Converting to gas from oil heat, and I have a chimney question.


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Old 11-17-10, 02:54 PM
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Converting to gas from oil heat, and I have a chimney question.

I'm in the process of converting from an oil fired boiler to a gas fired one. My question pertains to the exhaust venting.

The boiler I'm installing is a Weil Mclain CGI-3 pin. It is an induced draft boiler. I will also be installing a conventional (non-induced vent) 50 gallon gas fired hot water heater.

What is giving me a hard time is the venting/chimney connection. Currently, I have an electric hot water tank and an oil fired boiler. The oil fired boiler is connected to the tile lined chimney with a 7" single wall pipe.

Here's the current setup:



My question is as follows: How do I connect the new boiler and water heater? The boiler is induced draft, the water heater is not. They'll be put in the same locations, roughly, but the new boiler is less than 1/2 the size of the oil burner.

I have a 6" chimney lining kit, as I know the exhaust gases will eat at the existing chimney liner. How do I configure the vent piping for the boiler and the water tank? I'm concerned that the induced draft boiler will cause exhaust to blow back into the water heater.

Thanks for any help.
 
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Old 11-17-10, 03:27 PM
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Why not opt for an indirect hot water tank that is heated by your boiler?
 
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Old 11-17-10, 03:45 PM
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Budget is the main reason. Those indirect fired tanks sure seem expensive, last time I checked. Also, boiler capacity. Sized the unit for heating, not heating and DHW and I already own the boiler.
 
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Old 11-17-10, 03:52 PM
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Agree. I don't know of an acceptable way to do what you want - combine the mechanical-draft boiler and water heater into the same flue. In addition to an indirect, you could consider a condensing water heater, and vent it through the wall in a plastic pipe.

Fitting a new metal liner inside your fireclay flue is fine, but it is not required by NFPA 211, the fire-protection code applicable to chimneys. If your new boiler is significanly more efficient or a higher rating than the old one, a Level 2 flue inspection is required. If the new boiler is within the old boiler's efficiency and rating, a Level 1 inspection is required. The Code defines the inspection levels, but only accessible areas need be inspected, not concealed area.

The applicability of such codes is determined by local ordinances, which may specify other requirements or no requirements.

Is there space inside your chimney to snake a metal flue just for a water heater? I'm wondering if a 4" liner might be OK for the water heater.
 
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Old 11-17-10, 03:57 PM
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Capacity shouldn't be a problem unless your boiler is real tiny. The DHW can be set as priority so the boiler dedicates itself to heat the water and not the house. But the budget is the budget. I think there are direct vent hot water heaters out there. Have you checked to see if your boiler has an option to have a direct vent?
 
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Old 11-17-10, 03:58 PM
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I'll climb on the roof tomorrow and check. I was wondering the same thing; two different 4" liners, one for each?
 
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Old 11-17-10, 04:01 PM
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I checked online and it seems the direct vent water heaters are more expensive than indirect tanks. I would consult with a chimney guy. You may be able to vent them both in the same flue, but I wouldn't know for sure. It seems like it would work logically to me, but I have very little knowledge on this type of thing. The 6" lining kit may not be large enough either. Maybe some people in the chimney forum can shed some better light on this.
 
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Old 11-17-10, 04:03 PM
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The boiler can do direct vent, but not PVC direct vent. The supplier mentioned stainless direct vent piping, but I'm not sure what the particulars are.
 
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Old 11-17-10, 04:17 PM
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Originally Posted by The_Bishop View Post
I'll climb on the roof tomorrow and check. I was wondering the same thing; two different 4" liners, one for each?
I doubt that a 4" liner will be satisfactory for your non-condensing boiler. The liner needs to be at least the same size as the outlet flue from the boiler.
 
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Old 11-17-10, 04:17 PM
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Your biggest problem is going to be a lack of height from the top of the water heater to the chimney connection. You need a minimum of 12 inches of vertical run from the water heater vent before bending to enter the chimney.

You CAN have both an atmospheric water heater and a furnace/boiler on the same chimney IF the "capacity" of the chimney is high enough. Capacity is related to the interior cross sectional area and the effective height of the chimney. You would normally use a Y fitting with the furnace/boiler venting into the "run" of the Y and the atmospheric burner (water heater) venting into the side of the Y. The installation instructions for both the boiler and the water heater need to be carefully read to see if there are any restrictions or recommendations.
 
