GJ overfires than goes out

I have a GJ61 that I have trouble with overfiring. Since it is above the cutoff temp the timer does not run and it often goes out till it cools off enough to start the timer cycles again. This problem does not occur during the heating season since the calls for heat keep the furnance cool enough. This problem also only occurs after an extended call for hot water, such as when the entire family takes showers back to back. (Interestingly this problem seldom occured when burning Sherman coal, but was magnified when burning Harmony coal. I suspect the Harmony is a little hotter.)

I believe the issue is cause by my Aquastat well, but I want to run it by the experts here before I go to the bother of replacing it.

When installing this furnance 2 years ago the plumber did not have the correct well along and installed a shorter one. This causes the Aquastat to read about 30 degrees lower than the water temp thermometer. In theory this could simply be solved by setting the Aquastat temps lower, however I believe this also causes the Aquastat to respond slower. It seems to respond almost in sync with the water temp guage in a temp drop, but seems to fall behind on a temp rise.

Correct me if I am wrong....On a hot water run cycle the furnance should start when the water temp drops 10 degrees below the low Aquastat setting and heats until the temp reaches the low temp setting then it should stop.

What actually happens is the furnance starts about when it should, runs until the water temp hits anywhere between 150 - 170 then stops. By this time our demand for hot water is usually over and the water temp will slowly continue to build often until it blows the pressure release valve. Till it finaly cools enough for the timer cycles to start again it is often out.

Stat temps are as follows:

High = 180
Low = 135
Diff = 10

So....Is my theory correct? Could a shallow well be causing by stat to respond to slowly or is it something else? Any ideas would be appreciated.

I had the same problem with my hand fired boiler.I lowered the temps to 160 and solved the problem. With a larger family you may need a larger storage tank for back to back showers. I'm sure the more experienced people will help you out.

The aquastat settings & operation definitely become more important when burning Harmony coal. It burns very hot, and doesn't require as much air as coal from many other sources.

Where is the aquastat located in relation to the tankless coil? Ideally it should be located close to the coil so it reacts quickly. The aqaustat itself does seem to be questionable based on your observations. I had one that was doing the same thing on my oil boiler, I replaced it and the new one definitely reacts faster. If you can swing it, a proper length well and a new aquastat would probably be a good investment.

Also, do you have a bypass or "equalizer" pipe from the supply to return header? This helps keep the boiler temperature more even from top to bottom.

prophead wrote:By this time our demand for hot water is usually over and the water temp will slowly continue to build often until it blows the pressure release valve. Till it finaly cools enough for the timer cycles to start again it is often out.

The pressure relief should not be opening just because the temperature rises to 200 degrees or so. If you have an old style steel expansion tank, it might need to have the water drained out of it. If you have a bladder-style tank, it might have failed or need some air added. There is also the possibility that the expansion tank is undersized for the boiler/system.

Also, pictures are worth 1000 words. If you could post some pictures of the boiler, piping, controls, etc...it would help. Another forum member is running the same boiler, and might be able to comment on the aquastat location.

I didn't mention it, but if it heats water ....it is a boiler NOT a furnace. My pet peeve, but just a clarification you might consider. Back to the boiler issue........

Expansion tank size and type..... More volume and or more tanks is a GOOD thing.
Domestic hot water demand(how many kids, baths, etc.)
Tankless only? Do you have a domestic tank as a buffer between the coil and the demand side of the domestic hot water?
Well length? Is the aquastat bulb touching the bottom of the well?
It is not impossible for an aquastat to go bad, but unlikely.
Temp settings seem very normal. (But who knows if the aquastat is correct) You might lower the high limits and try it that way.
PRV should activate at pressure only, and as temp rises, so does pressure. I've seen many go to 250 or 260 on a temp gauge before popping off at 30 psig. (That gets YOUR attention)
Like everything in life, solid fuel prefers a slow increase and decrease, hence the outfires......gets too hot, cools too slow, timers are overridden.....blah, blah, blah.
I'm thinking more storage volume will help with expansion issues; a buffer tank for domestic will also help, and if you can access another aquastat, go for it. Can be a used one, and new wells are at any supply house.

