Help choosing between condensing tank (newer tech) & tankless (proven tech)?

Which water heater should I choose?

  • Condensing tankLESS style water heater

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MrStop

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I had been considering a condensing powered direct-vent (sealed combustion) tank water heater, but I am wondering if I should go tankless instead? I know the tank versus tankless has been beaten to death. However, does the equation change when the tank unit is substantially more expensive and newer/less proven then the tankless unit (assuming similar installation costs)?

BACKGROUND

As posted on the tank forum, I need to replace an older a slowly leaking water heater. We will be remodeling our home soon which involves tightening up the envelope. Because my water heater is located in the basement, my priority is to remove the combustion intake and exhaust air from the equation for safety and so I don’t need to bring in makeup air. Additionally, I would like to eliminate the flue chase (HVAC is already high efficiency). If I can improve energy efficiency, even better!

I was about to pull the trigger on a HTP Phoenix Light Duty 50 gallon water. It has great FHR, recovery and efficiency specs, it looks like it can be easily vented, and the stainless design should provide a longer service life. My concerns are that, like the Rheem RHE50 and AO Smith Vertex condensing water heaters, the service life is somewhat unproven and there seems to very little real world experience with them (at least in the long term). As the tanked units are considerably more expensive ($2000 for the HTP and $1,500-$1,600 for the others), I’m thinking a tankless unit (<$1,300) seems to be a better gamble (longer history, cheaper to replace if it dies). Additionally, I like the idea of the tankless unit taking smaller space.

I am aware of the idiosyncrasies of tankless water heaters (length of waiting time, cold slugs, dropping pressure, etc). I’m prepared to address this via a buffer tank and recirculating system. However, this adds system complexity and starts to negate some of the tankless benefits.

SITUATION

My house is 2 ½ bathrooms. It is occupied by two adults and two children (14 & 11). We typically shower, but the kids will sometimes take a bath (smaller/standard tub). We have low flow shower heads (2 GPM), a HE washing machine, and newer EnergyStar dishwasher. Plumbing runs are currently under about 20’ and we get near instant water today. I will likely convert to a home run system during our remodel.

Our water is served by the city with sufficient pressure. I have learned that I have hard-ish water (11 gpg) and I will be installing a water softener regardless of type of WH. Water supply into the home is ¾”. I’m in Southwestern Ohio, so I assume my cold month water temp could get down to 40-degrees or more, but I haven’t measured.

The current gas line is 1 ¼”. It runs ~41 feet and has 2-90* elbows where it terminates to supply a furnace (80K BTU), a Gas Grill (38K BTU), Fireplace (30K BTU), and the water heater. If I add in a tankless water heater (199K BTU), my total BTU load is ~347K BTU. Based on the charts, my supply line should handle 364-400K BTU. The meter shows a tag with a rating for 250/195 CFM. The regulator is marked as model B-31.

QUESTIONS

1. Should I go with a condensing tank style water heater, OR, a condensing tankless water heater? My assessment summary:
  • Condensing Tank
    • Pro's: Cold weather performance; More seamless operation without added equipment
    • Con's: More expensive unit; Less “proven”; Unknown life / reliability; Larger footprint
  • Condensing TankLESS
    • Pro's: Less expensive unit; More “proven”; Potentially longer life; Smaller footprint
    • Con's: Cold weather performance; Additional equipment to handle cold slugs / pressure drops
2. I read it is better to home run the gas line close to the regulator. Since my main supply line seems to be of sufficient size, is there a way to isolate the branch to the water heater at the end of the main run with a regulator or other device?

3. If I do go tankless, I think a 199K BTU would be best given my climate. Any preference or experience with the Rheem Series 95? Any concerns over the Richmond equivalent? The Richmond is in stock locally and on sale for about $200 less.
 
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Dana

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With high efficiency (bursty-fill) washer & dishwasher you'll run 2x as many ignition cycles on tankless (and at very low efficiency due to the low volumes per draw) than with other appliances. Real-world efficiency on a "98% efficiency" tankless runs in the mid-70s to low 80s, never more than 85% in anything but a commercial application.

Hands-down, with your family numbers & appliance types, go with the condensing tank. You won't have to swap out the meter or install a fat home-run gas line between the hot water heater and the meter the way you would with a monster-burner like a 199K tankless. HTP is a high quality vendor, with a lot of experience with modulating condensing burner equipment. It's only "light duty" compared to the full-on ruggedness of high volume commercial applications. Polaris is another all-stainless vendor with a track record but without the modulating burner of the HTP water heaters. Either will outrun the Vertex (basically a glass-lined version of the all-stainless Polaris) by at least 2x on lifecycle.

All else being equal I'd probably take the HTP Phoenix over the smallest Polaris (YMMV) but if your price numbers are real the Phoenix is even a bit cheaper than a Polaris (~$2500 for the 34 gallon version).

If you are heating the house with a high efficiency hydronic heating system, the answer is "none of the above". The right thing to do in 9 out of 10 situations would be to install an indirect fired hot water tank operated as a heating zone, with the zone controller giving priority to the hot water tank. "Furnace" to me means "ducted hot air", but there are regional dialect issues or something- it seems some folks from PA & OH always call a boiler a "furnace" too.
 

