Help with short cycling on a Weil-McLain GV-5 Boiler

SamTheMan

Have the subject line boiler in our house, it was installed in 1995. It is a sealed combustion unit and is used mostly for hot water (sidearm) and other auxiliary heating; so it has gotten little use over the years. Winter heating hasn't really started yet, so very little use right now. There is one zone which does come on almost daily, this boiler zone goes to a stainless heat exchanger, and right now that is the only zone running.
One thing I noticed while this zone is running, is that the boiler will run for a bit, then turn its flame off and purge, then immediately do a starting purge and restart the flame. Basically short cycling. After the initial warm up (which seems to run normally), the period of this cycling seems to be ~4 mins or so.

I stayed by the boiler while one of these short cycles happened. The zone valve did not close (that I could see), it stayed "on" the whole time. So I think it is the boiler that is causing the short cycling. Water temp seemed OK (up around 170F, which I think is normal for an old boiler that does not vary the flame size). Pressure is ~15psi at 170F, again I think normal.

A couple things I can think of which might cause this:

1. Partial blockage of the heat exchanger on this loop. Such that it can't transfer enough heat and the boiler water gets just over its "limit" and the boiler shuts down. If so, it must be right on the edge, cause the minute or so of the cool down and startup purges is enough to lower the water temp enough to let the flame run again for a while.


2. Maybe a bad temp sensor on the boiler side so that it is shutting the flame off too soon (at a lower temp than it should), and with only this one zone calling for heat it is getting over this lower limit of this sensor?


3. Perhaps unrelated, but I do notice when the boiler first fires up after being off for a long while (like most of a day) there is a loud humming noise. It goes away after a min or two, and does not happen again on restarts withing an hour or two.


Unfortunately this boiler is not modern enough to have an display to pull error codes or display states, so no obvious way (at least to me) to see why these cycles are happening while the zone valve is on.

Looking for suggestions as to what else might be causing these short cycles, and other testing I might do to hone in on the cause.

Thanks

EdTheHeaterMan

That heat exchanger may be what we refer to as a micro zone. Your GV-5 has a gas input of 140,000 BTUh. After calculating the thermal efficiency and other factors for piping and pick-up loss the actual output is about 106,000 BTUh. Since you can not adjust that gas input or NET output, the only adjustment you have to reduce the amount of heat that the boiler will produce is to turn the burner off... then turn it back on. This is the way automatic heating has worked for over 100 years since Mr. Honeywell invented the first thermostat in Minneapolis. I just made that up LOL

You are experiencing the use of one really big heater on one really small zone. Say it takes 8 to 12 minutes for the cold boiler to get from 70°F water to 180°F water on a call for heat from that heat exchanger. Once it reaches 180°F the high limit shuts off the burner but the zone is not satisfied. So, the pump(s) and zone valve(s) stays open while the water temperature in the boiler drops below the differential of that limit control (might be 5°F or 10°F). By the time the post purge of the boiler is finished, that boiler temperature has dropped enough to close the contacts on the limit control so the pre-purge picks up almost instantly. After the ignition cycle is complete, the burner ignites and all 140,000 BTUh's of gas fire up and make that 170°F water get to 180°F within 4 minutes. And the burner shuts off and the cycle repeats until the call for heat is satisfied.

So if the burner burns for 4 minutes and the turnaround time for the post purge>to pre purge>to ignition>to flame… is about 4 minutes then you are operating at a 70,000 input burner rate. If your boiler was actually firing 70,000 BTUh then there would be no cycling at all.

The only way to reduce the short cycling time is to increase the load on the boiler
The closer you get to that 140,000 demand, the longer the burn cycles will be.

hot_rod

The heat emitter is driving the operating condition of the boiler, basically.

EdTheHeaterMan

To put it in sell words... YES. @hot_rod

SamTheMan

Thanks EdTheHeaterMan. The short cycling seemed to get better over the winter, but has recently gotten worse and I did some more "investigating". Here is what I found:

The water temps coming out of the boiler is ~150F when the short cycling happens. So does not seems that the boiler's overtemp limit is being hit to cause the short cycling, unless of course that sensor is faulty.

