# Negative Pressure Gas Appliances



## UN1TED-WE-PLUMB (Oct 3, 2012)

We had the Navien rep in this morning giving a presentation on their tankless products. They're promoting that they have a venturi gas valve that creates a venturi effect or vacuum. What this enables it to do is to operate smoothly at full modulation of 199,000 btu on a half inch gas line and that gas pressure is no longer an issue.

My take on it. 

While it may be true that the venturi gas valve may supply enough fuel for it to run smoothly, the fuel still has to come from the system, which in my opinion, would deplete the volume from the rest of the appliances. I've never tested this theory and I will when I sell one. 

Second. 
When it is inspected, the system is now drastically undersized and my concern is that the inspector would fail it. I asked if it was specifically addressed in the code book and they assured me it was in the 2012 book. I've studied the 2009 book but not so much the 2012 book. I started reading it a few days ago and briefly skimmed through again tonight. I cannot find anywhere that mentions negative pressure gas appliances. I asked for the code section so I could look it up, he said he had copies, but then failed to give me any. I asked several times. 

Whats everybody's take?


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## plumbdrum (Nov 30, 2013)

You would still size your piping for total load. Don't go but what the manufacture says it could run at.

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## Plumbergeek (Aug 16, 2010)

I don't care what they say about using 1/2" gas line to the unit, which Navien & now Noritz are claiming as a selling point for retrofitting from a standard tank water heater to tankless. I'm still going to size it properly and use 3/4" to the ones I install. If you read Naviens manual they state 1/2" can be used if you are within 25' of the main line and the tankless should be the first appliance on the system. There is no way that the other appliances would not be starved if everything was firing simultaneously. 
Luckily for us in the south I can install units on the exterior next to the gas meter or run a new line from the meter to a basement install. The issue I'm seeing is all the Hacks see that 1/2" can be used and are sticking the tankless right where the old tank was without any concern for the gas requirements! I had a local plumber tell me he just installed his first Navien and ran 25' of TracPipe to it? I told him that 3/4" TracPipe could only carry 199.000btuh for about 5', he just shrugged his shoulder's..........


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## UN1TED-WE-PLUMB (Oct 3, 2012)

plumbdrum said:


> You would still size your piping for total load. Don't go but what the manufacture says it could run at.
> 
> Sent from my iPhone using PlumbingZone


I'm fully aware of that. I'm just trying to find out if what they're claiming is actually true.


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## mytxplumber (Jul 28, 2013)

I think this will rear its ugly head in time as more installs happen. This is more new and improved stuff or crap what ever you want to call it. The system still has a max capacity. I have been in many houses where the system was starved and the homeowners claim it was running fine. This is how people die and I don't want to be on the other end of a lawsuit. Negative pressure draws both ways to you still have to have capacity.


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## UN1TED-WE-PLUMB (Oct 3, 2012)

Plumbergeek said:


> I don't care what they say about using 1/2" gas line to the unit, which Navien & now Noritz are claiming as a selling point for retrofitting from a standard tank water heater to tankless. I'm still going to size it properly and use 3/4" to the ones I install. If you read Naviens manual they state 1/2" can be used if you are within 25' of the main line and the tankless should be the first appliance on the system. There is no way that the other appliances would not be starved if everything was firing simultaneously.
> Luckily for us in the south I can install units on the exterior next to the gas meter or run a new line from the meter to a basement install. The issue I'm seeing is all the Hacks see that 1/2" can be used and are sticking the tankless right where the old tank was without any concern for the gas requirements! I had a local plumber tell me he just installed his first Navien and ran 25' of TracPipe to it? I told him that 3/4" TracPipe could only carry 199.000btuh for about 5', he just shrugged his shoulder's..........



So true!


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## Gruvplumbing (Dec 26, 2013)

I'm not 100% sure but I think triangle tube has had them type of gas valves in there units for years now. Never heard any problems with them but it still calls for a 3/4 gas line also. And I'm pretty sure that it's not actually sucking the gas out of the system.


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## plumbdrum (Nov 30, 2013)

UN1TED-WE-PLUMB said:


> I'm fully aware of that. I'm just trying to find out if what they're claiming is actually true.


Why waste your time?

