Expansion Tank Needed For Tankless Heater?

[Haselsmasher]

When I converted from conventional tanked water heaters to tankless the guys who did the work left the expansion tank in place.

I'm now thinking there is not a need for it - as there is not a tank of hot water in place that could expand.

Do people see issues if I remove that expansion tank?

Thanks.

Jim

P.S.: This is a Navien unit with a .5 gal buffer tank - their "Comfort Flow" technology. I think this means it keeps .5 gal warm to some degree all the time. I *THINK* this leans toward needing to keep an expansion tank - but I'm not sure.

 

[Mr_David]

The water is only heated when there is flow. You do not need an expansion tank

 

[KULTULZ]

When I converted from conventional tanked water heaters to tankless the guys who did the work left the expansion tank in place.

I'm now thinking there is not a need for it - as there is not a tank of hot water in place that could expand.

Do people see issues if I remove that expansion tank?

Thanks.

Jim

P.S.: This is a Navien unit with a .5 gal buffer tank - their "Comfort Flow" technology. I think this means it keeps .5 gal warm to some degree all the time. I *THINK* this leans toward needing to keep an expansion tank - but I'm not sure.

I would refer to the INSTALL MANUAL before removing the tank.

There is a TECH here that does a lot of these Navien units. Hopefully he will come by.

 

[havasu]

I would tend to say it is not necessary. Navien is located in Southern California, and it is quite easy to just give them a call and they will tell you, right out of the proverbial horse's mouth. Please let us know what they tell you!

 

[Haselsmasher]

I would refer to the INSTALL MANUAL before removing the tank.

There is a TECH here that does a lot of these Navien units. Hopefully he will come by.

Good idea! D'uh! Why didn't I think of that?

There is no mention anywhere that I could find in the Navien installation manual about an expansion tank. As a point of comparison, I found online an installation manual for an A. O. Smith tanked water heater and it DOES specify an expansion tank. (I'm not saying that as being a surprise - but I wanted to see if installation manuals where an expansion tank is needed would cover it as a topic - or would it assume the installer knows it is needed for tanked heaters.)

This all seems to point to the fact an expansion tank is not needed.

Thanks!

Jim

 

[Haselsmasher]

I spoke with Navien Tech Support:

If recirculation is turned off COMPLETELY then an expansion tank is not needed. If recirculation is turned on - for either internal or external recirculation - then an expansion tank is needed.

Jim

 

[KULTULZ]

I spoke with Navien Tech Support:

If recirculation is turned off COMPLETELY then an expansion tank is not needed.

If recirculation is turned on - for either internal or external recirculation - then an expansion tank is needed.

Jim

OK...Please bear with me here as this is new tech to me...

This particular model offers a small tank to supply hot water until the tankless fires and supplies the hot water, correct? Is this what is meant as recirculation within the water heater or the house distribution system?

It seems to me... ... that the small tank has no real purpose unless a distribution hot water recirculation system is used as you will still have a hot water gap as the supply pipe is emptied of cold water?

Need help here gentlemen...

 

[havasu]

https://youtu.be/2iwIyTZfwGU

Since my old house (that I eloquently handed over to my ex-wife) has a Navien T/W/H, I can only talk about their great units. The buffer tank allows for instant hot water, while the unit fires up, so when you end up using the buffered water, all the water behind it will also be buffered to avoid the hot - cold - hot water many tankless units will give you. Since they also have a built in recirc pump on some models, it makes for a pretty good system.

 

[KULTULZ]

This has been bothering me...

I spoke with Navien Tech Support:

If recirculation is turned off COMPLETELY then an expansion tank is not needed. If recirculation is turned on - for either internal or external recirculation - then an expansion tank is needed.

Jim

If the WH has a buffer tank (not to be confused with an expansion tank), it is suggested that an outside thermal expansion tank be used as the heated water in the buffer tank will cause the water volume in the house system to expand (same principal as a tank WH) and that expansion either is absorbed by an expansion tank or may put excessive water pressure on the distribution system, faucets (usually rated @ 90PSI), toilets and the like, and may trip the TPRV.

Tankless w Buffer Tank and External Recirc Ill -Navien NR-240

 

[Haselsmasher]

This has been bothering me...



If the WH has a buffer tank (not to be confused with an expansion tank), it is suggested that an outside thermal expansion tank be used as the heated water in the buffer tank will cause the water volume in the house system to expand (same principal as a tank WH) and that expansion either is absorbed by an expansion tank or may put excessive water pressure on the distribution system, faucets (usually rated @ 90PSI), toilets and the like, and may trip the TPRV.

Tankless w Buffer Tank and External Recirc Ill -Navien NR-240

Your description in this post accurately reflects my interpretation of what the Navien tech told me.

I concluded, assuming the Buffer Tank always contains heated water (my understanding is it does) that one then has the exact same situation as with a conventional tanked water heater: (As you say) - the potential to have too much pressure in the line.

I believe only if you turn off internal recirculation (which means that buffer tank would not have hot water in it) then the external expansion tank is not needed.

Jim

 

[CuraLeakPlumbing]

When I converted from conventional tanked water heaters to tankless the guys who did the work left the expansion tank in place.

I'm now thinking there is not a need for it - as there is not a tank of hot water in place that could expand.

Do people see issues if I remove that expansion tank?

Thanks.

Jim

P.S.: This is a Navien unit with a .5 gal buffer tank - their "Comfort Flow" technology. I think this means it keeps .5 gal warm to some degree all the time. I *THINK* this leans toward needing to keep an expansion tank - but I'm not sure.

You need a the small thermal expansion tank if you are using the Navicirc feature that pumps hot water into the cold side. You need the bigger 2 gallon tank whenever you are using the dedicated recirc line because you are exceeding 2 gallons of heated water in a closed plumbing loop. There are no exceptions to this. If you want to try an experiment I have one for you. Hook a pressure gauge with a max reading red arrow on your hot line and let your recirc run all night.you pressure will jump 30 plus psi over you incoming.