Tankless water heater plus Tank?

Jack

I prefer the heat-flo tank for this application as it has four tappings. Cold in, hot out, pump suction to the Rinnai and hot into the tank. As well, it has better spacing of the tapings. You don't need an indirect. Pay attention to your pump sizing. Feed the cold first to the tank to prevent pressure drop. Give a big enough spread (20f)to your aquastat that the system can satisfy. You can mount the Rinnai (I represent them so I'm a tad biased) above the tank for efficient use of space.

peter_19

is that an oxymoron?


It seems like a silly combination, but does anyone have experience with combining a tankless water heater with a well insulated storage tank? I was looking at getting a Polaris or Phoenix water heater, but it sounds like they may have some serious quality/reliability issues. You could get a Takagi TK3 (about 84% efficiency I think) and a tank for less than half the cost of a Polaris. If you didn't want to give up the efficiency of the burner, you could use a TH1 instead (92% efficient). I think that would cost about the same as a Polaris, but might be a more reliable and last longer (well, the tank would probably need to be replaced, but I think Takagi appliances have a pretty long lifespan?).

Unknown

Ever considered one of these?

A water heater is a water heater,,, but this is obvious,,,,

Dave

Rich Kontny_3

THat

Looks like a sweet unit!

peter_19

Yeah, a mod/con boiler plus an indirect tank is definitely an option. That is a very cool unit, but I don't think it will provide the DHW capacity that I want. But a SOLO 175 plus a SMART 40 at 140* would probably do the trick. But I think that is a bit more expensive than a Polaris, and much, much more expensive than a Takagi TK3 + Tank (although that would not be a condensing unit, so you lose some efficiency). I don't think the efficiency gain is enough to offset the increased initial cost, though. Environmentally its better, but if you are going to put the environment ahead of cost then why not solar? I'm not ready for that (I'm not made of money), but I think the Takagi + Tank is well suited to adding in solar later on.

My other concern with a mod/con plus indirect is related to short cycling. To get the DHW service I want I need a decent sized boiler (the bigger the better for the DHW). But I live in the SF Bay Area and my RFH will often only be used to warm up the one or two rooms that get no sunlight, so I expect some very light loads from the RFH. So I think a tank acts a nice buffer to prevent that. But why use a mod/con boiler plus buffer tank, plus a second indirect tank, when I could just use a good water heater and have only one tank?

Mark Biro

Fine Homebuilding had a 4-page piece

Fine Homebuilding had a 4-page piece in the last several months (sorry, I cannot give chapter and verse) about adding a small electric water heater as buffer to an instant heater.

It caught my eye because a buddy had just mentioned the shortcomings of his Bosch... some cold slugs, inability to do real low flow, for real short periods...

It was suggestive. But became kinda complex after a while. And lacked, I thought, the full substantiation that it would have gotten on The Wall, for instance. ;-)

FWIW!

peter_19

Jack


You say you have done a bunch of these... have these all been combined with a radiant heat application? If it was only for DHW, why would you use a tank? Don't your customers question why they should pay for a tankless water heater and a tank, when they could just get regular water heater for much lower cost? What size tank do you usually use and why? Thanks for your response... I'm glad to hear someone else has had success with this seemingly crazy idea!

Jack

Ah, space heat...

The tankless/tank combos I've used have been for commercial hot water for special design circumstances. I like the tankless hot water heater for hot water. I think that for space heating you are better with a boiler.

Rich Kontny_3

Peter

For the most part the on demand tankless heaters are pretty much the same except Navian has a separate unit that works to compliment the tankless by using their "Heating Box" add-on. If you have good water quality,constant water pressure (or city water) and a heat load of 80,000 BTU's or less this combination system may be the most cost effective for you.

Since Navien innovated the "Heating Box" and had the first condensing on demand tankless I am sure the reps and sales people from the other larger tankless companies(Bosch,Takagi,Poloma and Rinnai) are not going to promote Navien as they are playing catchup with the one heat source does both concept.

I am non biased as I am a contractor and have no ties to any manufacturer. I have, however done my research with Navien to the point of installing two combination systems.
One is in my house and one is in a large new home.

The small electric tank is in response to our DHW systems that tend to have long runs that need the water voided to get the on-demand hot water. The old recirc approach caused short cycling when tied directly back into the tankless. The small electric buffer tank keeps the hot water in the system close to desired temp so you don't have to wait for the tankless hot water to get there. Once sufficient flow is met to kick in the tankles it(the tankless) takes over and you enjoy the benefits of it.

For some reason I can never get my photos to work with Dan's attachment feature. If you want to e-mail me direct I will send photos to you.

ScottRW

Rich

I'm interested in your photos, been thinking of this for my own house. I have more questions then answers though. If you could make them small files, I'd appreciate it. I'm out in the woods on a dial up.
Thanks in advance, Scott

peter_19

Yeah space heat is a complication... pretend I didn't even mention that. Just focusing on DHW, what do you see as the advantage of a tankless + tank versus a tank water heater? I imagine your customers must ask why they should want to use this combination instead of a "traditional" water heater. What do you tell them?

jackchips_2

jackchips_2

Rich Kontny_3

Pictures

If you want to e-mail me direct I can get you the pictures. For some reason I have trouble posting pictures on the wall w/Dan's attachment feature.

I have a IT friend who is going to look into this but until I find time and he does I have to do it this way.

Rich K.

kpc_17

well...

>one advantage is because there is no vent through the storage there is no/little heatloss.

>Because there is a large engine driving the tank up to 200K you can go w/ a smaller tank to recover quick.

> and because you have a tank there are times where on a huge demand you give the tankless a head start...under some demands you would have to go w/ multiple heaters...in that case you could go w/ only one heater and a larger tank.

Now the bronze circ that would need to drive thhat would need to be substantial..Taco 0010 or bigger...that would add to the cost.
my thoughts....kpc

Rich Kontny_3

The Resource Tab

Go to the resource tab above and right click, then scroll down to "Hot Tech Topics"once there go to Instanteneous Water Heaters as Boilers. Dan posted some pictures and an article there last year.

Eric Tardif

Venting

How do you vent these if there's no place to vent it through the wall? Could I use a chimney with 6" stainless steal lining? Or will the chimney ruin the efficiency?