Another NEST Question ... Circulator Cutoff Happening?

FredoSP

Hi All,

I know many of you think the Nest is better for playing ice hockey rather then heating and cooling but if you could put that aside for a minute, I have a question about my setup. This "issue" has been going on since last winter and I think I've narrowed down why it's happening and I wanted the experts here to confirm (or not) and maybe offer advice on how to stop it.

Nest third generation tstat for heating only, hydronic system. I notice when there's a call for heating that the red light on the Taco SR501 would turn on for Zone 1. The radiators would make the familiar noise of hot water passing though. Roughly a minute later I would walk downstairs and look at the Taco SR501 and notice that the green power light is off AND the Zone 1 red light is also off. I looked at the pressure and temperature gauge on the boiler and noticed it was around 125 degrees. I then go upstairs and look at the Nest and it's doing a two minute countdown, strange. At the end of the two minute countdown it reads "Power Out, no power to Rh wire." Roughly five/seven minutes later the Taco SR501 will have the power returned to it and the green and red lights will be illuminated and all systems go.

After about two months of this happening (last winter) I made it my mission to find out what was going on. I was reading everything I could and came to the conclusion that the sudden drop in power to the Taco SR501 was the result of something called "circulator cutoff."

Current Honeywell settings
Low Temp Limit: 130
High Temp Limit: 165

The way I understand it, the Honeywell L4081B low limit/high limit control was reaching its low limit or below and cutting off power to the circulator for that zone and also the Taco (I guess that's how it's wired). Thus the boiler giving the domestic hot water priority UNTIL the boiler was satisfied (hot enough) to send power back to the circulator and Taco.

Does any of this make sense? Am I correct with my educated guess?
If so, do I just live with it? This really isn't a major issue because the circulator cutoff only lasts for maybe five or seven minutes.

What would be a way to make this go away? I've included pictures because I know that helps out. I was thinking of maybe adjusting the high and low settings on the Honeywell aquastat but I was reluctant and decided to write this post.

Alan (California Radiant) Forbes

The Honeywell aquastat will not interrupt power or communication between the the NEST and the SR501.

Is your NEST wired to Rh, W and C on the SR501?

Jamie Hall

You're fundamental diagnosis seems quite reasonable. However, to be really sure one would have to look at the complete wiring. The domestic hot water priority call should, indeed, cut power to the circulator. Problem is the way you have it wired the aquastat cuts power to the whole control board -- not just the circulator. Which, of course, cuts power to the Nest -- which promptly dies after it's tiny battery goes flat. I'm not really familiar enough with the Taco to suggest a wiring change -- but a brief look at the wiring diagrams Taco provides for it suggests rather strongly that you may also need a 504 control to manage the priority switching.

Zman

I would not use the nest for a hockey puck, it would just shatter and screw up the ice. Whacking it with a hammer works nicely though

How is your nest wired? The green light on the 501 should never go off, I would start there.

Sometimes it helps to take the t-stat out of the equation. If you disconnect the nest from the SR501 and put a jumper wire across the "T-T" terminals, does the heat come on normally?

FredoSP

Thank you all for the suggestions. I'm leaning more towards the wiring at this point. I'm going to add a picture of the inside of the Taco, which I wired from a cross reference chart provided by Taco. The Taco was a direct replacement for an old Honeywell RA89A step down transformer, so I could have a Common wire going to the Nest because the RA89A had no common. I personally think the Nest likes a C wire so that's why I purchased the Taco.

From my pictures, I can see each circulators' electrical connection goes to the red old burner switch. Above that it's getting power coming in from a junction box. I'm sure those would be the places for me to explore.

If I understand things correctly, the Taco's job is to turn on and off the circulators when a call for heat is made. The aquastat's job is to control/maintain the water temperature (high and low limit) in the boiler and only tells the oil burner to fire or not. I'm not the person who designed or installed the system so that's why I'm trying to understand how these components work.

Alan (California Radiant) Forbes

If the power for your SR501 is coming from the junction box, turn off the power and make sure all your connections are tight. As others have said, the green light should always be on.

If the power is coming from some other control, it could be that power to the SR501 is switched off by that control and you never noticed it before with the old thermostat which didn't need continuous 24 volts.

HomerJSmith

You most probably have it mis-wired. The green power led should be on all the time. The red led for zone 1 should turn on when the thermostat calls for heat and at the same time turns on the circulator.