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## MACPLUMB777 (Jun 18, 2008)

While it may be true that the venturi gas valve may supply enough fuel for it to run smoothly, the fuel still has to come from the system, which in my opinion, would deplete the volume from the rest of the appliances. I've never tested this theory and I will when I sell one. 

What they are missing, is the volume that you need, the pressure doe's not count as much as the volume, To meet that 199,000 btu demand
many mfg's of high efficiency water heaters use a negative pressure gas valve but they have never claimed that they could work on a 1/2" gas line !


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## UN1TED-WE-PLUMB (Oct 3, 2012)

plumbdrum said:


> Why waste your time?
> 
> Sent from my iPhone using PlumbingZone




It's not a waste of time in my opinion. They claim that negative pressure appliances are specifically addressed in the code book for sizing purposes and I can't find it anywhere.


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## jeffreyplumber (Dec 7, 2009)

Unless the navian waterheater is magical. It will need to be plumbed the same as anything else that requires 199,000 btu. I mean it still burns gas doesent it? Navien is making the claim so I would ask them again but it makes no sence to me, I personaly wouldnt belive it unless its in a code book


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## jeffreyplumber (Dec 7, 2009)

BTW. My code book allows a 3/4 inch line to serve 200cfhr or 220,000 btu at 30 foot max. I wouldnt hesitate to follow code


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## UN1TED-WE-PLUMB (Oct 3, 2012)

jeffreyplumber said:


> Unless the navian waterheater is magical. It will need to be plumbed the same as anything else that requires 199,000 btu. I mean it still burns gas doesent it? Navien is making the claim so I would ask them again but it makes no sence to me, I personaly wouldnt belive it unless its in a code book




Exactly my thoughts.


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## Nathan901 (Feb 11, 2012)

I dont understand the underlying theory, they claim it's valves shape can cause a negative pressure or a siphon type effect?
Pulling gas out of the pipe faster?


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## bct p&h (Jan 19, 2013)

I think you have a bad or uninformed rep. The gas main still needs to be able to handle your total btu load. The negative pressure gas valve overcomes the need for a full size drop down to the heater. If you are doing a replacement in an older house, at least if they're anything like the ones around here, there is an oversized gas main and a ½" drop down to the water heater. My house for example, has an 1½" gas main and could handle me swapping out the tank type heater for 199,000 btu tankless. Because the gas valve can suck the gas out of the line, it can overcome the restriction of a ½" line for a short distance. That makes a replacement of a tank type water heater with a ½" drop a lot faster. 
The negative pressure gas valve can also overcome low or unstable gas pressures. Because it essentially creates a vacuum it raises the pressure seen at the gas valve and smooths out the high and low pressures of an inconsistent service.
There will be problems with their claims. All I can picture is someone not sizing out the main correctly and the unit firing with something on the stove. Now the hot water is working fine but the flame on is sucked out. They stop running the hot water and gas flow continues to the stove that isn't lit. So what happens when someone isn't paying attention and unburnt gas is flowing into the kitchen?


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## plumbdrum (Nov 30, 2013)

bct p&h said:


> I think you have a bad or uninformed rep. The gas main still needs to be able to handle your total btu load. The negative pressure gas valve overcomes the need for a full size drop down to the heater. If you are doing a replacement in an older house, at least if they're anything like the ones around here, there is an oversized gas main and a ½" drop down to the water heater. My house for example, has an 1½" gas main and could handle me swapping out the tank type heater for 199,000 btu tankless. Because the gas valve can suck the gas out of the line, it can overcome the restriction of a ½" line for a short distance. That makes a replacement of a tank type water heater with a ½" drop a lot faster. The negative pressure gas valve can also overcome low or unstable gas pressures. Because it essentially creates a vacuum it raises the pressure seen at the gas valve and smooths out the high and low pressures of an inconsistent service. There will be problems with their claims. All I can picture is someone not sizing out the main correctly and the unit firing with something on the stove. Now the hot water is working fine but the flame on is sucked out. They stop running the hot water and gas flow continues to the stove that isn't lit. So what happens when someone isn't paying attention and unburnt gas is flowing into the kitchen?


