Furnace and water heater sharing exhaust

Users who are viewing this thread

Jerome7

Member
Messages
87
Reaction score
0
Points
6
Location
95020
Hello all

I currently has a gas water heater and a gas furnace. They share the same exhaust leading to the roof (see picture).

The water heater is getting old and shows sign of rust and little leak, thus I am looking to replace it immediately. I am thinking of switching to a tankless unit since the cost of the unit is modestly more expensive. The installation is somewhat hair pulling though. I read the tankless unit requires PVC exhaust and can't be shared with other units. Is that correct? What are are my options to minimize the number of retrofit I need to do to my current hook ups?

I would rather avoid making another hole thru the roof. The unit is in the garage and the gas hookup is on an interior wall, thus venting sideway thru a exterior wall is also problematic.

Any suggestion, ideas would be very appreciated

Many thanks
 

Attachments

  • exhaust.JPG
    exhaust.JPG
    62.6 KB · Views: 5,808

Stuff

Well-Known Member
Messages
1,221
Reaction score
130
Points
63
Location
Pennsylvania
What is your reason to go with tankless? For most people there is little advantage but lots of issues.

Tankless also require more BTU so you may have to make changes to you gas lines as well.
 

Dana

In the trades
Messages
7,889
Reaction score
509
Points
113
Location
01609
Tankless units have MUCH higher BTU output, and are forced-draft, and have to use stainless (if non-condensing) or PVC exhaust venting. Galvanized B-vent or single wall would fail in short years from the exhaust condensate, and is disallowed. Most of the time tankless water heaters are side-vented out of a wall not vented through the roof (though roof venting is allowed, as long as the total length doesn't exceed the specified maximum.)

The large and modulating burners on tankless units usually require a dedicated 1-1/4" gas line from the regulator/meter and the water heater too. A tee off a run to the furnace would be prone to pressure fluctuation issues that interfere with the proper operation of the tankless.
 

Jerome7

Member
Messages
87
Reaction score
0
Points
6
Location
95020
Thanks for your replies all.

The primary reason for tankless is having lots of hot water as I have jets in my shower and myself, my wife, my daughter like to use them. Also less maintenance, no flushing, no replacing anode rod (I have a water softener so this anode dies more quickly)

@Stuff, would you mind giving few examples of issues? Installation or ongoing operating issues?
 

Dana

In the trades
Messages
7,889
Reaction score
509
Points
113
Location
01609
If you need high FLOW of multiple jets, a tankless is the worst choice, since it's flow rate is burner limited. If a 50 gallon tank isn't big enough to provide a long enough high flow shower, the answer is a bigger tank.

As for maintenance, tankless units need more maintenance than tanks, not less! The heat exchangers are prone to liming-up, reducing heat transfer efficiency and restricting flow.

With more information we could probably fine-tune the recommendations, but the likely best compromize would be a 75 -80 gallon stainless condensing tank water heater with a 75,000BTU/hr + burner, such as the Phoenix Light Duty. ("Light Duty" only relative to commercial application hot water heaters.)

Phoenix_LD_installation_SP.jpg


With major-gusher showers there is also pretty good "apparent capacity" payback and financial payback on drainwater heat recovery heat exchangers. If you have room down-stream from the shower for a 4" x 48" or larger section of vertical drain, a drainwater heat exchanger would allow you to drop back to a 50-60 gallon tank, since something like half the heat going down the drain can be returned to the incoming water stream.

power-pipe-dana-2.jpg


DSC_6441.preview.JPG
 
Last edited by a moderator:

Stuff

Well-Known Member
Messages
1,221
Reaction score
130
Points
63
Location
Pennsylvania
What size tank do you have now and are you running out of hot water now?

Swapping out your tank with another tank should be straight forward. Putting a tankless in means changing the exhaust, changing the gas supply, adding an electric receptacle, changing the hot and cold feeds, adding bypass/flushing piping and valves. That all assumes you have enough wall space for a single small tankless. If you want lots of hot water then you may need multiple tankless units. Would need to factor in ground water temperature as well.

