instantaneous water heater as boiler?

Constantin

... but many instantaneous water heaters are not approved for heating use. As best as I can tell, the Rinnai is only meant to heat potable water in the US. Why expose yourself to liability and/or warranty coverage issues?

... the Rinnai does not appear to be a condensing boiler. Given where natural gas prices are in most parts of the country, a 10%+ difference in fuel economy over the life of an appliance might make a big difference.

... the 10:1 modulation range that the gas valve offers is impressive, as is the possibility of installing the entire unit outdoors... I also suppose you can work around the minimum flow requirements (0.6GPM).

zac_2

The company I work for just hired a new guy who seems pretty experienced in hydronics (he's a limey who used to work for british gas) but he's bent on installing Rinnai instantaneous water heaters as a wall-hung boiler. He convinced my boss that it's an economical alternative to Lunas, Vitodens, etc. etc. ("cheap as chips" he calls them). However, I am very sceptical with this practice (it just seems wrong to me) but he always comes up with a good argument in their favor. so, is this really bad, and why?

Scott Gregg

Not a new idea

This is something that is gaining popularity here too. These tankless heaters are a cheap BTU. Using them as a boiler usually shortens there warranty but with the cost difference you could probably replace it 2:1 over a boiler and with the energy savings of the modulation still be way ahead of the game.

I have a 100-year-old farmhouse that I renovated. It had no space for anything but an air handler under the stairs. I used a Peerless Pavilion HVAC system with a hydronic coil and have a Noritz tankless (Mounted outside the house) providing hot water to the coil for heat AND unlimited domestic hot water to the house!

It solved a couple of problems (No space for water heater or boiler) and saved me money (a lot of money) on equipment. I got hydro-air heat and spent less on my up front costs.

No they are not boilers, but the darn things work. If you have the room use a boiler, if you need the cost savings up front or don't have the space, use the tankless. It will be fine.

These tankless companies know that many of their products are being used as boilers and some (Like Noritz) are even providing piping and wiring diagrams on their websites now to help installers put them in so they work well. I am seeing them used for a lot of small radiant jobs and combo HVAC/DHW jobs like mine. There is even a high-end condo project going up RIGHT NOW in Washington DC using Direct Vent tankless heaters at 236,000BTUs to provide hydro-air heat and DHW to the units (78 of them using Noritz N-084M-DV)

There is even a rumor that Rennai is bringing a model to the US market soon with the word “boiler” on it.

This could be the end of the munchkin type boiler and paying through the nose for a modulating low-mass boiler. Sound kooky? Ever hear of Patterson Kelly Thermific boilers? How about RBI, Camus, Lochinvar, or Aerco just to name a few. These are COPPER finned tube boilers used commercially with great success for years in the US. Their construction is not that different from these tankless heaters. Copper heat exchangers with fire under them. Some condense, some don’t but they all modulate.

It is a very exciting time to be in the hydronics business.

Constantin

Allow me to disagree...

I don't think it's the condensing boilers that have the most to fear from these units. IMO, it's the traditional CI systems on the gas side that have the most to fear from non-condensing, small, tankless boilers.

However, if the strengths and weaknesses of tankless units are not respected, you can find yourself in a world of trouble. According to Mr. Bean, getting such high-head boilers to work well in most domestic situations has allegedly allowed a number of hydronic companies to open up new lines of business, i.e. the remediation of badly installed distribution systems when used in conjunction with instant water heaters due to the high-head losses in the HX and other issues.

Installing a boiler outdoors is nothing new, and could be popular/viable anywhere else where deep freezes are not to be expected. Up here, where we have had -15°F for a week, I wouldn't put my primary means of heating outside the shell.

PM_2

delta T

Does anyone else worry about designing a system around a 20 degF delta T and using a heat source that is designed for a 50+ degF delta T? What is the head loss at these much higher flow rates (same BTU's, less delta T, must mean more flow)?

jwade55_3

Just went thru this.

With a customer that I was providing the circulation for. Had the bright idea to use a Rianni for a boiler, when he proposed this to Rianni, he found that he needed a brazed plate HX, the system is cast iron rads, and the manufacturer is concerned with any iron filings attaching to internal of their heater. After the contractor realized he would not only need a Hx, but another circulator ex tank and air separator, the "low cost" of the system kinda went away, solution was a T-50 munchkin wall hung, installed cost was a wash.

Scott Gregg

Not for CI rad jobs

I agree that tankles are not good for CI rads or old steel jobs but for new PEX and copper jobs you don't need the HX.

However, I know where there is one in Baltimore where there replaced 3 boilers with 6 of these tankless heaters and did not use a HX. (1960's building, steel piping CI rads) Guess what...it worked all last winter without even cleaning the filters to the heaters!

I would have never recomended this, but heck, it's done now and working.

Blackoakbob

Instant problems......

I still stop in my tracks when I see these wall hung water heaters. The nightmares come back of a Paloma Pak unit that was touted as "the newest thing is compact boilers". It was the most misserable piece of junk that I have ever put in! And believe me, I've sold some real crap that they made in the 80"s. That unit cost me alot of money and a very patient customer. I just turned down a job that wanted to use an instantaneous water heater. I just got so busy, the leaves are changing and the fall color is worth a trip north. Best Regards

Alan(CaliforniaRadiant)Forbes

We used

a Rinnai on a commerical job in 2003; radiant downtairs, Runtal radiators upstairs. The customer wanted the cheapest source of heat and I must say, the Rinnai fit the bill and has been working flawlessly. And if you can use the cabinet and get it in an outside wall like the one pictured, there's no flue work. It's also extremely quiet.

The only drawback is that you need a high head primary pump to flow through the restrictive HX (6 gpm @ 30' head).

zac_2

hmmm, some things to think about. I've always preferred the Baxi Luna for a wall hung boiler, you save a lot of installation time thus money as most of the piping and devices are contained in the boiler itself. I think the cost incl. labour would work out the same as a Rinnai. I suppose only time will tell in regards to the longevity and the reliability of these units though. Thanks for the advice!