Tankless Water Heater gas supply sizing

[SubnetMask]

Our water heater is in the process of dying. If it's just the pilot light, it'll run forever, but once the main burner kicks on, and I noticed that while the burner is on, eventually, the voltage from the thermocuple starts creeping down until it gets to 9.9mv, then after sitting there for a little bit, it drops to 0 and the gas is shut off (the gas is more than likley shut off due to the voltage dropping, as that's what it's supposed to do if the flame goes out). I realize that this is probably the thermocouple, which if I can get one, would probably be like $75, but the problem is that I, like MANY people, had no idea that the WH has sacrificial anodes that are supposed to be checked/replaced AT LEAST every five years. The WH is 13 years old, never checked or replaced and the anodes are GONE (confirmed - only the stainless wire at the center remains and it was crusted with limescale), for how long, who knows. so the entire tank is suspect at this point and I don't want to put any money onto it becuase while it *might* be fine, it might also be six months away from springing a leak from internal rusting. There's no way that I'm aware of to know for sure. And there's also a TON of scale buildup inside it.

That being said, I'm looking at the Rheem ECOH200DVELN-1 to replace it. Tank water heaters are far cheaper, and some say that the savings with the tankless heaters aren't as good as they say, but for us, the tankless would give us a decent amount of reclaimed space in our rather cramped mechanical/laundry room. Plus unlimited hot water. But really, space would be a big thing.

My problem & question is with gas supply. Although it hasn't been tested, I'm told by the gas company that the current supply pressure should be around 7.5 In W.C.. I have 3/4" supply line from the meter to all the way to my furnace, which is the last device in the line. The length of the run, to give a buffer, is 40' (It's actually more like 30, but I'd rather have that buffer). According to calculations, with the tankless, I'd need to supply 301,900 BTUs with the furnace, WH and dryer all running. That works out to just shy of 300CF/Hr of gas. Here's where I get lost. Two of the charts in the Rheem install manual say that I'll need either 1" or 1 1/4" pipe to get that flow rate. But they are both described as 'Inlet Pressure: Less than 2PSI' (that's real specific), one with an allowable pressure drop of 0.3 In W.C. and the other with an allowable pressure drop of 0.5. The third chart, however, is listed as 'Inlet Pressure: 8.o W.C. Inches or greater' (which is still below 2PSI and technically falls within the range of the previous two charts), and 'allowable pressure drop: 3.0 inches'. This chart lists 40' of 3/4 pipe able to carry 448CF/Hr. Plenty for all of the gas consuming appliances to run at the same time by a decent margin.

The existing appliances that would be staying are my dryer and furnace. The furnace has an inlet pressure range of 4.5-13.6, and the dryer 5.2-10.5, so that means acceptable inlet pressure range for the appliances that would be staying is 5.2-10.5, throwing out the lower of the minimums and the higher of the maximums.

So does this mean that if I have the gas company come out and increase the pressure to, say, 9.5 Inches W.C. (which is withing the acceptable range listed on the existing appliances and I was told by them they could do), will the 3/4" pipe be able to supply the ~300CF/Hr that would be required when everything is running, or is there more to it that's not listed in the install manual, and no matter what, I'd need to replace my supply line, and possibly have the gas company upgrade the meter?

 

[Jadnashua]

The gas flow capacity through a pipe varies by the effective length, supply pressure, and diameter. Effective length is determined by the straight-line distance with additions for each elbow. Depending on how it is run, the effective length could be way higher than the linear distance.

If you use 2psi supply, you can use smaller lines, but it needs a secondary pressure regulator to drop it to the appliance's 1/2psi max inlet (which typically has its own regulator, so maybe three). Typical appliances cannot take 2psi directly. 1/2 psi = about 14" WC. Because of flow (friction) that can limit what's actually delivered to the appliance during flow (static would be different). The regulator in the meter can limit the amount available at the end, so raising that pressure slightly can allow more gas to flow, up to the limit of the meter itself.

 

[Sponsor]

 

[Dana]

With big modulating burners such at a tankless it's always safer/better to run dedicated "home run" gas plumbing from the unit back to as close as possible to the regulator/meter as possible. If it's sharing a line with other even moderate-sized burners the pressure fluctuations of the other burner turning on/off can interact with the modulating burner, often to the point of it being detected as an error condition by the tankless controls.

Standard residential gas appliances is (as Jadnashua points out) are expecting nominally ~0.5 psi, or 14 water inches, with tolerance for the pressures lower than that. But 7.5" (0.27 psi) would be at the very low end of what they expect.

