 posts
592 upvotes
Ottawa
redflagjj wrote: Hi, sorry, one more question.

Do you know if an expansion tank is required for my tankless water heater with a check valve and a recirc pump? because I don't have one installed, and I "think" the pressure is causing a little bit of problem for my reverse osmosis and hot water is "kind of" backflowing into the cold water inlet. I am also worried that it will damage the various plumbing parts in my house.
If there’s a check valve on water main coming in, you need one. Otherwise probably ok. Can @Red_Army confirm this? He’s the rfd plumber

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