Oil fired boiler keeps cycling on and off after Nest calls for hot water baseboard heat

ericmhurtado

Hello,
I’m having a problem with the hot water heating side of my oil fired steam boiler. When the Nest for hot water baseboard calls for heat, it starts then shuts off after a minute, temp gauge reaches ~100, then cycles on and off until thermostat is satisfied but temp gauge never reaches high limit aquastat set point of 180. BUT when the thermostat that controls the steam side is satisfied, and THEN the thermostat for hot water baseboard calls for heat, the circulator runs, burner is off and the temp gauge increases to 140 before burner turns on for a minute, turns off and then starts to cycle until thermostat is satisfied. Why would this be happening? Could someone please help me or guide me in some direction?
I have a WM SGO 3 oil fired boiler, tankless coil, L4006a1967 low and high limit aquastat, low set to ~100 (getting dhw from electric heat pump water heater), high set to 180, Ra89a relay- please see posted pics
Thank you for any and all the help that you can provide

Jamie Hall

Pictures are good -- but since this a control wiring problem (and possibly interface problem) could you also post a complete wiring diagram?

ericmhurtado

Ok. Which wiring diagram?

ericmhurtado

Here are the wiring diagrams for the boiler and burner

Jamie Hall

Need to see how all the controls -- especially the Nest, which can be very problematic -- are wired together. All the wiring.

ericmhurtado

There are just two wires to the Nest, Rh and W, but I believe this could be the problem. I believe it is powered through a 24v transformer in the relay so it should work even without a “c” wire. But the transformer may not be providing enough power when the relay is switching on the Nest, circulator and the burner and it then shuts off and starts to cycle? Is it ok to replace the Nest with another thermostat that runs on batteries, such as the honeywell rth2300b, just to see if that fixes the issue?

ericmhurtado

I will try and get together all the wiring but bear with me as I’m not sure how everything is connected

Zman

If you want to eliminate the nest as a potential problem, just jump the wires and see if the problem persists.

Jamie Hall

Zman said:

If you want to eliminate the nest as a potential problem, just jump the wires and see if the problem persists.

Or, as you suggested, take the Nest off and substitute a good battery powered one -- Honeywell makes a few -- if you need the functionality or a good mechanical one... if you can find one any more.

HVACNUT

There should be a separate category for Nest problems.

ratio

Most Nest problems can be solved by using something else.

ericmhurtado

The nest is not the cause. I replaced it with a battery operated Honeywell and same thing happened. Now I’m thinking it could be the high limit aquastat. I plan on switching the high limit with the low limit and see if that solves the problem. Anything that I’m missing that would make it easier to troubleshoot? Thanks for all the advice.

SuperJ

If it heats until 100F and then cycles it sounds like maybe your using the normally closed instead of normally open contacts on the aquastat.

On a heating call as long as the call is present with your aquastat settings, the burner should start if the water drops below 100F, and shouldn't stop until it hits 180F or the thermostat satisfies.

The power stealing nature of the Nest can be a problem. I would recommend a C wire. There is a hack where you add a resistor to the W wire to C to dump some of the power stealing current. This can sometimes cure some weirdness from the Nest.
I don't have the details on what size resistor to use though.

Lance

It would be wise to consult the electron wizards. There are two I recommend; Votare and AmPiere.
Transformers get weak. On old two wire systems many of these had transformers with only 10 -20 Watts of power. I would look for a voltage drop during operation and monitor the amps which should be in the .2 to 1.2 range. Voltage drop can be caused by weak transformer, poor wire connection, a cracked wire, or even a shorting between wires that are activated. Especially the old cotton insulated wire which does not do well after 40 years or more. I have even seen undersized 20 or 22 ga wire over a distance rob power in the circuit. 30 feet distance of two wire stat cable equals a 60 foot circuit. upgrade to a 40 watt transformer can help sometimes with added loads. Remember you touched an old system. Anything could go wrong just by touching it. Good luck with your mystery.

ericmhurtado

Thanks to all for the helpful advice.
What I can’t seem to understand when putting together a wiring diagram is how the relay connects to the high limit aquastat? I see the circulators and the thermostat wires connected to the relay but not to the aquastat. What am I missing here?

HVACNUT

Your not missing anything.
Looks like the circ is energized on a heat demand and runs until the zone satisfies.
The aquastat will cycle the burner.

ericmhurtado

Thanks HVACNUT for sorting out my confusion.
But how does the aquastat or burner know that there is a call for heat? Through the boiler water circulating through the aquastat? Depending on the temperature, that would then trigger the aquastat to turn on the burner?