This original zone valve stays on throughout the short cycle, so it does not seem to be that the call for heat is going away.

Did a test with a different much bigger zone (1st floor heat) and still see the same results, short cycling with the water temp coming out of the boiler ~130F lower than the set temp (which is 170 or 180F as I recall). Again zone valve stays on thru the short cycle, so call for heat is not going away.

So I from these rather simple tests, I do think it is the boiler "deciding" to shut off the flame and short cycle. But I don't think it is due to the original "load" being too small and the boiler overheating, as this happens with a much larger zone too and the water coming out of the boiler is only in the 130-150F range. Unless of course the boiler's temp sensor is off or possibly reading a much higher temp at the heat exchanger than the thermometer at the water exit from the boiler.

So with these older boilers that don't have error readouts, how does was pinpoint the source of the problem? Could be the boiler's temp sensor is off and should be replaced. Could be some of the other sensors (air flow, etc) need replacing. Any thoughts on where to go next?

LRCCBJ

https://forum.heatinghelp.com/discussion/comment/1766296#Comment_1766296

I'm quite sure you are aware of this but we should look into the control and determine where the differential is set prior to concluding that the only way to reduce the cycling time is to increase the load!

I'll let you do it.😁

EdTheHeaterMan

Do you know which GV boiler you have? Series one (1) will not have a series number. All the GV boilers after Series 1 will have a Series number: 2, 3, and so on. Each different series is usually a change in the control system or some other design change This is the wiring diagram for series 1 & 2

and there are two temperature sensors that will cut off the burner while the call for heat continues. If you have a block temperature above 350° or the water temperature is above the adjustable setting on the water temperature limit, the burner will stop but the circulator will continue to operate. the flame light will then go out and the purge light will come on. I don't believe that this is the case for you, since the boiler water temperature is lower than the high limit setting. Double check the limit adjustable setting is at or above 170°

If your GV boiler is a different series, you may have a different issue.

EdTheHeaterMan

If you have the GV5 Series 3 or 4 then your control module is different and you have 2 circulators under the hood. the system circulator pumps the heated water out to the radiator system while the Bypass circulator keeps the boiler return temperature above 130°F to keep the flue gas from condensing inside the cast iron flue passages. That control works differently from the Series 1 and 2 control that operates only one circulator.

The current GV boiler is the GV90+ and is different from series 1, 2, 3, and 4. With the proper information on the boiler series number, ot the serial number that starts with CP xxxxxxxx I may be able to narrow down the place to look for your issue.

SamTheMan

Thanks again. I have a GV-5 Series 2 Boiler. Serial is CP 2851884. It was installed in July 1995. Converted to run on LP at that time, and had a hi altitude "pressure sensor" (diaphragm in a steel can with tubes coming out and a switch) put in too, since this is at 8300' elevation.

I do have a hi temp setting like the one you pictured above. A bit dusty but it was set to ~198F or so, just under 200F. Actually a bit too high as water boils at 196F up here. I did try turning the dial back and forth before setting it to ~192F. Its probe bulb is in the black cast iron "thingie" just after the circulation pump with like a 1-1/2" plug on the top (is that an output filter?), and seems OK. And the output water temp is not getting close to that set temp.

My changes did not make any difference, it still will go into the same short cycle pattern, repeatedly. I did watch and test a few other things this time:

When the flame goes out, the Flame light goes out and the Purge light comes on solid. During that time there is still a call for heat, and the circulation motor is still turning. And if I check that over temp switch, it still seems closed. (I measured resistance between the terminals when cold and off, and it was closed, eg near 0 ohms. I measure AC and DC voltage between the terminals when running and 0 volts, and it is the same during these short cycle shutdowns.) There may be something else to it, but I don't think this switch is at fault here. Or if it is it is just a momentary open that then goes away. I could try shorting the two wires that go to this switch and seeing if that is any different (while watching closely just for a test, of course) but I don't think that will make any difference. And at the end of this post-flame purge it shuts off for a second then starts another pre-purge/igniter/flame sequence.

