Indirect Hot Water Heater - Can Heat Exchanger Fail?

marcg

We bought a house with a HPT SSU-60 indirect hot water heater (label indicates manufactured in 2013), supplied by an oil-fired water boiler. The problem is that the tank is taking a long time to come up to temperature (> 2 hours) and runs out far more quickly than we'd expect. The boiler is at 180F and the tank set point is at 140F. According to the manual, with these temperatures, we should have 266 gallons in the first hour, and > 200 gallons per hour continuous, but we can easily run out over the course of two 10 minute showers.

I think the problem is that hot water circulating from the boiler is not effectively exchanging heat with the water in the tank, but I'm not sure if this is a possible failure mode. Here's where I'm at:

1) The boiler is working and holding temperature between 170 and 180 F, verified by the reading on the boiler temperature gauge, and the fact that the outlet from the boiler is hot.

2) There is one circulator for household heat and a second for the indirect tank. The controller is set up to give priority to the indirect tank. The indirect circulator is working (vibrates softly; I can hear water flowing in through the indirect tank; the copper supply from and the return to the boiler are hot at the tank).

3) The thermostat on the tank (aquastat?) is working. When I disconnect it, the circulator stops running and the sound of water through the tank stops. When I reconnect the thermostat, the circulator starts again and you can hear water running through the pipes at the tank. When the tank is above 120F (measured both by drawing water at the outlet and from a nearby sink - we do not have a thermostatic mixing valve), turning the thermostat down to 120 stops the circulator and the water noise; restoring the setpoint to 140 restarts the circulator and the flow noise.

4) Hot water is entering the heat exchanger -- the supply pipe is burning hot to the touch.

5) Hot water is exiting the heat exchanger -- the return pipe is equally burning hot to the touch.

Given all of this, I think the thermostat is appropriately calling for hot water to circulate through the heat exchanger in the tank, and the boiler and circulator are supplying this water. But somehow the household water in the tank is not absorbing heat from the exchanger at the rate I'd anticipate. If the problem were that the boiler is underpowered or the circulator is failing, then I'd expect the return to be cold, but it's as hot as the supply.

Is it possible for a heat exchanger to fail? It's not leaking - it's just not heating the water in the tank efficiently. Am I missing something obvious?

Thanks.

kcopp

Pretty good bet the heat exchanger coil is scaled up w/ lime/ calcium from hard water.
You can try to descale it but that may be a trick to get it back to what it was,

marcg

Thanks @kcopp. Looking around on this forum and others, it seems that the best bet is to try to thermally shock the scaling off, and if that doesn't work, then to try circulating CLR on it.

https://forum.heatinghelp.com/discussion/70122/cleaning-indirect-water-heater

https://forum.heatinghelp.com/discussion/163293/recommendations-for-ss-indirect-descaler

https://www.doityourself.com/forum/boilers-home-heating-steam-hot-water-systems/442749-indirect-dwh-coil-cleaning-procedure-restore-water-heating-effeciency.html

mattmia2

You should probably measure it with a thermometer. Is the inlet plumbed to the tank at the bottom and the outlet at the top? Did you measure the outlet temp of the tank? Without a mixing valve if the outlet of the tank drops 10 degrees your shower will feel cold. Can you turn down the cold water in the shower and have it stay warm?

hot_rod

How hard is your water? That tank may have a finned type coils and they tend to get plugged with scale
It depends on how hard, how much, and how hot you run them for mineral precipitation.

I’ve heard of shock methods, drain the tank, heat the coil, hit it with cold water.

It takes a lot of acid to clean a coil that has had that many years of build up around the outside.

HVACNUT

What circulator is used for the indirect? The SSU-60 specs 10 GPM at 7.9 ft. head. Is the supply and return full 1 inch copper? It doesn't reduce anywhere?

What's the ratings of the boiler? The numbers you're going by are based on a gross boiler output of 178K BTU's. That's a lot of boiler. 

What GPH nozzle at what pump pressure as compared to the BTU input of the boiler? If the boiler is under fired, that won't help.

And FWIW (great song BTW), you really shouldn't hear the water flowing through the pipes. If you say to yourself, "yup, that's running water", then there's air in the system and it must be removed because that would be a major hindrance to heat transfer.

mattmia2

I suspect you aren't running out but that the temp is just dropping as you lose the stagnation in the tank. An ssu60 should handle 2 10 minute showers without being heated at all.

As the tank heats you get a layer of very hot water at the top of the tank. As you draw that layer off you get water that is a bit cooler that is being heated by the coil but you will get that continuously. If you readjust the shower temp it doesn't run out. This is one of the reasons for the mixing valve.

The capacity ratings of output of the tank are dependent on the output of the boiler. There are notes in the manual about what size boiler you need to get those numbers. Does the burner fire the whole time the tank is calling?

marcg

@mattmia2 No, it's out. I turn the shower all the way hot, and the water is lukewarm at best and continues to get cooler. And once it's out, it's OUT for a couple hours.

