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dmengedoht 12-08-2010 09:05 AM

Tankless H2O Gas Ventilation
I read in Don Geary's article on that Rinnai indoor gas heaters cannot be exhaust vented into another appliance's exhaust pipes since the Rinnai has a forced air exhaust, and presumably it would backflow fumes out of the other appliance (furnace, etc..).

I am looking to install a R94LSi Rinnai in the cellar of my 1903 Victorian. The cellar is only 2 or 3 steps down in the ground... so there is plenty of combustable air to feed the heater. We have a furnace for the steam radiators that vents up and over into a hole bored into the wall of a chimney that goes up 4 floors, (cellar, 1st floor, 2nd floor, and full walkup attic with 20 foot ceilings). I know that the Rinnai exhaust should not be vented into the pipe from the furnace. Here is the question, finally: Can I bore another hole into the chimney in the celler, near or 1 ft away from the furnace boring, and vent into the same chimney? Will the venting be adequate, and not backflow to the furnace? The chimneys in the house are huge.

I really dont want to have the vent pipes go the full length needed to go outside. One side of the house would be unacceptable for aestetic purposes, and the other side of the house would be unacceptable for the length of the vent pipe run. Please advise. Thanks very much for the input... Happy Holidays, Dan

Grampa Bud 12-08-2010 10:41 AM

My friend you have a can of worms. Maybe we can help with some directive advice. First, you say 'furnace' for your steam radiators. Is this a hot water system or is it steam? Is your 'furnace' a
forced air furnace or a hot water boiler or an actual steam boiler? If you have a 1903 Victorian that hasn't had a hvac upgrade in the past 20 years, your chimney is probably all masonary. Meaning it has no stainless steel liner and after being scorched and cooled for many years inside its flue it is probably losing much of the brick and mortar strength. It probably has many small holes through the chimney walls that you cannot see without proper test procedures. The Rinnai tankless heaters are good units when properly installed. They are all 'power vent' units and their high efficiency heating units are termed 'condensing' units due to their lower operating temperatures. If you want a high efficiency unit you MUST have a power vent unit to help get rid of the condensation. You also MUST have a drain very near the High efficiency unit to collect the condensation that runs down the flue back towards the heater. On a high efficiency heater you MUST use either stainless steel flue pipe or if the manufacturer permits PVC pipe. This is because the flue condensate is a mild form of sulfuric acid that will, in very short order, make mince meat out of standard steel flue pipe and/or aluminum flue pipe and will accelerate the spauling on the inside of a standard masonary flue. The power vent should make it possible to exhaust out of the side wall of your basement as long as you are at least 12" above the snow line on that side of your home, but you will need a "thimble" to pass your flue through a combustable wall, even if it is only the rim joist of the floor and if you are thinking of putting on 2-90 deg. Ell's and a piece of flue to raise the height outside you must remember to insulate all of the outdoor part of the flue so the condensate does not freeze between cycles. Also read up on the installation guide because every heating unit that has exhaust gases has minimum and maximum sizes for thier flue pipes and how far horizontally and/or vertically the flue can safely be run. If you are outside those limits normally you would be screwed, but there is still hope in the form of an add-on power vent just for the longer flues and it can be controlled by the call for heat from your Rinnai or your flow switch.
Let us know how you make out. Grampa Bud

Jackofall1 12-08-2010 11:06 AM

Quite simply no you may not, as the unit is a direct vent unit.

If you read the venting instructions page 26, it is very specific, I wouldn't vary from this.

Nice unit by the way.

moopey 12-08-2010 11:42 AM

I would suggest maybe looking at a different manufacturer. one that's units are capable of venting with PVC. that may give you more venting options since you can go long lengths without the expensive cost of stainless steel.

I really like Rinnai units, but I will not use one at my house since I would have to most likely spend more money on the stainless vent material than the actual unit because of unit placement and desired vent location.

I know for a fact that Rheem/Rudd has a 94% or 95% efficient unit that can be vented with PVC and also very low minimum activation/flow rates. It's called the Prestige.

Jackofall1 12-08-2010 11:57 AM

I don't believe you will find any manufacturer of tankless hot water units that vent with PVC. If you find one please post it, I have been looking and can't seem to find any.

moopey 12-08-2010 12:04 PM

Rheem RTGH-95DV, Can be vented up to 35' with 3" PVC.

website link

spec sheet

Jackofall1 12-08-2010 12:16 PM

Thanks Moopey, thats cool, haven't looked for sometime, was in the market a couple of years ago, these must be something new.

Grampa Bud 12-08-2010 12:33 PM

You guys are right on track, but note that after you have put in 3 -6 different makes you will find yourself playing catchup just trying to keep up with the changes in design and materials on any one make & model. Laws change, ordinances change, rules change, engineering ALWAYS changes,
preferences change..... Eventually you will have to determine a few rules-of-thumb to know where the landmines are and so do not step. I have seen units clearly state PVC for flue and 3 months later say you have violated the warranty because Stainless was not used or as in the case of this Rinnai they indicate PVC if collecting condensate or Stainless if not collecting condensate. I have also seen concentric venting with Stainless on the inside pipe and a form of PVC on the outside pipe. Basicly watch your expected flue EGT and if it is 140F or less 3-4" PVC sched40 or sched80 will do just fine and at less cost than Stainless in the recommended 2-3" sizes.

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