A picture or diagram would help, but we'll check back later and see how things are going.
Whatever type of expansion tank you have, check it out for being water logged. The older style tanks have a drain in the bottom; hook up a hose and drain it.
Bladder tanks should feel light to the touch, tap it with something to see if you can determine water level(should be nearly empty). good luck!

As mentioned above: Expansion Tank !! Even if you have an operational one, it may be too small for your water capacity,
If you have a spare location on your plumbing , just add an additonal one.. after checking your current tank for being
waterlogged/full of water not air.

Greg L.

Thanks for all the replies....

The pressure relief should not be opening just because the temperature rises to 200 degrees or so. If you have an old style steel expansion tank, it might need to have the water drained out of it.

Temp guage pegs @ 260....does not blow until after the guage has been pegged for a while....the boiler will actually start to boil (you can hear it) and then the pressure builds to 30 PSI and off she goes....

I do indeed have an old style steel expansion tank and it does have water in it....I had drained it a while ago but it seemed to fill up as fast as I drained it!?

Where is the aquastat located in relation to the tankless coil? Ideally it should be located close to the coil so it reacts quickly. The aqaustat itself does seem to be questionable based on your observations. I had one that was doing the same thing on my oil boiler, I replaced it and the new one definitely reacts faster. If you can swing it, a proper length well and a new aquastat would probably be a good investment.

This could be it...The Aquastat is side mounted and with the correct depth well should get real close to the hot water coil, however with this short well it may not be close enough....I will post some pics...

I didn't mention it, but if it heats water ....it is a boiler NOT a furnace

How'd I do in this post?

Domestic hot water demand(how many kids, baths, etc.)

6 showers back to back

Tankless only? Do you have a domestic tank as a buffer between the coil and the demand side of the domestic hot water?

Tankless only

Well length? Is the aquastat bulb touching the bottom of the well?

Don't know....I just know the plumber was grumbling about not have a deep enough well and as a result the Aquastat is going to stand away from tank a bit and the temps could be off...

I will try to get some pics posted of the expansion tank, Aquastat, plumbing Etc.

When you drained the expansion tank did you have the make-up water shut off?

6 showers is doing to make the boiler work pretty hard, so some amount of temperature overshoot can be expected. Getting the aquastat to react quicker should help. If all else fails you may have to reduce the feed rate or hook up a dump zone.

Another question: When the fire goes out, do you find unburned coal in the pot or does all the coal get burned up? If all the coal is burned, you should check your draft control. The only outfire I have ever had was when I burned UAE coal and had my draft control covered with foil...all of the coal in the pot burned up.

Here is some pics from another post I made that may be of a help....I will try to get some better ones of the stat and plumbing.

http://nepacrossroads.com/about18876.html

Replaced the stat about a month ago with a used stat off of an EFM 520 a friend was pulling out. Placed the "new" stat in the same well and PROBLEM SOLVED! This "new" stat seems to be within about 10 degrees of the temp gauge on top of the boiler, a variation I would expect with the different locations. (The old one would be off as much as 30 degrees)

Now I need to pay the guy for the stat under a make offer agreement....Whats a used stat worth?

Something I use that seems to help with keeping the aquastats accurate is to put a non-hardnening compund on the bulb before it's inserted in the well. I like to use sil-glyde (for brake caliper pins) it's cheap, hasn't hardned (like some products that are "designed" for this purpose) and increases response and accuracy considerably.

sil-glyde: http://shop.advanceautoparts.com/webapp ... 12024852-p

Something I use that seems to help with keeping the aquastats accurate is to put a non-hardnening compund on the bulb before it's inserted in the well. I like to use sil-glyde (for brake caliper pins) it's cheap, hasn't hardned (like some products that are "designed" for this purpose) and increases response and accuracy considerably.

Done that....with the old stat it made no difference....