Why does my gas-fired water heater cut out?

January 29, 2005 5:38 AM

my new-ish gas-fired water heater has started to cut out after maybe 10 minutes use. if i close and re-open the tap it fires up again. it's pretty basic - a box on a wall with electronic ignition that runs off bottled gas (propane or butane, i suppose). can anyone explain why this happens (the 15kg gas cylinder is no longer new, but certainly not near empty)? thanks.
posted by andrew cooke to home & garden (15 comments total)

It might be "Hard" water which allows sediment to build up in the tank. Read This for a better explanation and some solutions.
posted by lobstah at 6:19 AM on January 29, 2005

This happened with my heater, when the central heating system had an airlock. It was a safety measure that was kicking in to prevent overheating / overpressure.
posted by viama at 6:38 AM on January 29, 2005

Check all the jets for dust (blow them out with a can of compressed air). I had a small gas heater that would do the same thing. the blocked jets caused the flame to not reach the thermocoupler (safety device that turns it off if the flame is out to prevent you from being gassed to death and/or blown up!).. the thermocoupler would then turn off the system..

I put up with this for two years before I figured it out and spent two minutes fixing it..
posted by HuronBob at 7:04 AM on January 29, 2005

thanks for all the replies - i wasn't expecting anything and these are helpful, although...

i don't think it's hard water, because it's a new heater, and there's water flowing ok, so i don't think it's for overheating (the water's not that hot either). peering through the little hole, the thermocouple doodah seems to be glowing red hot (sitting in a flame), but i'll try giving the jets a clean anyway - certainly something could be dirty, from the work done during installation. did it really run for that long before cutting out (i can get almost showered before it stops)? maybe the thermocouple is just broken, in that case (so even though it's hot, it's causing the heater to cut off after 10 minutes).

there appear to be two additional sensors - one on a water tube and one on the exit flue. i wonder if one of those is cutting things off for some reason?
posted by andrew cooke at 7:16 AM on January 29, 2005

Random thoughts and half-baked theories:
- Another bad-thermocouple symptom: it would have problems firing up in the first place (as the thermocouple would not heat up sufficiently to open the gas line). Sounds as if yours fires up okay though. If it's new it should be clean.
- I guess trace the sensors back and see where they interrupt the gas line. What's the exit flue sensor - not CO is it? I know zip about these things though.
- Your sensors might be okay so another place to check could be the valve/relay/moving parts/whatever which lets gas into heater (at the other end of the thermocouple).
- Also how's the ventilation around the unit/pilot light area?
posted by carter at 8:00 AM on January 29, 2005

Okay it might be an O2 sensor in the flue, that will cut off if O2 is low (and therefore CO is high). Maybe you should check how the unit is vented.
posted by carter at 8:30 AM on January 29, 2005

When in doubt, make sure all electrical connections are tight - wiggle and pull on every wire. If there's a loose connection, a wire may be heating up and expanding enough that it loses contact. I know my answer sounds flippant, but I can't tell you how much equipment that I've troubleshot has had loose connections.
posted by notsnot at 8:45 AM on January 29, 2005

i was thinking there might be an override on the thermocouple so that lighting works, but i was surprised at the length of time, which is why i was interested in HuronBob's experience - his comments strongly suggest that the cutoff time is quite long.

venting is not a problem, i believe (it's next to a CO alarm, installed by a qualified engineer who *said* she checked it for emissions, and the vent is clearly open into free air - i'm pretty paranoid about that).

on preview - thaks notsnot, i'll try that (it wasn't flippant at all!)
posted by andrew cooke at 8:47 AM on January 29, 2005

I would be especially cautious about this -- I lived in the Czech Republic, where these kinds of heaters are in every bathroom. In the course of about six months one friend died in his bathroom, another died in the living room of a cottage he was staying in, and another was washing her face one minute, and waking up in her own vomit on her bathroom floor the next.

Get a CO detector, keep it near (above? I think CO rises) the heater, and keep the batteries current. These heaters are dangerous.
posted by jennyjenny at 8:57 AM on January 29, 2005

I've got a regular gas furnace, high efficiency, in my basement. It's only four years old, and the pilot goes out on occasion. No drafts. Anything I should check besides blowing compressed air to clean it out or replacing the thermocouple?
posted by mecran01 at 9:19 AM on January 29, 2005

On the placement of CO detectors.
posted by aberrant at 9:20 AM on January 29, 2005

Andrew...

My heater would go for 10 minutes or more before cutting off...but, if you're seeing a clearly hot thermocoupler, i would doubt that's the problem......

but...a good cleaning of a gas appliance with compressed air never hurts!
posted by HuronBob at 11:07 AM on January 29, 2005

thanks!
posted by andrew cooke at 11:23 AM on January 29, 2005

just to follow up on this, i jiggled the connections and it worked this morning, so it may be that was enough. however, i also looked at the safety thermostat thing that should be in a flame and it (possibly) isn't - what i was looking at before were the conductors for the spark ignition. the thermostat thing has its own little tube for gas, with a little flame, and that flame is sometimes disappearing. however, it's also near the other (big, water heating) flames, so maybe that is enough. i'll keep an eye on it and, if the problem continues, start following that avenue. cheers.
posted by andrew cooke at 4:59 AM on January 31, 2005

oh, since this is still open, here's the solution...

at the top of the heater there's a tube/chimney that is a (loose) push fit into the chimney/outlet duct in the kitchen. teh fit was so loose that there was a significant flow of air going through the gap (since the outlet flue takes warm air at lower pressure, additional air was being "sucked through"). this, in turn, was causing some flow of exhaust gases from the burner to leave the main flow and go around the upper shroud (there's an inverted triangle below the chimney that catches the air). so some warm air was flowing around the upper shroud and then into the flue through the gap.

in itself, i don't think that was a problem. but there's a thermocouple on the edge of the shroud to detect escaping hot gases if the flue is blocked (in this case it wasn't, but if it was you're dumping all the waste gases back into the room). after 20mins or so that thermocouple warmed up enough, from the (small amount of) "escaping" gas that it trigger the safety cut-out.

all that was necessary was to seal the gap between chimney and flue (which i did by wrapping around som aluminium foil).

i worked this out by looking at how the whole thing worked, seeing the thermocouple, and then tacring the gas flow with smoke from a match. it was pretty interesting....
posted by andrew cooke at 2:35 PM on January 11, 2006