The only other "self monitoring" that it looks like this boiler does is via that "pressure sensor"/vacuum switch. Its sensing tubes seem to go down to the gas line/blower. Have not gotten a mirror and seen where they terminate. I am not sure what that is trying to sense, but is it possible that is on the edge of acceptable, and every so often it crosses that boundary and shuts off the flame, initiating a purge cycle which is followed by a restart? Just a wild guess, but given that the blower is old and getting noisy at the start maybe air flow is marginal?

Any other thoughts, things I should check, or ideas?

EdTheHeaterMan

You are probably correct with the water temperature and block temperature switch. I do not believe either of those two switches are your problem.

Understanding the sequence of operation is the first step to diagnosing your problem, understanding how to use an electric meter to check for an open circuit is another. The OHM setting is for testing resistance and or an open circuit when the part your are testing is not connected to the circuit. For example if you test this circuit to see if this switch is open, with an OHM meter you will not get the resistance or open reading you are expecting. You are reading the resistance of the entire circuit that the switch is connected to. (the yellow wires from the source to the load, thru the load and back to the source)

The Red Dotted line indicates the electric path to the meter thru the load,

To check the open circuit on a circuit when it is operating you need to test the contacts that are in question with a VOLT meter as shown here:

If the contacts are closed then the volt meter will read ZERO volts as above

If the contacts are open then the volt meter will read the source voltage as below

If you have the meter connected to the switch you want to test, when the burner is operating, then watch the meter at the time you see the flame go out, you will see the meter go to 24 VAC if that is the switch that opens. It may only last for a second or two, but you will see it.

EdTheHeaterMan

As far as the sequence of operation is concerned,

• It starts with a cold boiler so all the limit and temperature devices on the boiler are closed.
• The fan pressure switch is open to indicate that the fan is not operating
• Then the thermostat (or zone end switch) calls for heat
• The control receives the call for heat and starts the fan to pre-purge (the fan will not start unless the pressure switch is open). The circulator will also operate with a call for heat
• Once the fan starts, the pressure switch closes indicating the fan operation is successful and sends that signal to the control.
• After the alloted time the control starts the trial for ignition, by sending power to the ignitor (hot surface ignition HSI will glow)
• Once the trial HSI is hot enough the control will send power to the gas valve to allow gas to flow to the combustion chamber, past the HSI
• At the same time, the power to the HSI stops and the HSI is then able to be utilized as the flame sensor
• Once the flame is established and the flame sensor indicates to the control that the flame is there then the trial for ignition sequence is terminated. and the burner is allowed to continue the flow of gas.
• If any of the following happens then the gas vlave closes instantly
  • High limit opens
  • Block temperatiure switch opens
  • Fan pressure switch opens
  • Flame signal is interupted
  • Loss of power to the control
• Then the post purge sequense starts.
• Once the post purge is completed, the start up sequence will start again at the beginning as long as the limits and fan pressure switches are in the start-up position, and there is a call for heat.

SamTheMan

Thank you for the details here. Assuming the hi temps switches have been put to rest (will try just jumpering the switch just to be sure), it seems it would leave either the fan pressure switch or the flame sensor as possibilities. Fan pressure switch seems like it should be straight forward to monitor with a DVM. I will do that. But I am not sure how to go about monitoring the flame sensor, any suggestions there?

hot_rod

The early GV boilers had a Hoffmann 3 way thermostatic valve inside, a blue colored cast iron valve, #1.

If you have one of those it could be sticking, and not allowing sufficient flow to the system, just recycling back to the boiler.