@HVACNUT,@mattmia2 I agree that it's possible to not meet the specs if the boiler or the circulator aren't meeting the demand, but then I would expect the return line (if the circulator is the problem) to be cold or both the return and supply lines to be cold (if boiler is the problem). In fact, both lines are burning hot, and the boiler is maintaining temperature at 170-180F, while remaining off most of the time.

marcg

@HVACNUT re: hearing the water. It's not a loud sound. It's comparable to the sound I hear in the water pipes in the basement when someone runs the water upstairs or turns on an exterior hose. You have to be pretty close to hear it. The main way I'm sure that it's the sound of water through the tank is that when I stop the circulator by disconnecting the aquastat, the noise stops. I'm not sure if it's meant to be completely silent, but it's not banging. Does any noise mean air in the pipes?

marcg

@hot_rod we get our water from the NYC supply, which is supposed to be very low in calcium The heat exchanger is finned cupronickel, at least according to the product website for a newer version.

pedmec

I would measure the supply in and return on the boiler loop to the tank. You need to confirm that you have a temperature drop thru the loop. Even thou you said that you hear water moving it could be just the motor. The shaft and impeller are one piece plastic on some wet rotor circulators and the impeller will snap off of the shaft. So while the motor is turning you are not moving the impeller which means your not moving boiler water thru the coil. You could have conduction happening in the return which is why that is hot. If you have strap on probes or something to measure together on the loop in and out of indirect you will see the temperature changes on both sides as the water is moving. If you see the temperature drop on both sides but no temperature split then its most like scaled on the domestic side off the coil and needs to be descaled.

flat_twin

Does a broken dip tube fit the symptoms?

hot_rod

marcg said:

@hot_rod we get our water from the NYC supply, which is supposed to be very low in calcium The heat exchanger is finned cupronickel, at least according to the product website for a newer version.

90% of NYC water comes from the Catskills and Delaware aquafer. Considered moderately  hard at 62 ppm. So over time the various minerals attach to the coils and reduce efficiency and recovery. If it provided adequate hw when new and gas dropped over 9 years, scaling is the common cause. When it is replaced cut a window in the side to see what it looks like

If the tank is heavy when removed,?r that also indicates mineral deposits have settled

Excellent drinking water in NYC, due to the minerals. Whatever is in the ground is what is in the water you get, unless it is filtered out

Liquid Assets is an excellent book about NYC water system.

Larry Weingarten

Hi @flat_twin , A broken/damaged dip tube could be contributing to the problem, but the OP said the shower starts out warm and then cools down from there. If it were a dip tube problem, I'd expect the shower to start hot, at least for a minute or two, then start cooling. Still, not a hard thing to check while messing with the tank.

Yours, Larry

mattmia2

HTP superstore doesn't use a dip tube and has the inlet on the bottom. It does have a short tube on the outlet to help draw water off the top.

marcg

I tried descaling by shocking the heater. I drained the tank from the bottom, heated the exchanger dry by running the boiler water through it, then added cold water to the tank and drained again. Repeated 2x. A lot of sediment did come out, so the tank definitely at least needed to be flushed.

Unfortunately, reheat is still terribly slow. Coming up about 30F/hr (ideally would be >3F/min). The whole process is slow because the tank takes a long time to drain, but I'll give it another try when I have some time. I also ordered a pump and a gallon of CLR and will try to descale chemically. Any tips on this? HPT says to remove the aquastat and circulate through the fitting hole to the drain. But I'm wondering if I could just pump the CLR solution in through the drain and let it sit for a while.

In the process, I can answer some of the other questions - yes, fills from the bottom (no dip tube). Yes, hot water is definitely circulating, because the outlet tube was cold when I turned off the circulator to drain and got hot almost instantly after I turned on the circulator to do the dry heating.

kcopp

Like I said. You can try to do all the descaling and shocking you want.
You still wont get it back to anywhere near the performance it was.
Sorry to say you are probably best to replace it w a new model that has a smooth coil....one that wont get gunked up w/ scale.

mattmia2

I had one more thought. Did you actually feel the outlet of the tank or run a hot water faucet when you think the tank is out? Most shower valves meet anti scald codes by having an adjustable stop that prevents you from setting it to hot ony with the cold off.

marcg

After I flushed the tank, I measured the water temperature vs. time as it came up. Temperature rose at 30 degrees F/hour (started at 55F, after 1 hr, 85F, after 2hrs 115F), so the problem is definitely that the tank isn't getting hot, not a problem with the plumbing downstream

It seems very likely that the problem is that the heat exchanger is scaled. I'm going to try descaling, because I would like to make the indirect last at least until mid-2023 when heat pump water heater incentives kick in.

mattmia2

I would get 4 of this type of thermometer and strap them on the supply and return to the boiler and the cold and hot dhw connections:
https://www.supplyhouse.com/Mr-PEX-5240748-Strap-On-Pipe-Thermometer-for-3-8-1-5-Pipe-0-248F-Pack-of-2

It is also possible that you don't have enough flow from the boiler through the HX. Without actually measuring the supply and return temps to the boiler you don't know.