Rinnai On Demand Water Heater: Huh?

[mgbinspect]

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Got a question about this Rinnai Continuum 2902. I think I know the answer myself, but I want to make certain that my assumption is correct.

I've seen plenty of Rannai on-demand water heaters, but I noticed while outside near the exhaust that the unit runs often and rather oddly. It fires sporadically and whirs during operation and then abruptly shuts down again. I noticed a pump on the hot distribution line at the opposite end of the home in the crawlspace. So, I gathering that all this commotion is by design and keeps the water in the distribution system to a preset temp?

I had a hard time deciding, based upon the way the thing so abruptly would fire up and shut down, whether is was doing what it's supposed to do, or had a problem.

Anyone familiar with this unit?

[Nolan Kienitz]

I think I read somewhere in an installation guide for a couple of the on-demand units that they are not approved for use with the pumps to provide instant-on hot/warm water at the far reaches of the home.

Don't recall the brands at this time.

[mgbinspect]

Well, that's kinda why I posted. I've seen a lot of Rannais, but I've never heard one carry on like this one did. It fired up with an intermittent whirring noise that whirs up and then falls down, cycling a few times like that and then the thing cuts off. Then it might do it again a few moments later in a slightly different pattern. Then, cut off altogether for twenty minutes. The house was vacant and no one was running the hot water. Pretty weird.

[Richard Moore]

This is a line from a Rinnai trouble-shooting guide...

"Check for low water flow in a circulating system causing short-cycling."

I would guess from that that a circulating system is an option. Could there perhaps be air in the hot water lines of this vacant house causing erratic circ pump operation which, in turn, might trigger the water flow sensor at the unit? That's kind of a rhetorical question. Report it as behaving in a manner unfamiliar to you (or something like that) and recommend they have a Rinnai service tech check it out.

[mgbinspect]

Thanks Richard!

I just didn't know what to expect. I thought it entirely possible someone might just post, "Oh yeah, that's completely normal for this unit." One never knows until they ask...

I don't believe the little unin on the opposite end of the home was by Rinnai, and can't even say for certain that it was a cirulating pump. It could have been a sensing unit. There were a lot of firsts on this beast of a home.

[Jim Katen]

When you shut off the circulating pump, what did the water heater do then?

- Jim Katen, Oregon

[mgbinspect]

When you shut off the circulating pump, what did the water heater do then?

- Jim Katen, Oregon

I'm afraid I didn't shut it off, Jim. It was a curious looking affair, also unfamiliar to me - elongated and the pipe went right through the center of it (I'm used to seeing little pumps exactly like you see on boilers). In fact, it was overhead when I entered the crawlspace (black) and upon seeing it and making the assumption that it was probably a pump to maintain a preset temp for the hot water distribution system, I reached up and felt the pipe, but it really didn't even feel warm.

[Nolan Kienitz]

Here's a retro-fit style of water re-circulation system that adds a device at the remote fixture. May have been the additional item you saw.

Grundfos - Water Recirculation System (retro)

WATTS - Recirculation System

Device at remote fixture

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[fyrmnk]

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Got a question about this Rinnai Continuum 2902. I think I know the answer myself, but I want to make certain that my assumption is correct.

I've seen plenty of Rannai on-demand water heaters, but I noticed while outside near the exhaust that the unit runs often and rather oddly. It fires sporadically and whirs during operation and then abruptly shuts down again. I noticed a pump on the hot distribution line at the opposite end of the home in the crawlspace. So, I gathering that all this commotion is by design and keeps the water in the distribution system to a preset temp?

I had a hard time deciding, based upon the way the thing so abruptly would fire up and shut down, whether is was doing what it's supposed to do, or had a problem.

Anyone familiar with this unit?

Mine is not that brand, but it does that briefly one time when I turn on my main shutoff (I turn it off when going out of town, etc.), briefly tripping the sensor in the unit making it think water is flowing.

Maybe the pump is cycling on and off, causing it to do this with pressure surges?

[mgbinspect]

I'll tell you that it really almost more sounds like an intermittent variable speed draft inducing blower. That's what it sounds like. The noise is heard most clearly outside at the exhaust pipe or when standing right in front of the unit. The whir is almost like the high pitch you hear in a diesel truck engine (faint though) and it's very variable - not consistent. I'm used to a draft inducing blower just coming on full blast. That's why I wondered if it was some new technology. It's altogether foreign to me. I've seen many of the Rinnai units, but none sounded like this one. It's peculiar - a lot going on at once in there. Nothing sounds abbrassive as if anything's being damaged. Beats me?

[fyrmnk]

I'll tell you that it really almost more sounds like an intermittent variable speed draft inducing blower. That's what it sounds like. The noise is heard most clearly outside at the exhaust pipe or when standing right in front of the unit. The whir is almost like the high pitch you hear in a diesel truck engine (faint though) and it's very variable - not consistent. I'm used to a draft inducing blower just coming on full blast. That's why I wondered if it was some new technology. It's altogether foreign to me. I've seen many of the Rinnai units, but none sounded like this one. It's peculiar - a lot going on at once in there. Nothing sounds abbrassive as if anything's being damaged. Beats me?

Mine sounds the same way when it kicks on.

[inspector57]

IMHO that is a screwed up system which totally defeats the intention of the tank-less heater. The two systems are incompatible and I would think soon wear out the tank-less heater mechanicals due to incessant firing and shutting down just to keep the water hot in the pipes.

The energy savings of tank-less heaters is by eliminating the standby losses from an insulated tank full of hot water 24/7.

Instead, they are using a pump to circulate hot water through likely un-insulated pipes that will be reheated hundreds of times before it is ever used.

I have heard of using the tank-less with a small tank downstream that keeps the instant hot water part going without firing the main heater so that you have unlimited hot water that is instantly available. Some manufacturers I believer are offering a type of hybrid for this scenario.

I would be raising a flag to my clients for this type of installation. At the least, the pump could be on a timer.

[Tom Raymond]

Jim is right, if they want instant hot water there should be a tankles a dedicated unit installed close to that bath.

[Richard Saunders]

Most tankless that I see that have a recirc pump also have a 6 gallon electric water heater that the pump draws from. That eliminates the cycling of the tankless.

[charlieb]

I keep the installation manuals for on demand WH in a file on the laptop. It has its own nuances. Both of these diagrams are in the Rinnai manuals I have saved. I do not have one for the continuum and did not feel like looking it up.

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[mgbinspect]

It turns out that the unit that I passed off, at a quick glance, as a circulation pump was actually a Sterilight UV water sterilizier, so WWI there is not circulation pump on the system, which lends no less clarity to all the racket this water heater makes.

[Plumber26]

When the Rinnai is operating the blower does vary in speed depending on water flow, inlet water temperature, and amount of gas being used. Its is a very efficient unit in that it will use any where from 15k - 199k BTU's. Basically, it only uses the amount of gas needed to heat the amount of water traveling through it from the inlet temperature to the set temperature on the thermostat.

Now the noises coming from the unit if it were not in use is very strange and should not happen unless somewhere in the water system their is a pump. Did you notice whether or not they had a hydronic air handler that may have been connected to this unit? Rinnai does make air handling equiptment and the tankless water heater can be used w/ existing hydronic air handling equiptment of a different brand. (tankless version of Apollo, Polaris, or Lennox CompleteHeat set-up)

[Plumber26]

Another thing to note is to me that looks like 1/2'' gas piping entering the unit. I would recommend having installation instructions checked b/c gas piping may be undersized. Most Rinnai gas conections are 3/4'' but that does look like an older model.