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Old 11-17-10, 04:38 PM
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Mike: The outlet on the boiler is 3", according to the installation instructions it needs a minimum of a 4" flue.

Furd: I'm in the process of reading the instructions now to see what I can deduce.
 
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Old 11-17-10, 04:57 PM
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Furd, don't forget that the CGi is an INDUCED DRAFT boiler... I don't think it's allow to common vent it with another appliance...

Bishop, page 8, section 1c... third paragraph down.
 
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Old 11-17-10, 05:06 PM
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Originally Posted by Furd View Post
You CAN have both an atmospheric water heater and a furnace/boiler on the same chimney IF the "capacity" of the chimney is high enough.
True, but the boiler is mechanical draft, not atmospheric.
 
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Old 11-17-10, 05:09 PM
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Originally Posted by The_Bishop View Post
The outlet on the boiler is 3", according to the installation instructions it needs a minimum of a 4" flue.
OK, then you should be able to get by with two 4" liners - if there is room in the chimney. One can go down inside the existing fireclay liner. Or, just use the fireclay liner, as is, for say the boiler, and snake down another 4", if there is room, for the water heater.

I tend to agree with your wanting to go with a gas-fired water heater. They are reasonably efficient and have decent recovery times - and if you go with the common 40-gal size, inexpensive. (The recovery time for a gas heater will be shorter than your current electric heater.) My last gas water heater lasted 50+ years, but that is probably unusual.

If you've got a couple of teenage kids in the family, a 40-gal gas heater might be a bit skimpy. But if going to a larger heater, say 50 gal, get your pocketbook out.
 

Last edited by Mike Speed 30; 11-17-10 at 05:36 PM.
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Old 11-17-10, 05:09 PM
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By the way Bishop, the vent on your current boiler is improperly done... the location of your baro damper is inappropriate.
 
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Old 11-17-10, 05:11 PM
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The outlet on the boiler is 3", according to the installation instructions it needs a minimum of a 4" flue.
If you DIRECT vent the boiler with an approved stainless venting system, you can stay with 3" all the way out. If you vent into an existing chimney, the vent must be AT LEAST 4"
 
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Old 11-17-10, 05:18 PM
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Some boiler manufacturers will allow a induced draft boiler to be vented with an atmospheric appliance. The idea is that the 3 to 5 or 6" increaser changes the vent from a positive pressure to a negative pressure, and at that point the chimney effect takes over and expels that products of combustion.

If you can side wall vent it, and the distance is not too great think about going that route.
 
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Old 11-17-10, 05:34 PM
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Trooper, which code book? Also, (not that it's being kept) what's wrong with the baro damper? (So I know what to look for next time.)

TOHeating, I'll look into sidewall venting it, but problem is my basement is completely below grade, ceiling of the basement is roughly at ground level. Not sure how I can make that work.

I'm leaning towards leaving the electric tank as is for now, and going with an indirect fired unit once I can save up some more pennies. That eliminates a lot of my headaches, and simplifies the changeover so I can minimize the amount of down-time I have in my heating.
 
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Old 11-17-10, 05:42 PM
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Originally Posted by NJ Trooper View Post
Furd, don't forget that the CGi is an INDUCED DRAFT boiler... I don't think it's allow to common vent it with another appliance....
Forced draft or induced draft there is still a positive pressure at the exhaust gas outlet.
 
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Old 11-17-10, 05:47 PM
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Originally Posted by Mike Speed 30 View Post
True, but the boiler is mechanical draft, not atmospheric.
The boiler is mechanical draft but the water heater is natural draft.
 
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Old 11-17-10, 05:58 PM
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which code book?
No code book, the CGi install manual.

what's wrong with the baro damper?
See figure 1 in this PDF. Your exact installation isn't shown here, but the boiler shouldn't tee in like that with the damper on the opposite end of the tee.

http://www.fieldcontrols.com/pdfs/DC01575700.pdf

Forced draft or induced draft there is still a positive pressure at the exhaust gas outlet.
Right... and the water heater will have an atmospheric hood on it, and if there is a positive pressure, the boiler will backdraft out that hood, UNLESS, as TO points out below:

Some boiler manufacturers will allow a induced draft boiler to be vented with an atmospheric appliance. The idea is that the 3 to 5 or 6" increaser changes the vent from a positive pressure to a negative pressure
But, I personally would be EXTREMELY uncomfortable with this arrangement. Carbon Monoxide will only kill you once, but that's once too many for my taste.
 
