Ignition troubles Mr Heater

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Old 01-11-08, 08:57 PM
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Ignition troubles Mr Heater

Having troubles with my Mr Heater MHU 75 gas heater that I recently installed in my garage. System works "periodically" at best. Symptoms....Thermostat calls for heat, combustion air blower comes on fine. more often than not, after about 30 seconds of blower activity, it shuts off. The LED indicator flashes 3 times, which according to the manual means "pressure switch failed closed before CAB (combustion air blower I presume?) is energized or failed to open after CAB is energized".

I have been able to work around this problem by killing power and then re-applying power, to get the blower started again. Sometimes it will fire up and ignite the flame, other times I have to start "tapping things" until it fires up. Once it's going, it works fine for a number of hours. Eventually though, the 3 LED flashes start up again and I'm back to square one. This is a brand new unit...Does this sound like the pressure switch is acting "funny", and if so, where is it located? Any other ideas?

Thanks for your help!

Chris
 
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Old 01-12-08, 09:47 AM
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tapping !!!

I think that's the clue..."tapping"
I believe your pressure switch is defective. Maybe a collapsed rubber hose, or plain simply internal contacts that are not opening back once the call for heat is satisfied.

Next time, have someone by the t-stat while you're by the unit. disconnect one of the two wires feeding the pressure switch...have the t-stat call for heat...as soon as the combustion blower engages, re-connect the wire.

If the unit runs normal, the switch would indeed appear to be the malfunctioning part.

Boards have built-in a series of safety checks and checking for a flame signal and an air signal wnen "none" should be present (right before a call for heat that is) is enough for them to go on safety lockout.
 
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Old 01-12-08, 03:17 PM
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pflor,
Thanks for the advice, I will check that...Now for dumb questions...Where abouts is the pressure switch, what does it look like, what colors are the wires leading to it??

Thanks,
Chris
 
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Old 01-12-08, 03:48 PM
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Originally Posted by GEGUY View Post
. The LED indicator flashes 3 times, which according to the manual means "pressure switch failed closed before CAB (combustion air blower I presume?) is energized or failed to open after CAB is energized".
This is more lengthy of a flash-code explanation than you'll normally find. I am going to presume you have a 3 or 4 wire/terminal pressure switch as opposed to the 2-wire/terminal kind.

Perhaps yours requires the switch for 2 of those wires to first be closed (to complete the circuit) BEFORE the combustion motor starts, so that people do not simply bypass the pressure switch (as you coud easily do with a 2-wire pressure switch). If this aspect failed to where it was not closed (on 2 of the wires) when it should be, prior to start up, and yet there was 24 volt a/c power to the terminals, this would confirm that the contacts in the pressure switch are bad.

Pressure switches are a vacuum diaphram device that have both the wire/terminal connection and also at least one approx. 1/4 inch vacuum tube connected into it. It might be about 3 inches in diameter. Nothing else resembles it on the furnace/heater. You can't miss it, from this description.
 
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Old 01-12-08, 04:28 PM
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Manual

GEGUY:
save a copy of this if you do not already have it.

I'll try helping out later or early in the AM tomorrow [can't right now]

http://www.mrheater.com/upload/newsl..._hsu_45_75.pdf

Pressure switch appears to be a 2-wire unit (which makes things easier). See item-13 on pg-15 of the manual
 
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Old 01-12-08, 06:07 PM
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Chris:
From the manual, the pressure switch (P.S.) is a 2-wire switch with a normally open (N.O.) set of contacts which close after the combustion blower starts IF sufficient airflow develops. N.O. means that when the unit is on a break (no call for heat) the contacts of this switch are expected to be open.

This switch could experience one of these two types of failures: (a) that its contacts were closed prior to the call for heat, or (b) that the contacts fail to remain closed after the call for heat

You don't have failure "(a)" b/c the blower does start.
You have failure "(b)"...in other words, either the switch is defective, or air pressure (air flow) from the combustion blower is not developing to its normal level.

Connected to the P.S. [part #13 on pg-15] is a rubber hose [part #14 on pg-15]...make sure the hose did not become lose or disconnected and that it is not cracked...IF SO, that's your problem right there.

