Recommended tankless hot water heater configuration
I’m renovating and planning to install a tankless water heater, and am wondering if the advice given by Michael Chandler (article in FHB #192 – Why Add a Tank to a Tankless Water Heater? – https://www.finehomebuilding.com/how-to/articles/add-a-tank-and-pump-to-a-tankless-water-heater-to-save-energy.aspx) still holds with the newer condensing units.
I’m not looking for financial savings necessarily – however, I would like to reduce standby losses because we tend to be absent from the house for extended periods. I also like the convenience of “endless” hot water. “Instant” hot water and water conservation are not overriding concerns for me; I don’t need a recirculating pump. (I’m installing a pure home run PEX system, so a single loop is not really possible or effective, and multiple pumps – one for each fixture – is cost prohibitive. If I were doing a hybrid system, it might be doable, but I’ve rejected that route.)
I would like to avoid the problems described in the article. It seems that the Quietside water heaters have the equivalent of a tempering valve in them (although I don’t I would rely on it solely to ensure no scalding water), and the Navien NRxxxA models have a small tank built-in to mitigate the cold water slug, but I’m leary of the manufacturers’ claims. The dealer/contractor I’m working with installs Quietside (Samsung) units, and he mentioned that he doesn’t think any auxiliary tank is necessary.
I think that the small tank unit would help ensure that even low flow / trickle streams of hot water are possible, as well as fixing the cold water sandwich issue. (And, yes, I’ll probably insulate the hot water lines, even though they’re PEX, which generally retains heat better than copper.) And I understand that the pump helps with the demand / pressure issue. However, the diagram in the article (labeled “The fix – problem #2”) doesn’t make a lot of sense to me – I see three water connections to the tank water heater – 2 input (1 hot and 1 cold) and 1 output (hot), and I’m only familiar with 2 ports on a water heater.
We live in upstate NY in a condo unit, and the water supply line is shared (no separate meter / supply for each unit). It doesn’t seem that big (maybe 1″ diameter?) – so I’m concerned that pressure may be an issue… am I thinking correctly here? (I’m thinking that getting a true static pressure reading is not really possible without coordination from all the other condo units in the building, and the dynamic pressure may vary a lot, depending on everyone’s usage habits. My experience is that everything in these units were undersized for cost cutting reasons – minimal insulation, 100A electric service, sharing of plumbing supply and drain, shared vents, etc. So it wouldn’t surprise me if the water supply is considered “anemic”.) Also, the incoming water is reportedly ~40-45 degrees F in winter, so I’m expecting a temperature delta of ~75 degrees to reach “standard” 120 degree F hot water. (I believe our water temperature may vary by season more than for many regions of the country, however, since our water is pulled from Lake Ontario.) The dealer/contractor is recommending a Quietside ODW120A (120K BTU), which, according to the sales brochure can provide 2.9 gpm at a 75 degree rise. I’m of the opinion that that’s a bit small – we’re looking to install a large (2.5 gpm) showerhead, so that doesn’t leave us with much headroom – turn on the dishwasher and/or clothes washer accidently while someone is showering (or perhaps even opening a faucet on full hot), and the demand easily outstrips supply capacity, temperature-wise. (I’m not certain the available pressure / volume would be limiting here, but I’m *hoping* not because the “law of averages” and the fact that the supply must handle the needs of the entire building / 8 units.) So I was thinking about going with the ODW180A (180K BTU, rated for a 4.3gpm at a 75 degree rise) – is there any real downside, other than unit cost, for installing a larger unit? That would change the low end of the heater’s modulation range from 18K BTU/hr to 26K BTU/hr, so perhaps there would be some energy inefficiency when there’s demand for water requiring low temperature deltas? (Would the thermostat setting and pump configuration described in the article mitigate this – basically ensuring that the tankless always turns on at “full on”?) I am concerned that an undersized unit would cause the pump and tankless to remain on longer before the “steady state / temp” is reached (non-optimal), and the hot water requirements may still not be met.
Has anybody out there actually installed tankless + tank configurations as described in the article? (If so, which tank unit did you use, and was rewiring the thermostat as described in the article difficult? How did the configuration hold up for maintaining the “steady state” – was it difficult to setup and maintain?) What has been your experience?
Finally, I understand that the tankless unit needs an input filter and isolation valve kit to facilitate servicing. I am also under the impression that the tank unit should be periodically flushed – so should there be a “bypass” (controlled by valves) to allow the cold water supply to flow directly into the tank for flushing, without moving water through the tankless unit?
BTW – I’m not really considering a storage tank unit (sans on-demand heater) as a solution. I looked at a 40 gallon power vent unit as an option, but that would limit the available hot water (and require recovery time), and it was still expensive (albeit not nearly as expensive as a tankless + tank setup). Traditional vented gas tank units are not an option, as I want to get away from using the chimney flue (which is shared between adjacent units) – I’ll simply have them cap/seal the flue input from my side, when they install the water heater and direct vent furnace.
Sorry for the wierd username – I wanted to use “youngmj”, but can’t seem to create or edit my forum profile (“page not found” error); thanks for bearing with a newbie. I also apologize for the long post and thank you in advance for any feedback – there’s a lot to discuss and consider with these things!
Replies
I see three water connections to the tank water heater - 2 input (1 hot and 1 cold) and 1 output (hot), and I'm only familiar with 2 ports on a water heater.
What's commonly done is to add a tee at the drain valve opening.
Water Heater Configuration
Hmm... a tee - but at the drain valve? That's typically an output (commonly used to flush the tank), and the illustration clearly indicates water flowing in at two places - and one input is hot water from the tankless heater and the other is cold water from the supply source! I unfortunately still cannot comprehend how the system described and illustrated can actually work... I guess it may be time to consult a local plumber. (My concern is that many plumbers here are really ignorant of such hot water systems, and so push what they know as the solution rather than what is best for the situation.)
I connected a pressure guage onto the cold water bib for the washer, and it appears that the static pressure is ~65 psi. (At least, I took the measurement in the middle of the night, when hopefully all my neighbors' water usage was zero.) At that point, the line has been reduced to 3/8" copper, but I don't think that the pipe size should matter for a static reading. I believe that's a pretty healthy pressure from the municipal water system / pressure regulator, so what will matter is if there's still enough (dynamic) pressure at PDL, when everyone is using their water fixtures.
Thanks again for the insight!
tankless water heater
contact stiebel eltron.com and talk to the experts
professional advice cannot hurt
Just another water heater manufacturer?
I visited the website - http://www.stiebel-eltron.com
They are a German company that manufactures direct and storage (cylinder) water heater systems. While it is true that the US has less experience than most European and Asian coutries, I'm not sure that contacting a manufacturer (US based or otherwise) is necessarily going to answer my installation / configuration questions. I could as easily contact Samsung/Quietside (or Takagi, Noritz, Navien, etc.), and probably get answers that are no better or worse, or more or less relevant to my particular situation.
I didn't see anywhere on their site that suggested they offered installation consulting. Maybe I missed it?
I'm not meaning to be rude, but I am trying to sort through manufacturer claims and hype, and looking for practical installer experience - particularly an understanding of the reduced pressure problem and solution described by M. Chandler in the article. There may be other solutions out there, but the product brochures don't seem to address this issue (at least, not directly, in a way I can recognize / understand).