water heater corrosion

Al_11

What makes a water heater foul up with flakes of corrosion?
I have 2 w/h that are 5 yrs old. the one for hot water usage is fine, the other is for radiant floor heat in the basement and the burner and combustion chamber were loaded with flakes of corrosion. Both are piped to the same flu.

Constantin

Sounds like condensation...

I'll take a guess that the WH heating the radiant section is suffering severe corrosion due to heating a low-temp circuit. How is it piped to the radiant system? Is there a TRV or somesuch that allows the WH to maintain a minimum temperature? Or is the temperature in the slab controlled directly by the dial on the gas valve of the water heater?

Also, I would check for obstructions that prevent flue gases from escaping. For example, rust, etc. around the draft diverter is almost a sure sign that the flue is blocked.

Lastly, congratulations on having a WH last this long in a mode that it was not built for. When it fails, I'd suggest replacing it with a wall-mounted condensing gas boiler which will give you not only longevity but much better fuel economy to boot.

Mark Hunt

Have you


checked the draft on the scaled water heater?

Venting into the same flue is no guarantee that it is drafting correctly.

Mark H

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Alex Giacomuzzi

Flue Gas Condensation

If I were to venture a guess....... the flakes of corrosion are deposited all around the interior of your burner chamber and also on top of your burner itself. It possibly may even get so bad as to put out your pilot light from time to time.

What is happening is your water heater is experiencing "Flue Gas Condensation" right within the water heater flue stack and probably within your exhaust stack. This condensation becomes acidic at this point which attacks the steel and oxidizes away metal flakes of various sizes. These flakes fall back into the burner chamber and on top the burner.

Constantin suggested the operating temperature of your water heater as the culprit. This is absolutely correct. You are probably directly heating the concrete floor in your basement and using the water temperature dial adjustment as part of your water radiant loop control. This does not allow your water heater to reach a high enough temperature to fully vaporize the flue gasses and not have them condense in both your heater and your exhaust stack.

You need to repipe your basement circuit to allow the water heater to get up to say at least 120 deg F. and yet still be used as a heat source for your concrete floor.

Constantin

As an aside,

Please do not subscribe to threads on the Wall if you're using an obnoxious white-list spamblock like Earthlink, unless you add heating help to the whitelist. It is simply thoughtless not to do so...

Every time I post in this thread, I get a nastygram notification from Earthlink that you need to add me to the whitelist as a result of your request to be notified whenever new messages are posted. Not only does this increase the amount of garbage in my inbox, but it also ensures that you won't get any thread updates either.

I would also like to ask people who like to post with fake e-mail addresses not to subscribe to thread update notifications either, for the very same reason. After all, how is the system that runs the Wall supposed to notify someone that doesn't leave a proper address?

Mark Hunt

Agreed


Not very neighborly.

Mark H

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Al_11

water heater corrosion

the piping has a true mixing valve and the t-stat is set for hot, the same as the other water heater. The furnace, and both water heaters are all piped to a 5" B-vent flu. to me that is not large enough but only the radiant heat water heater has the problem.
thanks for your time
Al

Al_11

It seems to be drafting properly, using a match .
Al

Al_11

the system is using a mixing valve and the water heater is running hot, although your discription of it is correct it definately looks like condensation, but why?
Thanks Al

Chris Jorgensen

Corrosion

Is the heater allowed to get cold during the summer?

Constantin

Interesting...

While the tank may be set to high, that does not mean that the tank will actually reach those operating conditions whenever there is a prolonged call for heat. How does the heat loss calc compare to the output of the heater? Considering how inefficient such heaters are, it wouldn't surpise me if the thing is running 24/7 even in mild weather.

The way I understand the system, you have a TRV mixing the return water with the boiler water. This will protect the radiant floor piping system and keep the boiler warm - under normal water heater circumstances. Except, it's not heating potable water, it's heating a space.

What do you suppose happens once the water heater is running for several hours a day to cover the heat loss? Unless the boiler is on a variable-speed injection loop or somesuch, the return water temp is going to drop and drop as the BTU's get sucked out of the tubing. This "cold" water will then return into the boiler and keep it "cold" for hours at a time, cold enough for the flue gases to condense and cause the corrosion you're describing.

Normally, buffer-style water heaters fire for only a fraction of the day (usually in the morning and evening). The rest of the day, they stay toasty warm and hence the amount of time that their flue gases condense (which they will briefly if you fill them with cold water, then fire) is very short. In a heating application, condensation can happen 100% of the time, so it's not surprising that the boiler is suffering.

I saw the same effect in a ice cream parlor where the owner was using the heater as intended... heating water for the spoons, etc. The thing ran from the time the place opened to the time it closed. The owner complained that the WH would only last 1-year and that commercial vs. residential quality made no difference. He had the same problem as you, condensate rotting the things out, and could have solved it once and for all with a IDWH or a stainless WH like the Polaris which likes to condense.

Yet another reason not to use a regular water heater except for the smallest of heating loads. If the homeowner insists on continuing this practice, find a way to isolate the WH in such a way that prevents the internal temp from dropping below 140°F (shut the pump off). The only way to do that with a regular WH is to buy a commercial model, set it for 160°F and allow 20°F hystersis.

However, a small modulating, condensing boiler is likely to save you 40%+ on fuel heating the place over the extent setup, so that is what I would suggest. More money up front, bigger savings and better comfort down the pike.

John McArthur_2

Check out

Check out hotwater.com That site has excellant resources and troubleshooting. Good luck .... Bob

Al_11

I think you might be on to something. I can see what you are describing on the innitial start-up each fall. But once the slab gets near temp I would think that the mixing valve would start taking over and the return water to the water heater would start climbing. After the first start-up I would think the mixing valve would act like an injector and only add hot water as necessary allowing the water heater to maintain temp.
Assuming it can handle the load, I have no idea what the load calc is, I was just called in for service. If you are correct, we have an awful lot of water heaters going in the tank up here. In Minnesota using the water heater to heat the garage work-shop is very popular.

Constantin

I'm not so sure...

... you are correct in noting that the TRV acts as an injector on startup. However, as the heating load starts to approach the heating capacity of the boiler, the temperature inside the circuit is going to drop and then the boilers internal temperature will follow.

This is why a TRV cannot replace an injection pump - a pump with feedback has much better control over how many BTUs are moved out of the WH than the TRV, which simply limits the temp of the outgoing water by tempering it. Unlike the injection pump, the TRV has no control over the water temperature entering the WH...

If I were you, I'd give it to the HO in writing. Better yet, take them down there and have them pose in a picture pointing their finger at the problem. That way, if they ever have amnesia later WRT your warning (because this is a dangeous condition), their laywers will back down immeadiately.

As far as garages and workshops are concerned, it's all a matter of the load. If the load is 2-10kBTU, as most underground spaces are, the WH won't be stressed as much as a WH trying to heat a 50kBTU load. Perhaps any WH that heats a floor system should be generously oversized and run at max temperature unless its designed to condense like the Polaris. This is likely to be a pretty inefficient use of fuel...