Some of Navien's newer tankless designs have quite a bit more low-pressure tolerance, making them an easier retrofit onto 3/4" plumbing the previously had been serving a tank type water heater, but I'm not sure how well they would cut it if the regulated pressure where it enters the house plumbing is under 0.3 psi, dropping to some ridiculously low number due to the higher cfm it would need to be pulling through the plumbing. None of the other tankless vendors (that I'm aware of) have this internal compensation feature.

Another common issue when installing a tankless in a house with a big gas furnace and other gas appliances is the total capacity of the meter. Have you counted up the total BTU/hr of everything currently running on gas, and compared it to the capacity on the meter's nameplate? There are lots of houses with 250 cfh meters, and most of those simply can't swap a 40KBTU water heater for a 200KBTU water heater without upgrading meter capacity, no matter how fat the plumbing is inside the house.

 

[Reach4]

Standard residential gas appliances is (as Jadnashua points out) are expecting nominally ~0.5 psi, or 14 water inches, with tolerance for the pressures lower than that. But 7.5" (0.27 psi) would be at the very low end of what they expect.

Around here for natural gas the gas company normally provide 6 or 7 inch water column ( 1/4 PSI) after the regulator.

If you can get the gas company to provide 2 psi out of their regulator, you can use your own regulators at the appliances. You can put a lot more gas through a given pipe at 2 PSI. Run a gas whole-house generator and a tankless WH at the same time with your old pipes.

 

[Dana]

Sounds like there's a "new normal" going on out there(?) I'm not a gas-fitter- maybe I'm speaking a bit out of turn...

Code in my state stipulates: "All gas piping systems shall be low pressure, not in excess of 0.5 P.S.I.G. or 14 inch water column. Systems may exceed these requirements if designed and installed in accordance with 248 CMR 5.05(4)(B)." and many/most systems I see are set to something close to that, not 0.25 psi.

Going to higher pressure than 0.5 psi in my state requires an inspection & review by a certified professional engineer before the utility would be allowed to increase the pressure.

 

[SubnetMask]

Good call on the meter - the plate was painted over, but it looks like it's a 175 CF/hr. Not going to cut it by a long shot, so one way or another, the gas company would have to do an upgrade.

Dana, I'm not sure about appliances expecting 14" - my dryer, for example, specifically states an inlet pressure range of 5.2-10.5 - I can't imagine it would be terribly happy with 14". The Rheem tankless I believe also has a max inlet pressure of 10.5.

With the way the existing piping is run, I don't think it would be possible to run a second feed to the same location without it getting messy. Any way I go, adding another dedicated run will run into the existing piping at some point unless I have it way out from the wall, which would run into other things, or add a bunch of elbows, which you want to keep to a minimum. Replacing the existing main feed with 1 1/4", or even 1 1/2" for a huge capacity cushion, would be much more feasible. The other two appliances are a dual stage 52k/80k BTU Bryant furnace and a 22k BTU Maytag dryer.

To be honest, while it would be a pain, it wouldn't hurt my feelings to replace some parts - all of the inside shutoff valves are impossible to turn by hand - I have to get a big wrench to shut them off, and one is broken off and oriented behind the pipe up against the wall, next to a drain pipe - makes it FUN to get a wrench in there and turn off....

So it looks like no matter what, if we go tankless, I have to do some piping upgrades. I'm kind of thinking that the best approach for a tankless for my environment would be to replace the line from the meter into the house, which is mostly outside (The first 'tee', which goes to teh dryer, is 6" inside the house, and then the 'tee' that splits it to go to the current WH and the furnace is about 4' from there), to 1 1/2" and have the gas company upgrade the meter to be able to service the gas requirements, as well as set the pressure to about 9 W.C. Inches. While I understand that a home run for the WH would be 'ideal', it's just not feasible while keeping everything 'clean'.

 

[Dana]

Mind you, an 80K furnace is 2x or more oversized for 19 out of 20 houses in the US (making the 60K lower stage "only" 1.5x.) , as if people really EXPECTING temperatures south of -50F before the next ice age!

If you want to figure out what your oversize factor is, run these numbers on the January & February gas bills and report back. A 75-100KBTU/hr condensing water heater and a hydronic air handler appropriately sized for the space heating load is usually a better solution than an oversized furnace with a separate water heater.

But gets more complicated if you also have an oversized air conditioner to deal with too...