Jamie Hall

ericmhurtado said:

Thanks HVACNUT for sorting out my confusion.
But how does the aquastat or burner know that there is a call for heat? Through the boiler water circulating through the aquastat? Depending on the temperature, that would then trigger the aquastat to turn on the burner?

Exactly

ericmhurtado

Thanks Jamie. Does this mean that I can rule out the Honeywell RA89A relay as it doesn’t directly control the high limit aquastat? I’m trying to find out why the high limit shuts off way before it reaches the 180 degree set point.

John Mills_5

Usually unless you have cooling on a Nest too, running without a common doesn't work. These are power stealing and need heat & cool both.

ericmhurtado

SuperJ said:

If it heats until 100F and then cycles it sounds like maybe your using the normally closed instead of normally open contacts on the aquastat.

SuperJ, I’m not sure I understand. Are you saying that the wiring on the aquastat is reversed?

ericmhurtado

I think I have made some headway here. I have disconnected the low limit aquastat wires from the same terminals that connect the high limit and steam thermostat wires to the oil primary control, a Honeywell r7184p. Now the high limit behaves like a low limit and maintains a 180 temp because it’s connected to a terminal that is for “temperature control for tankless heater only” according to Honeywell wiring diagram. That’s why I thought it was short cycling. Now if I can find the right way to rewire the high limit to make this a cold start boiler.

Robert O'Brien

ericmhurtado said:

I think I have made some headway here. I have disconnected the low limit aquastat wires from the same terminals that connect the high limit and steam thermostat wires to the oil primary control, a Honeywell r7184p. Now the high limit behaves like a low limit and maintains a 180 temp because it’s connected to a terminal that is for “temperature control for tankless heater only” according to Honeywell wiring diagram. That’s why I thought it was short cycling. Now if I can find the right way to rewire the high limit to make this a cold start boiler.

You have a tankless coil for domestic? It can't cold start

ericmhurtado

I don’t need the tankless coil for dhw as I have an electric heat pump water heater. I’m assuming I can still cold start. Any reason why I can’t?

HVACNUT

First, the 2- L4006A aquastats are BOTH high limit aquastats. Break on temp rise, make on adjustabe drop differential.
Why there's one on the primary loop, I have no idea. I dont think the installer did either.

Turn off the power.

Remove the wires from the aquastat that's installed in the same pipe as the circulator. And remove the aquastat.

Remove the wires that come from that same aquastat at the Honeywell R7184P.

I assume the RA89A energizes both circulators with a heat call from the Nest. I only see one wire on terminal #4 so I guess the circs are spliced in the switch box?

You need 3 thermostat wires between the switching relay and the Nest. I think I see blue and green. Any chance theres a red and white tucked in there somewhere?

Remove the (obsolete) RA89A and install a Taco SR501.That will give you the 24v common you NEED for the Nest.

The L4006A in the coil stays at 160° (180° if the living space doesnt heat well). You can lower it when the heating season is over.

Inside the aquastat on the right side is a white wheel. Set it to 25. That's the differential.
This will keep the boiler at 160°. When the boiler temp drops to 135°, it will turn the burner back on to make 160° again.
It's not cold start, but with the steam on, it wont go cold anyway.

Even though your not using the tankless anymore, you should keep some temperature on the boiler, even when the heat is off. A warm oil CI boiler is a happy boiler. When the heating season over you can set the aquastat to 135° with 10° on the differential.

ericmhurtado

Thanks for the prompt response @HVACNUT
Here are my responses to your questions:
> @HVACNUT said:
> I assume the RA89A energizes both circulators with a heat call from the Nest. I only see one wire on terminal #4 so I guess the circs are spliced in the switch box? Yes as both the red B&G and green Taco circs energize on a heat call
>
> You need 3 thermostat wires between the switching relay and the Nest. I think I see blue and green. Any chance theres a red and white tucked in there somewhere? Yes there is actually. Unused

This maybe a stupid question but are there different sections for the water in the boiler thus the two aquastats? For example, the low limit would keep the boiler water IN that section at say 120 and for the high limit at say 180? No right?
Like I said a stupid question but please confirm this for me. Thanks

Jamie Hall

There are no stupid questions -- though there may well be stupid answers. But no, all the water in the boiler is in one big bucket. Two aquastats -- ore even more -- may be needed sometimes, since one may be a high limit, or set for domestic hot water, while another may be a low limit, or set for your hydronic heat, and so on.

