question about PVC concentric venting

njbillt

I'm looking at the installation manual of a Rheem condensing tank water heater (this is a power direct vent unit) and I think I see a conflict of information. The inlets and outlet are both 2". There's a chart that says I can use 2" PVC to vent up to 30 feet, but then it says I must use their 3" concentric kit. The illustration shows 2" going to a 3" reducer (expander,really) going in and out of the water heater. This makes no sense to me if it starts with 2", why go up to 3"? I called Rheem and they said I could use 2" all the way with a 2" concentric kit, but it was a boiler room answering service, and I'm not sure I got a qualified answer. I'd rather use 2" because I already have a 2" concentric, and it's just a smaller hole in the house. Can anyone think of a reason to use 3" PVC when the inlet and outlet are 2"? Thanks for any input.

icesailor

PVC Concentric:

Isn't the 2" through the wall part of a 2" concentric, 3"? I've always used a 3" vent flange to flash them to the side of buildings and I use a 3" flashing.

njbillt

no, it's definitly 3" pipe

The concentric it's calling for uses 3" PVC pipe. My furnace uses a 2" pipe concentric, and it's smaller. I'd rather use 2" pipe and concentric all the way, and the chart says I can. I'm just confused why they say I MUST use that 3" concentric. That doesn't make sense with 2" pipe. http://www.amazon.com/Rheem-SP20245-Concentric-Vent-Kit/dp/B004KLWPI0

icesailor

2" Vs 3" Concentric Venting:

All the Munchkins I did called for always called for 3". The supply house, the installations and reps said that 2" was fine as long as you stayed within the length limits. I was always direct venting immediately outside. I never had a problem. I saw problems with 3" concentric vents that were on small input appliances where on low fire, in my opinion, there wasn't enough back pressure in the venting to overcome varying outside draft/winds. I solved it by putting a 3"x2" PVC bushing in the intake and exhaust outside.

Purist/experts may say I was wrong to do that, but it solved the problems.

njbillt

Not sure about the munchkins ...

... but thank you for the reply. The only reason I could think for using 3" out of a 2" was less condensation, maybe? I never thought about the low back pressure with the 3". I think I'm going to use 2" all the way like Rheem said I could. I'm only going 15 feet. I don't think their nomenclature is up to date because even the specs say it has 3" venting, and clearly, it does not.

icesailor

Munchkin Venting:

I vented M80 Munchkins with 2" concentric vents. The boilers were located next to the outside wall. I vented 140's with 3" Concentric. I never vented a M80 that was 17' one way let alone 17'  round trip.

Rudd Power vented water heaters had 2" outlets but increased to 3" immediately. Now, they have a big restrictive washer at the top. The big restriction boosts efficiency by slowing down the flow of gasses through the heater. It's vented 2" now.

If you understand gas appliance vent tables and how they work, it is important that the vent be not too small. Being too big is just as bad. What you seldom see discussed is that the contracting flue gasses due to cooling, contribute to draft. Once the flue gasses become the same temperature as the surrounding area, there is no more aid due to heat contraction.

IMO

njbillt

okay, but what about my Rheem?

do you think I'm okay keeping everything 2" if I'm only going 15 feet? This unit is new to them and I don't think they have their nomenclature up to date. Even the specs say it has 3" vents, but they are 2".

icesailor

3" Vs 2" Venting:

I'm not telling you what or how to vent the water heater. The I/O manual will tell you that. If there is a 2" outlet and they tell you, you MUST vent it with 3"m and they provide you with a special increaser to fit on top of the blower, that you must use it. New Ruuds can be vented with 2" as long as you don't exceed a maximum length.

Munchkins said to vent with 3" but it also said you could use 2" if it was below a certain input, and didn't exceed a determined length.

What is common practice where you live? Have you asked the Plumbing/Gas inspector what he/she will approve? They are the ones who have to sign off on it. 17' is a long way off. The AHJ makes the rules.

njbillt

Yours last is the best reply so far

"What you seldom see discussed is that the contracting flue gasses due to cooling, contribute to draft. Once the flue gasses become the same temperature as the surrounding area, there is no more aid due to heat contraction."