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Old 11-17-10, 06:24 PM
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Forced draft or induced draft there is still a positive pressure at the exhaust gas outlet.
Right... and the water heater will have an atmospheric hood on it, and if there is a positive pressure, the boiler will backdraft out that hood, UNLESS, as TO points out below:
Some boiler manufacturers will allow a induced draft boiler to be vented with an atmospheric appliance. The idea is that the 3 to 5 or 6" increaser changes the vent from a positive pressure to a negative pressure
Read carefully what I originally wrote.
...IF the "capacity" of the chimney is high enough.
A three or four inch pressurized vent into a six inch flue will in most (but not all) cases also allow for an atmospheric vent into the same flue IF the atmospheric vent enters a Y fitting from the side. The Y fitting along with the natural draft of the main flue will keep the pressurized vent from back drafting down and out of the vent hood on the atmospheric vent.

It isn't the preferred method but it does work and is safe IF all things are done properly. I dare say there are tens of thousands of these installations in current use.
 
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Old 11-17-10, 06:26 PM
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FWIW, I like the idea of keeping electric for now and going indirect with saved pennies. All that stainless venting adds up in cost, and a good indirect will far outlast and outperform a standard gas-fired water heater. One instead of two combustion appliances is a good thing, too.
 
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Old 11-17-10, 07:45 PM
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NJTrooper, got it. Don't know how I missed that paragraph.
 
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Old 11-17-10, 08:46 PM
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That is only true for the direct vent though...

What Furd and TO are saying is that by upsizing the flue pipe, you may be able to safely vent an atmospheric vented water heater into the same flue IF DONE PROPERLY.

I personally would choose not to do that though... I don't think I could sleep at night... but that's just me...
 
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Old 11-18-10, 06:57 AM
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I agree with Xiphias. Leave the electric for now, then get an indirect tank (Triangle Tube, Weil-McLain, or Willams are the only ones I'd use) when you can afford it. Just prepare some stubs in your boiler piping where you can later attach the pipes to the indirect.
 
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Old 11-18-10, 11:13 AM
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Ok, that leads to *another* question, then. My current heating has no zone valves, it's one large pipe with those 'takeoff' fittings for each baseboard loop. If I was to use an indirect tank, I'd have to zone it, correct? That will be a bit of a pain, as that main loop runs the length of the house and some of those takeoffs are hidden behind the sheetrock ceiling.
 
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Old 11-18-10, 11:22 AM
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Is there just a single piping loop - with baseboard supplies and returns teed off the same pipe? If so, then it sounds like a monoflo system.

But, whatever. If it is a monoflo system, you'd want two zones: one for the enitre monoflo loop and one for the indirect. The indirect would be set as priority. It would take two zone valves.

The fact that some of your existing loop is concealed behind walls, etc., shouldn't be a problem.
 
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Old 11-18-10, 11:30 AM
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Yes, it would be a single loop, with the baseboard supplies and returns teed off that loop.

I didn't know that there were zone valves that large.

So, if I have this right, I'd install a T in the beginning of the main loop, and that would be the takeoff for the indirect. Then, in the main loop return, I'd have another T, with a zone valve to close off the return from the main loop placed before the T, and another in the return from the indirect tank.

Does this sound correct, or am I off base?
 
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Old 11-18-10, 11:59 AM
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If I follow you correctly, you've got the right idea. The zone valves can be on either the supply or return of a loop, whichever is easier.

How large is your main loop? Zone valves are available up to 1-1/4" from Taco and probably other manufacturers. If you used a 1-1/4" valve on a 1-1/2" line, it wouldn't be the end of the world.
 
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Old 11-18-10, 02:39 PM
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Don't forget that your CGi-3 may be marginally sized for an indirect... you said that early on, and it seems to have been overlooked. Isn't the -3 something around 50K BTU?
 
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Old 11-18-10, 03:16 PM
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Yeah, DOE rating is 51k. Besides that factor, the indirect tanks are $$$! Minimum investment on my part will be at least $1000. Holy crap!

I'm going to call Weil Mclain Monday, get some clarification on the venting issue. From what I'm reading, I can't vent both appliances if it's a positive pressure vent, but if I increase the vent pipe to 6" right at the boiler connection, it's not a positive pressure vent anymore, it's a natural draft.

The plumber I spoke to today didn't see a problem with using 6" vent pipe right at the boiler, with a T that sweeps in at a 45 degree angle to add in the water heater. Says he's installed them that way before with no issues.