If the hose is not the problem, I want you to perform the following test, but you need someone by the thermostat while you're by the heater unit...
- have your helper call for heat from the t-stat
- as soon as the induction blower starts, put a jumper across the 2 terminals of the P.S. You should have a brown and a red wires connected to the 2 contacts at the P.S., put a jumper between them.
- What I expect to happen now is that the combustion blower continues running, the spark ignitor starts sparking and the gas ignites. Just the way things should happen if all was normal.
- Stop the call for heat and remove the jumper

So...here's the scoop:
(1) the switch is defective, or
(2) there's problems with your vent pipe/cap

You said in your 1st posting that you installed the unit. I'm suspicious of the vent pipe you have used. Please take a few pictures (from the distance as well as close-ups)...from inside and outside of the house (the vent pipe and vent termination cap of course is what I'm interested in seeing).
An undersized and/or partially blocked vent can cause this problem. I need to know what type of material you used (for pipe and cap), its size (diameter), number of fittings (ellbows, reducing couplings, etc.), length of the vent system.
 
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Old 01-12-08, 10:28 PM
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Pflor,
Yes, I installed the venting, ran down to the local big box store and got the venting kit. Fairly simple, 4 inch diameter, one elbow out the back and a straight shot up through the roof. Has the vent cap on it. Will post some pics tomorrow (pretty late right now!). My first attempt at such a project (my field of expertise is security systems), so I wouldn't be surprised if this is the culprit. Will follow other directions as well though and keep you posted. Thanks for all your help!
 
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Old 01-12-08, 10:41 PM
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Ok, so I got antsy and ran out and did the jumper test. It was as you described! Put the jumper across and she fired right up...Hose looked fine...

As far as the venting goes, is the problem along the lines of restricted air flow?? I know you need pics, but just some observations before I can post them tomorrow...Out of the back of the unit, I have a 90' elbow hooked straight into a straight shot up through the roof. Total distance of the straight shot is about 8-10 ft terminated by a vent cap.

All the piping is standard 4" inch as I stated earlier. Pics tomorrow.

Chris
 
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Old 01-13-08, 04:20 AM
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Glad to read things worked out for you.
It all appears to point to a defective pressure switch, but I'd still love to see those pics.

If indeed you have to buy the switch, here's their website for ordering parts
http://www.mrheater.com/productdetails.asp?catid=65
There's also an 800-number all the way on the bottom of the screen (just keep on scrolling) you can call to order.

Of course the place where you bought this heater may have it too.
 
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Old 01-13-08, 09:42 AM
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pflor,
about to head out to the garage now and take some pics...
Will keep you posted.

Thanks!
 
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Old 01-13-08, 10:13 AM
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o.k. thanks...
I'll be out of the house a great deal of the aft. b/c of activities with my son. Look fwd to those pics, will be getting back in touch aft. I come back.
 
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Old 01-13-08, 10:23 AM
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Now this is strange...Just got out to the garage. Heater has been working fine all night (can tell by the temp in the garage). Just killed power and turned it back on, called for heat. Working just fine! Could our little jumper trick have "unstuck" something? It hasn't worked this well in weeks...About to take those pics...
 
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Old 01-13-08, 10:27 AM
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Just the luck of the draw
 
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Old 01-13-08, 10:37 AM
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pflor,
Just turned t-stat down, called for heat, kicks right in..Strange..

Here's those pics
http://www2.snapfish.com/thumbnailsh...4/t_=108864914
 
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Old 01-13-08, 03:05 PM
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I'm back...
Nothing unusual with your vent. I'm just curious though, what's that copper line with the manual shut-off valve (looks like a ball valve) in front of the vent on picture "vent-2"? It is not a gas line, is it?

I'd recommend you buy the pressure switch. They are cheap and one never know when they'll be required on short notice. Maybe you'll never use it...then again...you may soon.
 
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Old 01-13-08, 08:29 PM
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Originally Posted by pflor View Post
I'm back...
Nothing unusual with your vent. I'm just curious though, what's that copper line with the manual shut-off valve (looks like a ball valve) in front of the vent on picture "vent-2"? It is not a gas line, is it?