This is the input I was looking for, from someone with experience. Unfortunately, I don't know how to decipher it. Is there an advantage to using 3" over 2"? I can use either, but I'd have to buy the 3" concentric. By the way, Rheem doesn't supply special increaser to fit on top of the blower. They just picture it that way in the manual. The Rheem tech said they probably did that because a lot of installations may be replacing a heater with 3" vent. So you can see why I'm not comfortable with his answer? I have not yet asked our building inspector. From what I understand he just puts stickers on things.

icesailor

Concentric venting:

The old Rudd PVP (Power Vented) water heaters used a 2" X 3" Increaser that fit over the outlet of the blower. It was a specific fitting that had a slot cut out of it. You HAD to use the fitting to start the vent. The blower took the place of a draft hood and just set over the normal vent on a gas water heater. There was no restriction at all. With the new energy efficient heaters, there is a big washer over the hole with a hole that restricts the flow of exhaust. I just looked at it as a way to make the hot exhaust stay in  contact with the tank for a longer period of time. As I remember, the blower is now and still 2". I read installation manuals pretty carefully, even if I think I know what they say. I don't remember anything about it MUST be 3", it was a replacement, straight up or out (the old one was straight up through the roof) and I replaced it with 2". It was approved.

As far as your situation, I can't advise you on going 17' to termination. But I look at the venting like it is a properly sized chimney flue. Too small and there is too much restriction to achieve proper flue gas velocity. Too big and the velocity gets too slow. The length of the exhaust flue is directly related. If the appliance is (say) 100,000 BTU per hour, it is just as bad to put it into a flue that is too small as it is too big. As far as my expansion issue, I look at it like a 2 stroke dirt bike. The exhaust opening in the block is small. The opening at the end of the muffler or whatever sound device they have is small. But, near the exit of the block is a big expansion pipe. A 2 Stroke engine will not run efficiently without one and may burn up the piston. When the cylinder fires and drives the piston down, you get power. When it starts up, it has to exhaust the gas and bring the new charge in at the same time. The expanding exhaust needs a place to go with little back pressure. When and as it comes out the port, it continues expanding in the expansion chamber. As it is heading out the exhaust and is contracting but the next charge is coming. The contraction of the charge leaving and the new charge firing, helps the exhaust out of the exhaust pipe. If too much back pressure or not enough free flow of new air/fuel, power drops off because of the restriction. That's where the "Power Band" is. Where it's most efficient. The "sweet spot" The same "Sweet Spot" when you vent into a chimney.

If you understand where I came from and have gone.

icesailor

Model Heater:

What model Rheem heater are you trying to vent?

njbillt

ECOREH50

It is this model. You will see in the spec's it says: Use 2 or 3 inch diameter PVC



http://www.rheem.com/product/residential-gas-water-heaters-professional-prestige-series-high-efficiency-condensing-power-direct-vent



On page 9 it shows it vented with 2" pipe, but then please see page 13 where they use 3" and the reducer (expander). The table on page 13 says 2" vent minimum 7 feet and max 30 feet, 90° elbows or concentric with an *. Look at the top of that chart and you'll see the * refers to "*Use only Rheem 3 inch concentric termination kit (SP20245)."



http://www.globalimageserver.com/fetchdocument-rh.aspx?name=professional-prestige-high-efficiency-condensing-power-direct-vent-high-efficiency-condensing-power-direct-vent-use-and-care-manual



I am practically at sea level here in NJ, and it will be natural gas. I'm just looking for advice because I have a 2" concentric and pipe. I'd have to buy the 3" material.



Thank you all for the input.

njbillt

I replied with the model number and links to the intallation manual

bump?

ChrisJ

Nope

Please be advised I'm not a pro, and honestly didn't even know what a concentric was until just now.



But the wording is very clear.  Like you said,

*Use only Rheem 3 inch concentric termination kit (SP20245). The use of any other kit is not approved.



That means, you cannot use any other kit besides that one.  Even if the 2" would work I wouldn't do it because it could end up biting you later some way some how.  As far as the water heaters outlet only being 2", the reason you go to 3" is to make up for the restrictions caused by elbows and piping length so the water heater's outlet size doesn't really matter.



Is there any reason you want to use a concentric instead of the standard two pipe setup?  I only ask because that seems like you could do it no problem at all using 2" and the holes would be even smaller, just twice.