I'll see what Weil Mclain says Monday, I guess.
 
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Old 11-18-10, 03:57 PM
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For 50,000 BTU even a 3/4" zone valve would do no problem.
You can also zone with circulators and leave the monoflow on a summer winter switch.
I have gotten away from zoning with circulators except for large BTU systems, where zone valves make no sense.
 
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Old 11-18-10, 05:04 PM
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With 51,000 BTU's you are doing better than a 50 gallon conventional water heater.
 
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Old 11-18-10, 05:25 PM
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Originally Posted by drooplug View Post
With 51,000 BTU's you are doing better than a 50 gallon conventional water heater.
I think so. My relatively new, cheapo, 40-gal, gas-fired water heater is rated 40,000 Btu/hr input. The two of us never run out of hot water - including showering, clothes washing, dishwashing, etc. - or even with company staying with us.
 
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Old 11-18-10, 05:57 PM
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Unless your boiler is running constantly during the coldest weather I suspect that you have enough capacity to use it with an indirect. You would have to use a priority setting on the indirect and you may even need to use a maximum time default back to space heating but only during the coldest of weather.

From your pictures the biggest problem in using the gas-fired water heater is that you simply don't have enough height to the chimney connection. You need a minimum of 12 inches vertical rise from the water heater vent before entering the chimney. You might be able to make a new flue connection higher up on the chimney and then cap the hole presently in use.
 
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Old 11-18-10, 07:45 PM
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Yup, the indirects (the good ones at least) aren't cheap. About the cheapest I've seen is a TT Smart 30 for $880 (SMART30 - Triangle Tube SMART30 - Smart 30 Indirect Water Heater). OTOH, a gas-fired water heater is something you'll replace every 5-15 years (usually after it fails catastrophically and causes a flood), whereas a good indirect (a stainless steel tank-in-tank design) should outlast you.

As to what Furd said - IMHO, if you have a priority on the DHW for your boiler (and you should, with a boiler that small), you MUST have a maximum time cutoff on the priority. This is a vital safety measure - if your DHW circulator/valve fail, or if the DHW aquastat fails closed, or even if there's just a short in the wiring, the system will never come off priority otherwise. If you are home, you'll get cold. If you aren't, you can get into a pipe freezing situation. I can't think of any situation where it's a good idea to have priority for the DHW without having a time-out on that priority.
 
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Old 11-19-10, 03:21 PM
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The plumber I spoke to today didn't see a problem with using 6" vent pipe right at the boiler, with a T that sweeps in at a 45 degree angle to add in the water heater. Says he's installed them that way before with no issues.
You need a minimum of 12 inches vertical rise from the water heater vent before entering the chimney.
After reviewing Furd's posts about the common venting, and the W-M install manual, I agree that it's do-able. (I still wouldn't do it myself...) but, you do need the vertical portion above the water heater to prevent flue gas spillage into the home, and still maintain an upward pitch of the flue toward the chimney. Don't neglect this point!

When you talk to W-M, or check the manual, think about the possibility of running the 3" pressure vent up the chimney. You might be able to fit that AND the WH vent in that chimney flue... but only IF W-M says it's OK to run the 3" vent that long a distance, and up the chim...
 
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Old 11-19-10, 05:13 PM
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That's all on my question list.
 
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Old 01-04-11, 07:22 AM
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I figured I'd give a followup on the project, now that it's done and inspected and passed, with no problems.



I have a 3" to 5" adapter at the boiler, which then meets up with the 6" Y that is connected to the new liner. Weil-Mclain confirmed that if you increase the flue size directly at the boiler it is considered an atmospheric vent.

The other leg of the Y reduces down to 4" for the flue pipe connected to the hot water tank. From the top of the water tank to the bottom of the chimney entrance is a hair over 12" even if it doesn't look like it from the pictures. The 4" flue has a very steep rise to the 6" Y.

When I was assembling the flue piping, while everything was cold, it was still drafting hard. Before firing up anything, I smoke tested the hood on the hot water tank and it was drafting fine. I fired up the tank first to check the draft and it was very strong.

I then fired the boiler while checking the draft on the hot water tank, and it didn't change at all. It didn't blow back, but kept pulling the smoke right in from all sides of the hood.

This picture is from before I sealed the chimney with the high temp cement, so rest assured it's closed up.

I'd like to thank everyone for their help and insight!
 
 

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