With tankless there is endless hot water not lots. If everyone is taking a shower at the same time then everyone gets lukewarm. Also issues of "cold water sandwich" as the heater turns off between uses and if below the minimum flow rate. Some people are fine with the limitations while others can't live with it.

In Europe where conservation is king and most minimize water usage they love tankless. Years ago I was staying at a B&B with an old tankless and the lady turned off the standing pilot every night to save that little bit of gas. If you got up too early you got a really cold shower.
 

Jerome7

Member
Messages
87
Reaction score
0
Points
6
Location
95020
@Dana, Thanks for the awesome pictures. I have seen the drain heat exchanger in some other threads here but I unfortunately don't have access to the shower drain.

@Stuff, thanks for the to do list. I was only aware of the exhaust and repiping the water lines to allow water to enter from the bottom of the unit. I have an electrical outlet near by and I saw most models are compatible w/ a 1/2" gas line. What is the purpose of the bypass for valves flushing?

If I size my unit properly (like 6-9 GPM) wouldn't it provide me w/ enough hot water flow?
 

Dana

In the trades
Messages
7,889
Reaction score
509
Points
113
Location
01609
A 199,000 BTU/hr (the maximum sized residential unit) condensing burner puts out about 190,000 BTU/hr into the water, and at wintertime incoming water in your area (assuming that's Gilroy CA?) of about 50F. A decently hot (nowhere near scalding hot) shower head temp would be ~105F, for 55F temperature rise. At 190,000 BTU/hr with a 55F rise the max flow is 190,000/55F= 3455 lbs/hr, or (/60=) 57.6 lbs per minute. At 8.34 lbs/gallon that's 57.6/8.34= 6.9 gpm. Even that is probably optimistic.)

With a 2-2.5 gpm shower head and six 1.0 gpm side sprays ( or four 1.5 gpm sprays) and your screwed just on raw BTU. Even if your shower flows duck under the 6.9 gpm by a bit, you'll have screaming family members every time somebody else turns on the dishwasher or washes their hands while someone is in the shower. There is simply not a satisfactory margin on maximum flow.

A bigger tank can deliver 15 -20 gpm peaks without any trouble- you could run two simultaneous major gusher showers w/sidesprays for several minutes with an 80 gallon tank.

With a 76 KBTU condensing burner, the recovery rate on an 80 gallon tank is about the same or faster than a typical 40-50 gallon standalone. There are bigger burner versions available, but a bigger burner wouldn't really buy you much in this application. With a 76K burner you usually don't have to upgrade the gas supply plumbing, with 100KBTU+ you often do, which adds quite a bit to the installed cost. A full stainless 80 gallon unit isn't cheap, but it'll last 20+ years in your case (and when your furnace craps out you might replace it with a hydronic air handler running off the water heater.) Shop around- this one is a re-badged 80 gallon HTP at a somewhat discounted price.

A drainwater heat recovery unit doesn't have to be anywhere near the shower (or the water heater for that matter) to work. It just has to be down-stream of the shower drain's flow. It's usually only impossible if the shower is on the lowest level of the house, draining through the basement or foundation slab, though ripping open finished walls in living space to install one can be a major PITA.
 

Jerome7

Member
Messages
87
Reaction score
0
Points
6
Location
95020
Thanks all for your advices. That was a great write up Jim. Do you have a DIY blog? I would definitely subscribe.

Ok no tankless for me. Looks like not something a DIY guy like me should undertake and quite expensive to get a pro to do the work. Home Depot tankless install starts at $4000.