EdTheHeaterMan

Have you ever had a professional maintenance service? it is on page 19-20 of the manual https://www.weil-mclain.com/sites/default/files/field-file/gv-series-2-manual_1.pdf

You will need a meter that measures micro volts and micro amps for testing a flame signal. Not a normal tool for the homeowner. And I have rarely used one because I can usually find if the flame signal is there by other methods. If I suspect the flame signal is the problem, I clean the flame rod (not in your case because that boiler has no flame rod and uses the HSI for the flame sensor). ALSO the HSI is very fragile and will break if you try to clean it, The burner cone can be cleaned, and should be cleaned if there is a lot of debris or carbon build up on it. It should be replaced if there is any metal missing.

Another fix for a poor flame signal is to run a dedicated ground wire from the HSI mounting screw to the grounded mounting screw on the control. Look on the control for the mounting screw that has a metal connection the mounting screw touches.

SamTheMan

Well I believe the problem is fixed. I inserted two copper wire leads to the pressure switch electrical contacts. I monitored those wires to see when/if the switch was open or closed. Ever since I did that, no more short cycling. Basically it follows the sequence Ed gave above. Pressure switch is first open, then fan starts and switch closes, then purge and ignition follow. No more short cycling, water temps keep going up over time up to ~175F as it should, flame does not go off until call for heat ends. Basically working as it should.

Not sure what I did, because nothing in theory changed, I just unplugged the spade connectors on the pressure switch and reconnected them with a small piece of copper wire in them (to probe the switch with). Either there was some oxidation/corrosion on those connectors, or my unplugging/plugging movement jostled something inside the switch. And yes, it continues to work fine after the test wires were removed too. In any case, I am glad grateful for the help, and things are back to normal now.

Thanks to everyone!

Now that the boiler seem to be working normally again, it raises another related question, which maybe should get its own thread, but let me start here:

The original question I had before these issues, was should I invest time, parts, and money in this 29 year old boiler, or just bite the bullet and replace it. It's not been easy to get an unbiased opinion from the guys who repair/install boilers as they all seem to want to sell me a new unit. But this boiler (until this just recently) as been very reliable, and has seen little use during its 29 years. It is not used for central heat (except when we are gone on vacation, which is rare), there was a sidearm hot water tank but that was largely unused for the last 5 years as another water heater was put in place. So maybe the equivalent of one full winter heating and 24 years of DHW use. When I last had a pro look at the boiler, he said there was no sign of rust/oxidation in the water; and he did not take the blower off and check the burner cone but perhaps that was because he did not find soot in the exhaust (?).

I am curious what the folks here that have experience with these boilers think. Keep the old GV5 going or cut bait and go with a new possibly more efficient boiler? (The efficiency is "nice" but really that extra 7% will never pay for itself in fuel savings over the life of the boiler.) Thoughts?

Jamie Hall

Keep the old girl going as long as you can. Nothing new will be as reliable, and the fuel savings will never pay for replacement.

hot_rod

Reliability and parts availability. Two determining factors to consider.

EdTheHeaterMan

https://forum.heatinghelp.com/discussion/comment/1801470#Comment_1801470

@SamTheMan said "Well I believe the problem is fixed. I inserted two copper wire leads to the pressure switch electrical contacts. I monitored those wires to see when/if the switch was open or closed. Ever since I did that, no more short cycling. Basically it follows the sequence Ed gave above."…

"Not sure what I did, because nothing in theory changed,"

Sometimes the actual contacts between a spade connector and a spade tab will have corrosion build up over years in a damp space. Even if the space isn't always that damp. By removing and replacing a connector on a spade connection or a pin connection will remake the contact more securely. The friction of the wire moving in and out may have caused a better connection.

That is the second thing you should always try on an electronic system repair, like computers and sound equipment or control systems. (the first is to cycle the power off and back on… Or …"Is it plugged in") Remove the wire or circuit board and reinstall the wire or circuit board to remake the electrical contact/friction connection.

I believe that is what happened to you there. Good luck with your system. Hope that is the end of your problem. If Not, you know were to find us!