Bingo

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## supakingDFW (Aug 19, 2014)

bct p&h said:


> I think you have a bad or uninformed rep. The gas main still needs to be able to handle your total btu load. The negative pressure gas valve overcomes the need for a full size drop down to the heater. If you are doing a replacement in an older house, at least if they're anything like the ones around here, there is an oversized gas main and a ½" drop down to the water heater. My house for example, has an 1½" gas main and could handle me swapping out the tank type heater for 199,000 btu tankless. Because the gas valve can suck the gas out of the line, it can overcome the restriction of a ½" line for a short distance. That makes a replacement of a tank type water heater with a ½" drop a lot faster.
> The negative pressure gas valve can also overcome low or unstable gas pressures. Because it essentially creates a vacuum it raises the pressure seen at the gas valve and smooths out the high and low pressures of an inconsistent service.
> There will be problems with their claims. All I can picture is someone not sizing out the main correctly and the unit firing with something on the stove. Now the hot water is working fine but the flame on is sucked out. They stop running the hot water and gas flow continues to the stove that isn't lit. So what happens when someone isn't paying attention and unburnt gas is flowing into the kitchen?



Exactly!


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## Coolcanuck (Jan 30, 2012)

Code overrules manufacturer. That's in the code book.


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## plbgbiz (Aug 27, 2010)

Coolcanuck said:


> Code overrules manufacturer. That's in the code book.


In my never-to-be-humble opinion, this is where a qualified Plumbing Professional should overrule both.

Navien is promulgating a lie. A properly designed natural gas piping system cannot have 200K BTU's tacked on without creating a problem. Otherwise, all gas sizing charts that have been in place for decades, oversize the pipe by 200K BTU. 

http://www.plumbingzone.com/f7/new-navien-tankless-22881/

http://www.plumbingzone.com/f31/navien-ch-240-asme-21422/index3/#post333311

http://www.plumbingzone.com/f31/navien-ch-240-asme-21422/index3/#post333419

http://www.plumbingzone.com/f31/navien-ch-240-asme-21422/index4/#post333817


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## Will (Jun 6, 2010)

When I took there training(Naviens) they pretty much said when I asked them that your unit will starve off the rest of the system. They responded they they don't care about the other fixtures, only getting gas to there system. It's a load of crap IMO. I will still size it right to work properly no matter what they claim.


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## plbgbiz (Aug 27, 2010)

1 cubic foot of gas=1,000btu

If a burner is rated at 150,000btu, then 150 cubic feet of gas is needed. Delivery pressure and pipe size are irrelevant to this equation. No matter the deliver system, without 150 cubic feet of gas, 150,000btu has not been accomplished. Granted, I may be over simplifying, but not by much. 

Notice the photo from Navien's install instructions. Since when does the order of fixtures in the system matter? Since the tankless companies realized a fundamental problem with selling their product. They marketed a money saving pipe dream that seldom if ever came true because of adding 100,000btu (or more) to the gas piping system in a home.

Solution? Re-engineer the gas valve so that it will draw more gas than the piping was designed for. The result is if a tankless unit is at the end of the pipe run, and demands 100,000btu over the original design, the stove and furnace have at least diminished function and possible complete failure. This is proven by Naviens own documented instructions.

Also, they say you can run 1/2" pipe but the minimum pipe size for the gas connection is 3/4". Why? Because the marketing department never had lunch with the engineering department. The instructions also acknowledge that while 1/2" is approved, you should not use a 1/2" flex connector because unwanted noise from the pipe is likely. Why is that? Because they are sucking more gas through a 1/2" pipe than it was designed to deliver.

Their own diagram states only the meter needs to be sized properly. 

The whole marketing scam with Navien (and now Rinnai with others to follow) is just that, a scam. They are just not good enough salesmen to convince me that up to 200K BTU of gas demand can be added to a system without increasing the pipe sizes from the new fixture back to the meter. It is plumbing 101.

On the other hand, they are probably more than qualified to sell it to the public and once again leave it to us to be the bad guys.

Next maybe we can discuss how Navien immediately voids the warranty if you have not tested and verified that the water conditions meet near Utopian levels of hardness, etc.

As I stated in another post on this topic, I will not assist them in the lie.

SCREW YOU NAVIEN!


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## Will (Jun 6, 2010)

I want to like tankless units, but no matter how you look at it the only thing you gain from them is unlimmited hot water(when it works) for a premium price. Higher upfront cost, higher cost to operate, higher cost long term. I'm thinking of installing one in my home, but even I don't want to pay for them lol


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