Recirc system with tankless?

[KidCanada]

Hello all,

Brand new member here although I have been lurking on the sidelines for a few weeks - trying to educate myself about tankless technology and the options available. Reading this forum has been most informative and it certainly appears that there are two distinct camps on this technology.

I have been intrigued by tankless in the past but have always written it off as a solution for my house b/c it hasn't been compatible with a recirc system - up until now? If I read correctly, it sounds like some of the new Navien tanks may be compatible with a recirc system. If this is the case, then my interest is piqued.

Here is my situation: Big (4000 square ft) old (100+ years) drafty, wood house on Vancouver Island BC. We currently have a gas fired, direct vent 50 gal HW tank which is roughly 12 years old and probably nearing the end of its life cycle. We are pretty water conscious so it has always met our HW needs and we almost never run out of HW. I am looking to replace it with something more efficient (I am amazed at the amount of wasted heat that comes out the exterior vent!). Looks like a tankless setup would cost about the same as a new gas direct vent setup (status quo for us) so that is a non-issue. We also have a huge gas line coming into the house so I don't think that is an issue either. Similarly, we are blessed with very soft water here on Van Isle so the scaling issue would also be minimal. As you can see, lots of the typical tankless dealbreakers don't apply in my situation. Except for the recirc thing - up until now?

I installed a recirc system when doing some bathroom renovations on the 2nd floor of our house a couple of years ago and it has worked like a charm. Because it is a big house with long runs, there was an amazing amount of water wasted every morning before the first shower got hot (pre-recrc pump). Now, with the pump, I have it on a timer which only runs it for 1 hr in the morning and 2 hours in the evening which is the only time HW is needed up there. Instant HW when needed with no wasted water - awesome! Now the question is : ' can I maintain that setup (and it's advantages) with a tankless HWH?'
Furthermore, correct me if I am wrong, but that would also solve the minimum flow issue with the high efficiency clothes and dish washers (both of which we have).

I would love to get some feedback from you plumbing gurus out there on this topic. Thanks in advance....
KidCanada

[Dana]

A recirc would guarantee that the short bursts by the washer would be hot but it would still be short-cycling the tankless, so the heater's efficiency during those fills would still be at most 60%. That's not exactly a problem, since the volumes on high-efficiency washers are small. A 20-30% hit in efficiency on a small draw won't be easy to measure on the fuel bill- it's the big volumes like tub-fills and showers that make the largest difference.

Takagi's have been used with recircs for years- they recommend limiting the recirculation flows to ~ 2gpm flows to minimize wear on the heater. But since you're not running it 24 hours/day it won't much affect the longevity for you even if it's 6-8gpm. If the total volume in the loop is less than 5 gallons it's a bit of an efficiency hit on cold starts but not a huge one during maintenance burns, and would not have a measurable effect on the fuel bill (it's only measurable in the lab, but it's something to note if max-efficiency is your stated goal.)

If you insulate the loop to R4+ (3/4" wall or more closed cell foam) odds are good the recirculator will only run once per on-period at the beginning, and only rarely as a temperature maintenance burn.

If the difference in installed cost between an 0.82-0.84EF Noritz/Rinnai/Takagi and a 90%+ condensing unit is $1000 or more, if your family normally showers rather than takes tub baths you will get a better return on investment (and use less fuel) installing a drainwater heat recovery system that returns 50%+ of the drainwater heat to the incoming cold water. (Not always easy to install, if the heater isn't already close to a drain downstream of the showers.) Natural Resources Canada maintains a list of third-party tested drainwater heat exchangers by effieciency here. There may be provincial & federal subsidy for installing one too. (I'm not sure what B.C. offers relative to other provinces.) Drainwater heat recovery extends the apparent capacity of tanks during showers too- it's like having another ~30KBTU of burner (that uses no fuel). It does nothing for tub fills however- the drain and potable have to flow simultaneously for the heat exchange to take place.

There's more to moving from a 50gallon tank to a tankless than the cost of the unit- in many/most installations the gas plumbing has to be upgraded to support the much larger burner, and for non-condensing units the cost of venting can be substantial if it has to go any distance. Get some quotes.

[Scott D. Plumber]

I Agree with Dana that recirc can (and does short cycle tankless water heaters adn that is one big reason why in most cases the warranty suffers if you pump through it. That said, an "on Demand" recirc system like the ACT Metlund d'mand system meets all the needs without hurting the warranty or giving you a big energy hit for running a recirc system when you don't need it. www.gothotwater.com. THey also now have a expansion tank that also acts as a small storage tank that will buffer the "cold water sandwich" effect that can happen when one takes a consecutive short draws from a tankless water heater. All major models recognize this system and it is the only one I know that DOES NOT take the warranty to 3 years. As for Navien, yeah, they have that stuff built in however they have too many temperature control issues, etc for me to be a fan. I'd stick to the big 3 Rinnai, Noritz Takagi and use a metlund pump.

Condensing is another animal. Don't get too hung up on it for tankless. They are more efficient in the long run, however, if you have a car in the driveway that gets 40mpg and one next to it that gets 46mpg and they are both "off" and only run for about an hour a day, how big a difference is there really?

Is it better, yes! Is it worth the cost difference? Maybe, depends on the customer's point of view.