I'd recommend you buy the pressure switch. They are cheap and one never know when they'll be required on short notice. Maybe you'll never use it...then again...you may soon.
pflor,
Thanks for all the help, will go ahead and get that switch. Had to fly south for work, but just checked in with the wife and the heat is holding steady at 50'. I'll keep my fingers crossed!
 
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Old 01-14-08, 01:44 AM
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Glad things worked out well for you.
Good luck 'n see U in the forum some day.
 
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Old 01-14-08, 12:27 PM
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Originally Posted by GEGUY View Post
Now this is strange...Just got out to the garage. Heater has been working fine all night (can tell by the temp in the garage). Just killed power and turned it back on, called for heat. Working just fine! Could our little jumper trick have "unstuck" something? It hasn't worked this well in weeks...About to take those pics...
If this is the case and not some coincidence, then I'd guess that the electrical contacts in the pressure switch are bad, and not the diaphram part of it. Many electrical components fail and often some jump start gets them to make better contact at least for a while. Water heater thermostats can be that way, or pressure switches on for wells, or even bad starters on cars. Anything that has contacts.
 
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Old 01-14-08, 12:42 PM
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Originally Posted by pflor View Post
Glad things worked out well for you.
Good luck 'n see U in the forum some day.
If you're ever perusing the security systems forum, I'm your man!
 
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Old 01-14-08, 01:43 PM
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Thank you, you're very kind.
 
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Old 03-18-13, 11:16 AM
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Thank you GeGuy and pflor - years later advice well received!

Thank you for your posts GeGuy and pflor. Years later it came in extremely handy when my Mr Heater MHU75 would not fire up. Through research and diligence came to your blog string and the solution was to "jump" the Pressure Switch (part # 60030). Fired up just as described. Ordered new pressure switch from Enerco (out of warratee) for $18 plus $9 S&H. Approx 2 weeks to arrive, will post back once receive and installed.
 
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Old 03-30-13, 10:04 PM
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same problem

Mine has a 3 blink error code also. I have read the fixes but have an absent minded question - when you say 'jumper' are you saying take a piece of copper wire so both ends touch both prongs on the pressure switch?

I tried cutting the power on my heater but the code still persists. I haven't done anything else. I ran my heater all winter here and it just stopped working the other day.

If I understand this procedure correctly, I need to turn the power onto the heater, but not have the t-stat calling for heat. Attach a jumper to the 2 connectors and then call for heat. If/when the unit fires up I should remove the jumper and let it run as normal, hoping that tripped it. If it doesn't I can assume my pressure switch is defective and should call the mfr.

I believe I can remove one of the wires as well to see if it fires up and assume plainly that the switch is defective.

Is that correct? Thanks and appreciate the advice.
 
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Old 03-30-13, 11:14 PM
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Welcome to the forums.

You are working with a gas appliance. You can't be absent minded in your methods. You must follow the directions precisely in post #6. You will need two people.

Check the rubber hose first for integrity issues.... fell off one end, kinked, etc.

Removing the wire from the pressure switch will cause the unit to go into a "no start" condition or if it was started it will shut off.

when you say 'jumper' are you saying take a piece of copper wire so both ends touch both prongs on the pressure switch?
That is correct. It is low voltage so you wont get a shock if you accidentally touch the wire.
 
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Old 04-21-13, 10:17 AM
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As PJmax states follow in sequence. Need to follow a linear pattern. When unit first starts the power vent starts. You do not have the jumper on yet. Once the power vent starts you jump the pressure switch. After 30-40 seconds the ignition sequence should start and burners should ignite. As soon as you pull off the jumper the burners should stop. If this happens your problem might be a few issues similiar to those listed in the thread above. That is, the exhaust flue is blocked and /or the tube from the power vent nipple to the pressure switch is blocked and/or has a hole and or your pressure switch is bad. Keep in mind the pressure switch assembly is a bit of a safety function in that if your vent is blocked a suction will not be created through the small tube to activate (ie close) the pressure switch.

As posted above I thought it was a bad pressure switch, I orders a new one and replaced. However, this did not provide a solution. After further investigation, I found that the nipple on the power vent became fused shut by some hard creosote like substance. It is a small hole so it did not take much. I cleared with a 1/16 to 1/8 drill bit (on a cordless drill) through the nipple. This re-established the vacuum required to close the pressure switch. The unit nowworks fine.
 
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