Replacing my current tank (it's made by AO Smith) should me straight forward. However looking at my current tank, there are few things that I don't get (yet):

1) looking at the top of it, I don't see the anode rod. There are only the hot+cold water lines, the exhaust, and relief valve.
2) there is a second relief valve on the side of the tank that isn't connected. Looking at other units, the relief valve is always on the side (picture 2)
3) There is a timer on the right side of the tank (picture 3 and 4). The black foam (with red stripes) is covering a 1/2" copper pipe. It feels like water is continuously running in that pipe. This pipe is connected to the flushing valve at the bottom. My guess the silver tube connected to the timer encloses electrical wires. How does this work? What is it for? Furthermore, this is a gas heater, and gas doesn't have a lower rate during off peak hours. Do I need to connect this to my new water heater? Could anyone please shed some light? I understand the other connections.

Many thanks

sorry my pics are not oriented properly...please tilt your head to the left :)
 
Last edited by a moderator:

Stuff

Well-Known Member
Messages
1,221
Reaction score
130
Points
63
Location
Pennsylvania
1 - anode rod access is most likely just to the left on top where you see the insulation showing
2 - looks like that tank had option of where to attach relief valve. Installer's choice.
3 - That timer is attached to a circulation pump so that you have "instant" hot water at distant faucets. It appears you have a dedicated return line which is a good thing. Most set timer to not run pump at night or when away. Some newer timers have 7 day schedule.
 

Reach4

Well-Known Member
Messages
38,797
Reaction score
4,412
Points
113
Location
IL
1) looking at the top of it, I don't see the anode rod. There are only the hot+cold water lines, the exhaust, and relief valve.
Some AO Smith water heaters use a "combo anode" which is on the output nipple.

On your new WH, you might consider loosening the anode while the unit is new, if you plan to change the anode ever. They over-torque those in the factory, and you would need a big impact wrench later. You can use teflon tape, because the threads cut through the tape and make electrical contact. I use a powered anode. Protects as well as magnesium, and does not react to help produce H2S. Costs significantly, but I intend to move mine to a new WH if I need one.
 

Dana

In the trades
Messages
7,889
Reaction score
509
Points
113
Location
01609
The port marked "RELIEF VALVE" stamped into the steel is the usual location designed for placement of the the (necessary for safety & code compliance) temperature & pressure relief. Only one needs to have a t & p valve installed. The paper labeled "RELIEF VALVE" port on the top where the actual t & p is installed is typically where the sacrificial anode gets installed, but any other port on the top works.

If you replace it with a stainless water heater (rather than another glass-lined mild-steel tank), it doesn't need a sacrificial anode.

The timer on the pump looks like an enable/disable for a recirculation system (?) . Ideally this would only run intermittently just to keep sufficiently hot water near the hot water taps when needed, but if it's running continuously whenever the timer is "on" it is a huge waste of energy (both water heating energy and pump power electricity). It may have been somebody's kludge/hack of a po'man's recirculation system.

The foam pipe insulation on the plumbing at the top of the tank violates the required 6" clearance between the exhaust venting & dilution hood to combustibles.
 

Jerome7

Member
Messages
87
Reaction score
0
Points
6
Location
95020
Thanks all. I watched some recirculation pump video on YouTube and now it makes sense to me. But I noticed 2 things

1) i saw them installing a thermal sensor on the further faucet. I looked in my kitchen and bathroom and I don't have any. Is it measured at the pump?
2) i saw many setup w/ a expansion tank. Some have it on the hot water line, some on the cold water line. Which one is correct. Mine doesn't have one and no problem thus far. Is it required?

@Reach4, I also have a water softener thus my anode probably won't last that long. I was also thinking using a powered unit once this one is dead. Don't see many being online though.

@Dana, stainless seems really expensive unless for the electric models. A powered anode is a cheaper solution.

BTW, the flush valve of the new unit probably won't be exactly at the same location and will force me to move the pump a bit. Is it okay to use a flexible pipe? Just for the portion that connects to that faucet at the bottom. Also what is the typical lifetime of pumps. Since I have to move it and solder the pipes, it could be a good opportunity to install a new pump as well.

