Common venting condensing boiler & water heater

Housedoc

Greetings Yall,
I am working on a builder's personal home that will also be entered in the Home Builder Association's beauty contest in the fall, called the Parade of Homes. The house is extremely well insulated with R-30 walls, R-5+ windows, closed crawl space and encapsulated attic. The builder wants radiant floor heating on the main floor only. The conditioned floor area is about 2,600 sq. ft with a heat loss of 17,000 btuh on a design day. On the domestic water heating side, the master suite includes a "human car wash" large enough for a bible study group with four shower heads, + 3 more full baths. The challenge I face is how to match the heat loss and the domestic water heating loads with one natural gas appliance. After some thought and leaning on the opinions of several people smarter than me, the consensus was to forget it. That leaves me with two appliances, one for the micro heating load and the other for water heating.

For heating Im considering a modulating boiler such as Lochinvar, Knight or Cadet, or the Trinity Tx by NTI. Both turned down to 7K-8K input. The domestic water heating would be a condensing tankless style from the plumber as in a Navien or Rinnai.

Of course two gas appliances require two venting assemblies. Ideally, I would love to couple both appliances into a common vent/combustion air with only one roof-sidewall penetration. If the two appliances are the same manufacturer, that usually flies. My question is, does anyone know if the PVC venting of two appliances with different manufacturers can share a common venting scheme?

Ironman

No, in fact, IDK of any manufacture that allows common venting of category 4 appliances that are of a different type or model number - even of their own.

Also, IDK of any SINGLE on demand water heater that would be able to handle the gpm needed for a car wash type shower.

I think you best option would be to use a small mod/con boiler that has a 10 to 1 turn down with an indirect water heater connected to it that has enough storage capacity for the shower. You're probably looking at a 119 gal indirect, maybe more.

We need to the gpm requirement of the shower.

Housedoc

Thanks BB,
I was looking at Section 500 of the International Mechanical Code and there is an entire section describing the do's and don'ts of combining vents. Then I found a PPT of the topic where Elite Software's gas vent software shows a calculation for doing it, absent any mention of a manufacturer. It would seem to me that the physics are not proprietary. In theory, what's the difference of flue gas coming from brand X versus brand Y appliances, if both are condensing?

The shower has four (4) heads at 2 GPM each = 8 GPM. But that would not be the hot water demand unless someone has stainless steel skin. Most folks top out around 105 degrees. That's likely about 60% of 140 degree hot water and 40% of 55 degree water. In other words about 5 GPM demand at 80 degree rise.

Ironman

It has to be in the manufacturer's instructions to be common vented. And they must be followed exactly. Remember, category V appliances are positive pressure in the vent. What would happen if one appliance created a substantially greater pressure than the other?

5 gpm would deplete a 119 gal tank in 20 minutes (you get about 80% of its storage capacity). It would take an 80k btu mod/con about an hour to recover a tank that size assuming a 77* delta T.

hot_rod

CentroTherm has a cascading vent system. I think you need to work with the appliance manufacturer for approval.

nicholas bonham-carter

Are there any floor coverings which would make the underfloor radiant heating under radiated for the main floor?--NBC

Ironman

Take a look at the HTP Versa Hydro: condensing boiler, large storage/buffer tank, micro zones are no issue, all in one appliance.

http://www.htproducts.com/versahydro.html

hot_rod

nicholas bonham-carter said:

Are there any floor coverings which would make the underfloor radiant heating under radiated for the main floor?--NBC

Wool carpet with a thick foam pad is probably the toughest floor covering for use with radiant. R-2 or higher is possible.

Housedoc

Radiant is on top of the subfloor. The only R-value that inhibits transfer is 3/4 inch hardwoods and a couple rooms with tile.

Bob Harper

Centrotherm's cascading vent system cannot be listed under UL 1738 since that std. does not include a test for cascade systems. As pointed out, when one appliance is firing, a breech or failure in another appliance can lead to hazardous if not deadly spillage of CO. The vent safety systems are designed only for that individual appliance and are not interlocked with one another. Also, there is no std. test for commissioning these vent systems. I urge you to take Ironman's advice on using one modcon to do it all.