MrStop

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Thanks Dana.

With high efficiency (bursty-fill) washer & dishwasher you'll run 2x as many ignition cycles on tankless (and at very low efficiency due to the low volumes per draw) than with other appliances.
Would a buffer tank on the tankless help with the short cycling?

I have a ducted forced air high efficiency gas furnace, not a hydronic system. I think I would like a hydronic system for the garage, but that would probably involve removing and repouring the slab.



I have read your other posts in this forum speaking of the gas line requirements for the tankless units. Specifically, the need to run a pipe back to the meter to minimize interactions with the modulating burner. I'm curious about the science / theory behind this. Is it the length of pipe, separation, or the regulator itself? Is there not another way to minimize this without adding another line?

BTW - Are you the Dana D from GBA?
 
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Dana

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I am indeed that Dana.

Very large modulating burners are sensitive to pressure fluctuations, which can affect the temperature control or even cause the sensors to trip an error condition for the embedded controls, which is the primary reason for making it home run. If another big load (such as an 80K furnace) turns on/off on ling branching from the line feeding the tankless it will cause a pressure-wave blip at the tankless, since the regulator can't react to stabilize the blip until the wave reaches the regulator, which may be many feet away on the plumbing path.

If you run it straight to the meter where the primary pressure regulator is, that pressure glitch will hit the regulator VERY near the branch point to the tankless, and the size of the pressure wave that reaches the tankless will never be big enough to cause control or error-condition issues, since it is substantially damped by the regulator.

The size of the plumbing has to go way up due to the very high maximum draw, which can cause the gas pressure to drop too low for optimal operation at high-fire. You can only slurp so fast through a given sized straw, and that's even with uncompressable liquids. With gas you have even bigger pressure drop issues. A 199KBTU/hr burner is about 5-6x the amount of burner needed to heat my house (and probably yours too, but a 2-stage or modulating 80KBTU/hr furnace probably isn't super-overkill.) To serve an 199KBTU/hr burner 50 equivalent-feet away from the regulator requires 1-1/4" gas plumbing:

gas-natural-piping.jpg


But to serve the 76KBTU/hr burner on the HTP you're good to 80 equivalent-feet with only 3/4" gas plumbing.

With either you can't ignore the equivalent feet factor for every ell & tee along the path. Even 50 equivalent-feet can be physically not all that far away from the meter if there are lots of twists & turns along the path, but most installations don't have to go much longer than that.

An HTP Phoenix or Polaris will have plenty of "spare" BTU to heat a garage slab in an insulated garage, but it has to be run through a heat exchanger- it's a real hydronic design problem. If there is even a remote chance of freeze-up in the garage you'd have to use glycol/water solution on the heating side of the heat exchanger.
 

MrStop

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I am indeed that Dana.

I thought so, based on your very detailed answers.:)

Very large modulating burners are sensitive to pressure fluctuations, which can affect the temperature control or even cause the sensors to trip an error condition for the embedded controls, which is the primary reason for making it home run. If another big load (such as an 80K furnace) turns on/off on ling branching from the line feeding the tankless it will cause a pressure-wave blip at the tankless, since the regulator can't react to stabilize the blip until the wave reaches the regulator, which may be many feet away on the plumbing path.

If you run it straight to the meter where the primary pressure regulator is, that pressure glitch will hit the regulator VERY near the branch point to the tankless, and the size of the pressure wave that reaches the tankless will never be big enough to cause control or error-condition issues, since it is substantially damped by the regulator.

Thanks for clarifying.
 

MrStop

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Hopefully my final question so I can get off the ledge. I contacted the local distributor to check on who performs service on the HTP units. They also said service would be "performed by local plumbing company." The distributor is newer and haven't sold any units yet. Also, I can't find any signs of local installers/ servicers. How is HTP's technical support in helping unfamiliar trade persons or homeowners service their units should there be a problem? I'm quite competent at fixing/building anything as long as there is good parts and technical support/literature.
 

Dana

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I have no direct experience with their tech support, but they have active live-chat support through their knowledge base page, which doesn't need a subscription or credentials to use. You might even ask THEM (on their chat line) prior to buying whether they would talk you through debugging & repair should anything go south on it, given that the distributor is so new that they have yet to sell a single unit.

If you ask, report back what they say- I'm curious!

They also have contractor forum page which may be useful for figuring things out. Like any other web forum you'd have to register as a user to post questions or answers.
 

MrStop

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I can't say that I have been or was too impressed with the answers from HTP's live chat-support. They suggested that I contact their technical support department. I didn't really want to get into that.

However, I did look at the forum and they have a great set of troubleshooting videos. As a worst case between that and the manual, I should be able to troubleshoot. This thing looks like a breeze to work on as everything is totally accessible.

The Phoenix arrived at my house today, here's some unboxing pics I took: https://terrylove.com/forums/index....d-direct-vent-water-heater.59061/#post-439185
 
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