Leak in domestic coil or something else?

dseebauer

We have an oil fired boiler (about 7 years old) for hot water radiators with a domestic hot water on demand coil. A while back the system started leaking on the floor (out of the safety valve turns out). The service tech immediately started replacing all sort of other parts (high vent, expansion tank, safety valve, $ worth) and it did not fix the problem. When we called them to come back they claimed it was either the auto fill valve ($) or there was a small leak in the domestic coil ($). I replaced the auto fill myself and the problem slowed down, but not totally gone. Does it make sense that a hole in the domestic coil could in fact cause the overall loop pressure to rise and cause the blow off leak? I thought they were completely isolated systems. Second question, could poor water quality damage the coil. House has a well and it pulls up a ton of iron (brown red water if left unfiltered). Thanks in advance.

Jamie Hall

Ah yes. The wonderful parts cannon.

Let's try a little diagnosis first, eh, before we start shooting?

There should be two separate water lines going to your boiler from your domestic water supply. One goes to hot water on demand coil. The other one goes to the boiler autofeed valve.

Now. There should be a manual valve on both of those lines.

First, close the manual valve on the line feeding the domestic on demand coil. Open a hot water tap somewhere (no water should flow — if it does, you have other problems). Adjust the boiler heating side pressure to where it belongs and close the manual valve on the boiler autofill valve. Then let it sit — off — yes you'll have a cold shower — for 24 hours. If there is any change, one of the manual valves is leaking, and you'll need to figure out which one. Now. Open the manual feed valve for the domestic hot water — but leave the boiler turned off (more cold showers, sorry about that). If the boiler water pressure rises, there is a leak between the domestic hot water coil and the boiler. Sorry about that. If it doesn't, the problem is probably the autofill valve either misadjusted or the expansion tank not properly charged — or the autofill valve leaking by.

EBEBRATT-Ed

What @Jamie Hall said

This may be a little simpler since your using it for DHW. Shut the valve in front of the boiler feed PRV and leave it closed. Leave the water on to the DHW coil and run the boiler normally.

Then record the water pressure at the gauge. Over time the pressure may rise and fall depending on the boiler water temp Just keep an eye on it.

If the dhw coil is leaking water into the boiler side you will see the pressure rise and it will steadily continue to rise until the relief valve opens.

dseebauer

Thanks for the rapid response. I should have mentioned that we did do the test of shutting off the autofill and the pressure did in fact rise. I am assuming that this means the DWH coil is really leaking into the boiler side. For my own education, does the DHW coil sit in a bath of boiler water (heat exchanger)? If so, it makes sense the leak would increase pressure. I always envisioned it as a dry heat exchanger similar to what my gas fired wall mount on demand has to ensure no cross contamination. I will run few more tests as described before agreeing to the expensive repair. My brother suggested to just let it leak and manage the water coming out. Not a lot. Gallon or so every few days. Any real danger to that? Harm to boiler or to hw supply. Sort of acting like a second auto fill ;-).

EBEBRATT-Ed

If you let it continue you will be adding fresh oxygenated water to the boiler which will shorten the boilers life.

City or well water has oxygen in it. This will over a long time corrode a steel or cast iron boiler. In the boiler when the water is heated the oxygen is driven out of the water and is vented out the air vents. The boiler water being a closed recirculated loop then stays relatively corrosion free.

If the coil is leaking it should be an easy fix (especially in a 7-year-old boiler)

In older boiler the bolts that hold the coil ten to get rusty and corrode and are difficult to remove. I doubt you will have that problem. A decent tech should be able to change the coil in 2-3 hours without a problem.

And yes, the coil has city water going through it (cold water in hot water out and the coil is surrounded by boiler water.

Coils are not very expensive. If you get your boiler model # you can look on Everhot thermaflow or call them

HVACNUT

Is the leak from the boiler relief valve on the back of the boiler, or the discharge for the Back Flow Preventer near the feed valve?

If there's valves on the domestic hot and cold at the boiler, check the pressure, close the valves, go to bed. When you get up, check the pressure. Same pressure, bad coil. Pressure rise, something else. But I believe all the other parts were replaced at whim anyway.