HVACNUT

Not stupid at all. There are no stupid questions, just stupid answers.
Like I said, they're both high limit aquastats. There is no low. Theres only the differential for the aquastats and the pressurtrol. But the pressurtrol is on a different circuit.
The installer failed at his attempt for temperature control in the HX primary loop in and out of the boiler.
All switching is in parallel back to the primary. Both aquastats and the steam thermostat.
The aquastat in the piping is useless the way its wired. It should have been wired to N.O. dry contacts in the switching relay, if the RA89A had that option, which it doesn't, to shut off the Taco circulator alone at high limit.
But because everything is wired in parallel, the boiler is either making steam or maintaining 160° from the coil aquastat so the temperature in the primary loop is gonna be what its gonna be regardless of its setting. Unless the setting is higher than the setting in the coil aquastat.
You've got your third wire for a common to power the Nest.
Do as above and wire the SR501 using the "cold start" wiring diagram in the instructions. It will be basically the same as the RA89A.
Couldn't hurt to get some new 18/2 thermostat wire, some plastic zip ties and reroute the wire from the aquastat to the primary to get it away from the boiler front plate.

@Jamie Hall Didnt mean to step on your quote. I started this at 5:30. Played with the dog, coffee with the missus. Time flies.

ericmhurtado

I can’t thank you guys enough for all of your help.
I’m going with your advice on getting a taco sr501.
To confirm: wire common on Nest to the 24vac/comm screw on the sr501? Wire the aquastat to the 5 and 6 on the sr501?
This setup would appear to solve a lot of my issues all at once. I can get a common for my Nest, aquastat ONLY controls burner on a call for heat and I can turn off the boiler in the summer. After having it thoroughly cleaned.

HVACNUT

Nice!
5 and 6 on the N.O. -[ ]-

ericmhurtado

And the only thing wired to the oil primary would be my steam thermostat? Does the sr501 have a 24v transformer and would it need one for the nest?

Thanks

SuperTech

> @ericmhurtado said:
> And the only thing wired to the oil primary would be my steam thermostat? Does the sr501 have a 24v transformer and would it need one for the nest?

The SR501 has its own transformer and has a 24 volt common terminal to power up a thermostat. I recommend choosing a different thermostat than the Nest.

ericmhurtado

Ok thanks @SuperTech . I have read that they’re better suited for a forced air system rather than a steam/hydronic one like mine. Unfortunately the nests came with the house and I’m not willing to spend ~$200 and up for a similar one and I’ve gotten used to them.

pecmsg

ericmhurtado said:

Ok thanks @SuperTech . I have read that they’re better suited for a forced air system rather than a steam/hydronic one like mine. Unfortunately the nests came with the house and I’m not willing to spend ~$200 and up for a similar one and I’ve gotten used to them.

Then I see continuing issues!

NEST stats have nothing but problems. Do a search for NEST Issues or better yet just Replace it!

Jamie Hall

And you don't have to pay as much as you quote for a good, programmable thermostat. Ditch the Nest and head to the big box store -- or Amazon -- and get a usable thermostat.

pecmsg

https://www.ecobee.com/

SuperTech

The owner should be the only "smart" thing involved in control of the boiler. I haven't had any issues with the ecobee or Honeywell thermostats if you want the ability to access it from your phone.
I prefer a basic non programmable digital thermostat over the Nest for boilers, even the mercury T87 yo yo thermostat is a better choice once you set the heat anticipator.

STEVEusaPA

HVACNUT said:

First, the 2- L4006A aquastats are BOTH high limit aquastats. Break on temp rise, make on adjustabe drop differential.
.

Probably.
But either way, I would never send 2 thermostat wires to a primary control, nor have them that close to the front door of the boiler where heat will affect them, and maybe even melt the coating.

Op needs to cut the pipes going into the coil and NOT cap them both. If the coil is leaking, remove coil and put on a new blank plate with new gasket.

But the whole wiring issue is suspect to me. The only safety is on the pressuretrol which may not shut down the burner if the zone is calling, or may not turn off the circulator if it's steaming.

OP should get someone over there. Despite the good advice received here, sometimes you just need to pay a pro to come over and fix it right. Sometimes the best 'help' is to get some 'help' in person.
I would not want a runaway boiler, or a circulator sending very cold water into the boiler while it's steaming.