Short Cycle on MZ25s
After many years of excellent service the boiler started short cycling. The DHW is still hot and the house warm. But, the boiler is starting every thirty or so seconds and running for only part of the purge cycle or a few seconds of burn unless we need heat. When I disconnected the DHW thermostat it stopped short cycling. Do DHW thermostats go bad?
I doubt that the DHW piping is dumping enough heat into the room to make it cycle. I did add a little very well insulated DHW buffer tank this summer and the boiler was recently switched over to the winter setting. It does the same thing on the summer setting now anyway so that is not the problem.
The DHW has been great, but could the tank siphon fast enough to defeat the delta? No, that makes no sense since it worked OK all summer long.
The thermostat part number in the manual is not a part manufacturer's number so I cannot find a replacement easily. Monitor wants to hook me up with a dealer instead of taking my money. Very professional of them.
I pulled the thermostat and there are tiny little markings but nothing like a manufacturer's name that is visible. BTW, the bottom few inches of the probe were coated with a black wet powder. Is that significant?
Well, it turns out that the DHW tank was partially full of air. The "Automatic air vent" is no longer automatic. The system still short cycles after manually bleeding the air out of the tank. This tank is full of water from the boiler and the DHW is heated by a finned coil that passes through the tank. The tank was only partially full of hot water but the DHW was still hot enough since we have a tempering tank.
Sure, thermostats/aquastats go bad after many years of service- it's the nature of electromechanical parts.
Time is money, and they don't want to waste it talking a homeowner (who may or may not have a clue) through a debug, but the dealer might have that type of patience- call them!
What does the suspect part look like? (Can you post a pic?)
Attachment 14476The front of the thermostat is covered by a white plate that is part of the boiler housing. There are two screws holding the unit to the plate. Between them there is a dial with Degrees C from 30 to 90 (30 is at the bottom and 90 at three o'clock) in ascending order clockwise around a brass flat screwdriver slot that is slightly eccentric so that it is possible to tell which degree mark towards which it is pointing. Around the brass slot, there is a grey metal ring. The control is about three centimeters square and 1.5 deep. There are three male spade terminals on the bottom. (Only two are in use.) On the back is one Phillips screw. The sensor is a one meter or so long copper tube about 2 mm in diameter. It may have a bulb on the end of the tube since something hard hits the inside of the fitting on the tank when the tubing is pulled.
Photo? There is not much to see unless I pull it out again, and those screws are tiny and it is late. It is taking so long to transfer the photo I will post it later if/when it comes from the camera.
Well, the air vent and thermostat were ordered today. I also ordered the realllllly expensive water treatment. Apparently, if you don't use the manufacturer's chemicals, you have no warranty on the boiler chamber.
A boiler installer told me today that the boilers in Europe are replaced every seven or so years. I truly hope that is not true.
Nonsense, think 20-25 yrs. with professional annual service as mandated by most European municipalities.
The new thermostat and air vent are installed. Two liters of treated water were also added. The boiler is doing the exact same thing.
What is the same thing, you ask?
First, the thermostat calls for hot water, then, the boiler starts the purge cycle, then, thermostat stops calling, Next, boiler shuts off. Sometimes it only takes a second or so, sometimes longer.
Removing the thermostat from the circuit ends the cycling. Shorting the leads to the thermostat brings the boiler on normally.
Perhaps the thermostat bulb should be higher in the tank? Not that it was moved when this all started to happen, but now I know it is at the bottom of the tank since I put it there.
I am at my wits end with this and am afraid that the boiler will wear out from all of the start-and-stop.
Thanks for the good news. How old is the oldest MZ that you saw?
The boiler is now wired so that the heating circulator pump runs whenever the boiler is in winter mode. It was wired so that the circulator only ran when the thermostat was unhappy. Now the short cycling is not happening as often.
The big difference between this year and the last years was the installation of a new highly insulated tempering tank on the domestic hot water that I mentioned before. The tank is equipped with a thermostat of its own. The tank's thermostat is controlling a circulator which moves the tempering tank's water through the domestic hot water coil in the boiler. There is a check valve to prevent cold water which also enters the coil at the same point from flowing into my home's hot water piping. The only time I have seen the tempering tank's circulator on was when the boiler was off during the time it was being rewired and a lot of hot water was used. The tempering tank is around twenty gallons. Oh, I almost forgot - the tank and boiler have a tempering valve between them and my home's hot water pipes.
Do you think any of the connections could be causing the water in the coil to cool enough to create the short cycling?
How old...any answer -even from a full-time condensing boiler service man - would be anecdotal. Full time space heating pumps are fine, but should cycle off when the indirect water heater calls for heat. Try to find a company that specializes in high efficiency condensing boilers. Contract them to service your boiler every year in the spring - thus avoiding the mad fall rush usually occurring after the first frost, when most folks realize that it is going to get cold again.
Thanks for your suggestion.
I opened and cleaned the burner chamber and it looked good. All of the fittings are tight. Replaced the air valve on the DHW tank, the tank's thermostat too.
What should the Honeywell VC8111.11 actuator for the three way valve look like when it operates? The plastic indicator only moves a little (about half way in the slot) when it switches the valve. Is that normal? I think it always moved like that.
I'm thinking of moving the tank thermostat's bulb higher in the tank so that it is not so close to where the cold water pipe enters the bottom. Hopefully the buffer tank will be able to temper the initial cold pulse from the delay that might be caused.
I replaced the Honeywell valve and the boiler is working fine now. The old valve was gummed up so I cleaned it and am reserving it as a spare. It was sticking part way and dumping heat from the boiler into the house every time the DHW thermostat stopped calling for heat.
I also changed the automatic air vent again. I think the water treatment chemicals were gumming it up as well. There were quite a bit of tan crystalline deposits on the automatic air vent port. I added a plastic hose to the bucket under the TPRVs and the automatic air vent's port is staying clean now. The moisture is preventing the formation of crystals.