 

[SubnetMask]

While you're probably right about the furnace being over-sized, it's not something I'm going to worry about at this moment, since I'm not going to replace it because it's too big. Now, if/when it needs to be replaced, that's a different story, and I will certainly keep your comments in mind, but for for now, I need to plan the gas supply based on what I have.The A/C, on the other hand, I'm not sure isn't under-sized - it struggles to keep the house comfortable in the heat of summer, and doesn't do a terribly good job at it while struggling - but then the system may not be balanced right.

Like I said, considering what I have to work with (I tend to be a stickler for keeping things neat/clean - I don't like messy installs - you should see me spaz when I see the typical Dish/DirectTV installs lol - those are ALWAYS a big NOPE - go back and do it again... RIGHT this time... for me - I won't let ANYONE do ANYTHING on/in my house because I know their work is NOT up to my standards), while a home run for the WH would be ideal, I think my best bet is to oversize the supply line to 1 1/2", which on the low side, according to the Rheem charts, can supply 747 CF/Hr at 40' (better to over-size the supply than under size it, and for the few dollars difference in price between 1 1/4" and 1 1/2", if I'm going to be doing it...), and bring the 1 1/2" in until it tees off to the WH and furnace, like my diagram below (please excuse the crudeness of may hand drawing). My crude drawing is actually the way it's piped now - except the feed is 3/4", and the dryer and WH are reduced to 1/2" after a 2" nipple in the 3/4" tee, and the furnace is reduced to 1/2" just before the line enters the furnace (truthfully, no reason I wouldn't pipe it the way it is now once off the 1 1/2" trunk). I would think that supplying everything with 1 1/2" that is reduced for the appropriate input sizes of the appliances would minimize the potential for something firing up 'annoying' the modulation of the WH. I mean, on the lower chart, the 1 1/2" line can supply 747 CF/Hr at 40' - so a lowly 80k BTU furnace - assuming it ever really fires up at 80k (from what I've read they tend to only fire up to stage 2 under certain circumstances, so maybe it never/almost never fires up to 80k) - is only going to need a whole 78 CF/Hr - just over 10% of what that 1 1/2" trunk can supply.And if I have them set the pressure to 9" WC (up from the 7.5 they said was the likely pressure based on their typical residential service) at the same time, which is within the acceptable input range for everything.... I can't imagine I'd have supply issues. If I'm completely off my rocker, please tell me - I've already learned a lot in the last few weeks that I had no idea about - both water heater (maintenance) specific and gas supply related - I'm not against learning some more.

One other related question - Any reason I couldn't have the gas line behind the WH? The way it's run now, which even I upgrade to 1 1/2" supply, the line comes in at my eye level - no matter what, I'd need to mount the WH off the wall to clear the gas line, like I had to do with my central vac. With the WH, I'd have to mount it off the wall for the vent(s) to clear the gas line if I were to mount it below the line - but I'd rather mount it higher for more 'ground clearance'. Truthfully, I can't see it being an issue, considering the nature of the tankless water heaters, and the fact that, as installed years ago, the line to the furnace passes within 1/2" of the exhaust stack for the furnace... And we're talking about black iron piping here, not PEX (or similar gas-rated plastic piping).


View attachment 44786

 

[Dana]

In the 40' measurement, are you including the "equivalent length" (mentioned by jadnashua in the first response) of every tee & ell along the path? See TABLE A2.2. It adds up.

Oversizing the pipe to 1.5" would be wise- not a guaranteed solution, but better than cutting it close. I don't believe there are any code issues with routing the gas plumbing behind the water heater, as long as the manufacturer's clearances to walls etc are met.

There's no incentive for retiring a function hot air furnace early. Unlike oversized cast iron boiler it's not an efficiency issue, but primarily a comfort * issue. Most 2 stage furnaces have a simple time-out algorithm for stepping up to the second stage (sometimes adjustable, some times not.) If the call for heat isn't satisfied within xx minutes it kicks it up a notch. The time it typically on the order of 10-15 minutes. When sized reasonably for the actual load that works out pretty well for using overnight setbacks, shortening the recovery ramp. When oversized it often just gives you a scorcher-finish to a call for heat. One of the guys I work with has an 80K condensing 2-stager that came with his condo, that has about an 18K design load. He found the noisy hot-blast finish on winter mornings pretty annoying, and disconnected a wire in the controls to inhibit it from stepping up. Even at the lower stage it's something like 3x oversized for the load. When it's time for replacement, whether tankless or tank, running a right-sized hydro-air coil off the water heater can work out pretty well.

