Indirect Water Heater Can't Keep Up

[Radio Flyer]

I have a 13-year-old Vaughn S35TPP indirect water heater (see photo) heated by a Budrerus oil-fired boiler with a Grundfos recirc pump set on high speed. I have the water heater aquastat set to 145° with a 10° differential, and I have a mixing valve set to about 122°.

People say an indirect creates "endless" hot water, but my experience is the opposite. After a twenty minute shower with a low-flow shower head, the hot water is gone and recycling takes at least half an hour.

Should I try cleaning the coil? If so, what would I use to clean it? CLR or maybe vinegar? I'm assuming I'd need to install new O-rings when reinstalling it, right?

To remove the coil I'd need to cut the unit out, since three of the connections are sweated and fixed above (see photo). Afterward, do I then sweat on slip couplers to reinstall, or is there a better method? And should I have some kind of heat loop at the cold inlet?

Thanks for any advice!

 

[Radio Flyer]

 

[Sponsor]

 

[WorthFlorida]

Is this a new problem or always have been a bit short on hot water. The endless hot water is claimed by tankless water heaters, without a tank.

If you remove the coil and it is extremely covered with a thick layer of minerals, vinegar won't do much unless you can make it soak for a few days and change the vinegar daily. First thing is shut it down and drain the tank. Open a hot water side of a faucet to allow air in as you drain the tank. When you open the valve and little or no water comes out and it take a screwdriver to poke in the tank after removing the valve, may indicate that the bottom of the tank has sediment that might be partially covering the coil.

If you have a gauge of the outlet temperature of the Buderus to the indirect tank or a temp gauge as the heated water enters the tank, get a read on it while running the shower. Part of the problem might be the water is circulating too fast for the furnace to keep up with the demand. So if the temperature outlet temp is not up there say, 120-145 degrees, lower the speed of the circulator.

My brother has a home in West Dover and it's incredible how cold the well water is. It is somewhere in the forties. He has a Buderus propane tankless with an indirect water heater and the few times while there with a house full of family members, I never noticed it running short on hot water.

 

[Reach4]

I would suspect the liming up would be in the potable water side of the coil vs the part facing the boiler water. My thinking is that the boiler water had a limited amount of lime to deposit, where the water being heated would have more minerals to deposit all of the time.

I would think you would rig up something to use a small pump to recirculate the treatment solution, as is done with tankless water heaters. I have no relevant experience.

 

[Seiyafan]

Next time the boiler fires up, feel the pipe going into the water heat and coming out, there should be some temperature difference, otherwise it means the boiler is able to produce the heat needed but very little is being transferred to heat the water.

 

[WorthFlorida]

A water heater provides 66% of heated water at the set temperature. . That is you have a 35 gallon tank of heated water but as you draw water, ice cold water is entering the bottom of the tank and mixing with the hot water, therefore, you have about 22 gallons of water at set temperature. Obviously the recovery rate is a factor of how many BTU's of heat is entering the tank. With a shower head of 2 gallons a minute for 20 minutes and not knowing the recovery rate, your temperature issue may be within the set up.

https://www.fastwaterheater.com/water-heaters/articles/potential-causes-reduced-hot-water/

 

[Jadnashua]

The tempering valve and higher temp in the tank SHOULD mean you'll have more hot water at 120-degrees than a simple tank without those things. But, it's more likely that the interior of the WH has accumulated a bunch of mineral deposits that are now insulating the heating coils from the supply, slowing down the heat transfer. Plus, those mineral deposits will decrease the available volume in the tank. Soft water would have helped, but over the years, those minerals will build up. If you want to try to recover functionality, assuming things aren't leaking, hooking up a setup like is used to demineralize a tankless system may work for you. It might take awhile, and a few recharges of the cleaning solution. That would require disconnecting the supply and outlet pipes, then hooking in a means of adding a pump to get that acidic solution to circulate, eventually, dissolving the mineral buildup. If the piping has gotten thin, it might clean off enough so it leaks, and you have to replace the tank, but that would be the extreme case.

You might try turning off the inlet, then draining the tank while measuring how much water you get out. That would give you an idea of the volume of mineral deposits in it. It will be off some, unless you have a valve at the outlet, as it would include the water in the hot water distribution network, or at least some of it.

 

[Dana]

Is the burner on the boiler cycling on/off while serving the water heater or does it burn continuously? (If cycling it's a heat transfer problem, which could be insufficient flow on the circulator &/or liming on the heat exchanger.)