I can't see the wiring on the 501 but I suspect that the circulator is powered by the honeywell and the low temp adjustment is keeping the circulator from turning on because it senses the boiler water temp is below the setting.

If the 501 is powered by the honeywell and the circulator is powered by the L4081B then you would lose power to the 501, the circulator, and the thermostat as the low relay wouldn't power the relay for the circulator until the boiler water was above set point on the low setting. That is why you are losing the power to the circulator until the water heats up.

The Fix:
Power the 501 from the red burner power switch box instead of from the honeywell and have the 501 power the honeywell.

I see one flexible conduit from the 501 to the burner switch and one flexible conduit from the burner switch to the honeywell, so the wiring connections are done in the red burner switch box.



The wiring would remove the L4081B circulator relay out of the circuitry and the circulator would turn on when the Nest calls for heat and turn off when the Nest reaches set point. It would not be governed by L4081B, but by the 501. When I said, "wiring to remove circulator relay from the circuit", I meant the L4081B low relay.

The alternate wiring would allow the L4081B to turn on the circulator. The SR501would be powered at all times and control the L4081B, turning it all on and off. You would have a delay turning on the circulator until the boiler water temp was above the low setpoint.

The Nest C connection would be connected to the Common on the SR501.

I hope this is right and that it helps. Easy Peasy.

HomerJSmith

A whole bunch of photos just showed up on my computer. My drawing is probably correct, however where the connections are made could be different.

You have 2 red burner switch boxes? Closeup pics are great, but pics further away tells me how everything is connected.

HVACNUT

You'll need a constant hot (120v) to power the SR501. 
Remove the jumper between H and 3.
Provide a constant 120v (after the service switch) to H.
Connect the hot from the L4081B to terminal 3 on the SR501.
This way the control will always be powered, therefore the Nest will always be powered. And the circulator will still shut off when boiler temp drops below the low setting. 

FredoSP

Hi All,

Thank you for all the suggestions.

I'm not sure if this means anything but I forgot to mention that I have another "vintage" tstat that is line voltage and is probably wired directly to one of the circulators? I don't think this has any effect on the low voltage Nest tstat because I don't think any of those wires come on contact with Taco SR501. I'm not sure if this new piece of information has any effect of the suggestions that were already given to me.

I also have another emergency shut off switch at the top of the steps going to the basement. I thought that was "odd" but then remembered something I read online that if there was a fire in the basement, this switch would prove useful so I won't have to run through flames to shut off the other emergency switch by the burner - maybe its code in my area?

I've also included some pictures of the system from a couple of feet away so it helps give a better overall view.

Zman

My first guess is that your 501 is getting it's power from something switched. Where does it get it's power?

HomerJSmith

Zman, of course you're right, but the question is what and why. I think the L4081b low temp is controlling the SR501 rather than the 501 controlling the L4081b, but I don't know everything or I wouldn't be making so many mistakes.

HVACNUT

The OP showed that the 501 is wired through the load side of the Lo, or Circ. side of the L4081B. That's is a N.C., open on temp drop contact. 
On the 501 he needs to remove the jumper between H and 3 and run a new constant 120v to terminal H. Everything else stays as is.

HomerJSmith

Lo, or Circ. side of the L4081B controls the circulator. The question is whether one wants the L4081b or the SR501 to control the circulator. I address that in my drawing.

Constant power to the SR501 per my drawing.

HVACNUT, my drawing shows that. One would remove the both jumpers, the L4081B between the Hi cutout relay and the low cutin relay jumper and the SR501 H to 3 jumper.

Mcxstar01

Hey guys - 

I’m in a similar boat to FredoSP. I have two zones with a Honeywell aquastat sending power from the burner switch to the circulator relays (SR501 for one zone and a legacy Honeywell for the other zone) for the two zones, the circulator relays then respond to a demand from each zones thermostat to energize the circulator pumps.

I was going to perform the fix proposed by HomerJSmith and rewire to have the aquastat  between the SR501 and circulator instead of having the aquastat between the burner switch and SR501. 

Before doing so I have a question, why do I need that aquastat controlling the zones? In theory I think I know why… you want the boiler/water at temperature before sending it through the zones. But I have another aquastat on the boiler keeping the boiler at temperature by controlling the burner. If that aquastat is doing it’s job to keep the boiler at temperature then the water should be at the right temperature to go to the zones - am I wrong?

Welcome the feedback guys, thanks!