Thanks much. Your expert opinion are very appreciated
 
Last edited:

Dana

In the trades
Messages
7,889
Reaction score
509
Points
113
Location
01609
1) The UP15-18SU/LC may or may not have a local (to the pump) aquastat heat sensor accessory installed, or it may be installed but not working. (I'm not sure if the timer & aquastat accessory is an either/or option on that system, or if they can be combined.) The aquastat turns off the pump when the returning water is above some temperature. If it's out of calibration or absent, the pump will run whenever it is enabled with by the timer. It is guaranteed to be an energy-pig if only controlled by the timer (about 85 watts of pump, and lots of heat lost through that R2 pipe insulation.) If it seems to be running constantly even when the pipe is already very-warm/hot where it comes out of the wall (peel the insulation open and feel it), it has no functioning aquastat control. If it has aquastat control it will stop pumping when the pipe is hot, and only turn on again after the temp drops to a merely warm temp.

Any bronze-impeller Grundfos should be good for a couple of decades, and they're not cheap. You may need/want to spend some money on an aquastatic control, but replacing it now would be silly.

2) The hot water & cold water side are all at the same pressure, and the expansion tank can go on either.

A powered anode will not make a glass lined tank last as long as a stainless tank. A typical commercial 80 gallon glass lined tank with a bigger burner can't be counted on to go longer than 10-15 years independent of anode. The Westinghouse badged HTP Light Duty is under $2800 from the web stores ($3K+ at retail outlets) and would be good for better than 20 years, in your application, independent of warranty. At glass lined 75-80 gallon water heater with a ~75,000 BTU burner a 6 year warranty runs about a grand.

A big electric tank can last a long time, but has MUCH slower recovery times. The 85 gallon Marathon only rates 61 first-hour gallons, compared to 154 first-hour gallons for the 80 gallon Phoenix Light Duty.
 

Stuff

Well-Known Member
Messages
1,221
Reaction score
130
Points
63
Location
Pennsylvania
1) a thermal sensor on the further faucet - Normally needed only when you don't have a dedicated return line. It looks like you do have a return line so sensor should be at pump. Some pumps advertise only xx watts so do not include sensor/switch so waste electricity.
2) Usually the expansion tank is skipped if on municipal water with no pressure regulator/backflow valve. Theory is system pressure can push back to the street if needed.
 

Jadnashua

Retired Defense Industry Engineer xxx
Messages
32,770
Reaction score
1,190
Points
113
Location
New England
Most potable expansion tanks are not spec'ed for use on the hot water side...those designed for hydronic heating obviously, are, but they're not built the same inside...hydronic systems don't constantly have dissolved oxygen being introduced, potable water does, so, those on the potable side must be more corrosion resistant to have a chance to last.

My recirculation system only seems to run the pump about 45-60 seconds, maybe 4-5x per hour. Note, some keep the pump running, but use a valve to shut down the flow. That's less efficient, but still works.
 

Jerome7

Member
Messages
87
Reaction score
0
Points
6
Location
95020
Thanks a lot for the explanation all. I think I will just keep the same setup and swap my tank w/ a Whirlpool 50 gallon from Lowe's. And get a powered anode when the original one is gone.
 

Reach4

Well-Known Member
Messages
38,797
Reaction score
4,412
Points
113
Location
IL
And get a powered anode when the original one is gone.
You might want to loosen the anode when new. They really torque those down at the factory. Add that to effects of time, and you need a 5/8 drive torque wrench to loosen the anode later. A breaker bar will just rotate the water-filled tank.

In torquing down the powered anode, use teflon tape. The threads cut through and you still get electrical contact. I figure 15 ft-lbs is plenty of torque.
 
Top
Hey, wait a minute.

This is awkward, but...

It looks like you're using an ad blocker. We get it, but (1) terrylove.com can't live without ads, and (2) ad blockers can cause issues with videos and comments. If you'd like to support the site, please allow ads.

If any particular ad is your REASON for blocking ads, please let us know. We might be able to do something about it. Thanks.
I've Disabled AdBlock    No Thanks