What is the BTU/hr heat rate of the boiler? Less than 100 BTU/hr rate isn't going to deliver the endless shower experience for a full flow shower, but should keep up with a single l0w-flow shower.

How big is the biggest tub you need to fill?

A storage temp of 145F is on the cool side for a small-ish indirect. Most are rated for up to 180F, but bumping to even 160F should make a noticable difference.

 

[Radio Flyer]

Working on this now. Here's the condition of the coil. I suppose it could be worse, but looks like a cleaning is an order. Some scale in the tank also, but not tons. I wonder whether cleaning will resolve the issue or not.

Boiler is a direct vented Buderus G115WS/4. 98,000 BTUs gross and 85,000 BTUs net IBR.
Circulator pump was set on high. Grundfos UPS 15-58-frc. Boiler was cycling on and off frequently when hot water was trying to recover.

 

[Jadnashua]

Is that mineral scale or rust on the inside of the tank? If it's rust, it might be time to consider replacing the tank.

FWIW, the indirect I have says don't set it higher than 140-degrees F, but many can handle more. The tank on mine is SS, so in theory, anyway, shouldn't rust.

Yes, cleaning the heat exchanger should help. When the boiler is turning on/off, that means that the return is getting back hotter. You might also try running the pump slower...that will let it draw more heat on each cycle...think passing your hand through a candle flame...move it fast enough, and you don't even feel the heat. Running it fast also draws more power, and in the extreme, can erode the pipe from the inside over time.

Go to a place like Costco, buy a bunch of vinegar, and submerge the thing...let it sit probably overnight, while maybe a brushing a couple of times to loosen things up. The vinegar (weak acid) should remove most of the mineral deposits.

 

[Dana]

Working on this now. Here's the condition of the coil. I suppose it could be worse, but looks like a cleaning is an order. Some scale in the tank also, but not tons. I wonder whether cleaning will resolve the issue or not.

Boiler is a direct vented Buderus G115WS/4. 98,000 BTUs gross and 85,000 BTUs net IBR.
Circulator pump was set on high. Grundfos UPS 15-58-frc. Boiler was cycling on and off frequently when hot water was trying to recover.

If it's cycling it's a heat transfer problem, which could be either insufficient pumping or heavy scale/lime or both.


The net IBR number is meaningless here, since there is no appreciable distribution losses (assuming the tank is next to the boiler, not on the other side of the house.) Assuming it hasn't been re-fitted with a smaller nozzle the boiler is spitting out 109,000 BTU/hr, and if the installer set it all up correctly the tank should be taking all of that heat. I haven't dug up & read the specs for your indirect, but YOU should.

109KBTU/hr is more than enough to support a full-flow "endless shower" or two water-sipping low-flow showers.

 

[Radio Flyer]

Just to (finally) follow up, what looked like mineral scale on the coil turned out to be mostly just sand and grit. There was also a one-inch layer of sand on the bottom of the tank and some paper-like scale on the sides.

I brushed off the coil and the sides of the tank and then inverted the tank while spraying inside with a garden hose to get out the sand. I also vacuumed the bottom of the tank. Vaughn tech support told me to just lube and reuse the O-rings, which I did.


The good news: I now have ENDLESS hot water! I assume removing the sand packed in the coil fins made all the difference. Should I now set the circulator pump to low??


The bad news: I must not have removed all the sand from the tank. I think some sand may have been sitting around the access hole when the tank was inverted and couldn’t be removed.

After I hooked it back up, some of my faucet aerators blocked up within seconds. The flow didn’t just slow down. It stopped completely. I cleaned the aerators, but for the next few weeks all the aerators in the house kept getting partially blocked.

It appears I’m over that now, with the remaining sand settling to the bottom of the tank (I hope). (However, I still need to figure out how to backflow or otherwise clean the sand out of a motion-sensing kitchen faucet that has reduced flow before the aerator.)


Conclusion: This all meant that I must have sand coming from my well. I installed a Rusco 3/4" spin-down filter with 100 mesh element on the main cold water inlet with ball valves for a bypass. I’d say that every month it collects a teaspoon of sand. Hopefully this solves it.

BTW, the folks at Vaughn technical support were a terrific help. Definitely worth talking with them. Thanks also for the help and input here!

 

[Jadnashua]

What does the installation manual say about the pump speed? I'd try it on low and see what happens. It shouldn't cycle if the heat transfer is correct. Running it slower means there's more time to extract heat, so the return water will be cooler keeping the boiler on so it doesn't short-cycle. If that doesn't work well, I'd bump it up to medium, but I don't think high will be necessary or work well.