Duct balancing issues are the bane of ducted HVAC systems, especially when ducts are used for both heating & cooling, since the individual heating loads for the rooms are rarely directly proportional to their cooling loads. Dialed in perfectly for heating can leave some room sweaty, others chilly in summer, and conversely in winter when the ducts are dialed perfectly for the cooling season. Sometimes it's bad enough to be worth tweaking balancing vanes seasonally. Duct leakage can also muddy up the waters, especially if the ducts run outside the thermal & pressure envelope of the house, say in an attic above the insulation layer. A ducts leaking in a vented attic can drive major air infiltration levels whenver the air handler is running, using "the great outdoors" as part of the return path, screwing with the flow balancing issues to boot.

 

[SubnetMask]

So I went back and re-measured the piping, 'more accurately' this time, and from the meter to where it currently splits off to the WH and furnace, there are 6 90 degree turns, including the first turn in the meters manifold, and 35 linear feet of pipe, so that brings my 3/4" run to a 'total length' of 47.36 feet. This is not including the branch for the dryer as that tee is straight through as far as the supply trunk is concerned...

So if I take that and adjust for 1 1/2" pipe, that brings my 'total length' for 1 1/2" pipe to 59 feet. The lowest chart in the Rheem manual lists 60' of 1 1/2" pipe as capable if supplying 600 CF/Hr, still double what I need for the BTUs, and if the pressure being set at 9" W.C. would put it into the third Rheem chart, then it would be capable of 2,085 CF/Hr. I still don't really understand the charts because they're rather vague - they're all 'less than 2PSI', with the third one being 'over 8WC Inches but less than 2PSI' - if having the pressure set to " would land it solidly in 'Table 4', then the 3/4" at 50" would land it at capable of supplying 387 CF/Hr - which may be 100% fine without changing anything but the meter and the supply pressure - but I don't know how the tables work, so that being said, I'd rather over-size the main trunk. It would be nice if they just gave some solid, easy to work with numbers: 'X' static pressure + 'Y' pipe size = 'Z' CF/Hr.

Another side question - while going about all this, regardless of if we scrub the tankless idea or not (Which I'm pretty sure we're not going to at this point), I want to add a whole house filter to address sediment and scaling (I don't want to add a water softener, space of course being one of the concerns). Rheem has one (their 'AllClear Water Treatment System') that they claim 'distorts' the calcium carbonate so that it can't stick to itself, and due to its design (not so much their claim), it would also catch any sediment coming in from the main (there was a decent amount of sand that came out of the bottom of the current WH). But they're proud of it and I don't know if it's meant to be 'whole house' or just for the WH supply. Culligan has something that is marketed as whole house, and supposedly can also protect against scale, but they don't say how. Does anyone have any suggestions for a filter of some kind that can address these if one of those two wouldn't be up to the task? Main requirements would be not too tall - I can't have a 3' tall canister due to where I have to put it (The culligan I believe is 13" tall, which would be ok), and that it be clear.

Another quick question - what's the deal with the 'sediment traps' in the gas line? Is sediment really that big of a deal in natural gas systems? My system has four - one at the first at the first turn after the meter, then one at the last turn before the water heater, furnace and dryer (each, of course). I have no problem putting them in, of course. Just seems odd that they seem to be at practically every turn in the system - as if we were talking about a water well that routinely sucks up tons of sand and you need to deal with it at every turn (should see how much sand comes out of my sisters well!). Really just curious about this one.

 

[SubnetMask]

So I had the gas company come out and scope out the situation and quote a solution, and a workable solution has been figured out. The plan is for them to replace the meter and increase the pressure from the meter to 2PSI, then place a ~400CFH regulator right before the line enters the house to knock it back down to a WC level that is acceptable to the appliances. At that point here would be about three linear feet, maybe four, of 3/4 pipe after the regulator with one 90* turn that would have to carry the ~300CFH for all appliances. The charts I have (from the Rheem manual) place 10' of 3/4" pipe at either ~25CFH under the requirement, 60CFH over or 649 over, depending on which one applies (<2PSI .3" allowable drop, .5" allowable or >8" and 3" allowable drop).

I'm a little concerned that even at the short lengths involved, the capacity of the 3/4" pipe might be borderline. In a perfect world, I'd like to increase those few feet of pipe to 1" while re-doing it, but while the regulator is available with 3/4" in and 1" out, they only carry 3/4" in/out, and if I could even find a (reputable) place (other than fleabay) to get one, I've only seen them for WAY more than they apparently cost from the gas company.

What do you guys think? Will less than five feet of 3/4 be plenty up to the task?

 

[GuySavannah]

I think you'd be pushing it just a bit with less than 5 feet of 3/4... but that's just my opinion without seeing it first hand.

-Guy from savannahwaterheaters.com