Taco SR501 not turning off consistently
The upstairs zone would not turn off after the set point was reached and then exceeded. Disconnected the two wires from the thermostat and it continued to power the pump. Purchased and installed a new Taco SR501. With the thermostat reinstalled the pump ran when the setpoint was raised above the room temperature, but again it did not turn-off and kept heating. Read Note 1 on the SR501 instructions indicating that some digital thermostats using only a 2 wire connection need a 1000 ohm resistor between the thermostat white wire and the common to function properly. After installing the resistor everything seemed to work fine at firsyt, until it didn't, again it kept the pump running and overheating. This is now an intermittent issue, as sometime it works and sometimes not. Any ideas would be appreciated.
Comments
-
Morning Jack, , please give Taco Technical Services a call during normal business hours Mon-Fri 8am-5pm EST 401-942-8000 and just ask for Technical Services.
0 -
To finish the troubleshooting, disconnect the t-stat wire from the SR501, if it stops, something is up with the wires from the SR501 to the stat.
To double-check that, put a jumper across the R and W wires (remove the resistor), does it turn on? disconnect, does it turn off?
Dave Holdorf
Technical Training Manager - East
Taco Comfort Solutions
1 -
As I have said elsewhere, the resistor trick is a very delicate kludge trying to balance giving the thermostat enough voltage and power to continue operating vs. convincing the controlled device — the SR501 in your case — that the control circuit is either open or closed. 1000 ohms does seem to work sort of most of the time, but it isn't magic. If you can't supply a dedicated common to the thermostat for some reason, you can try putting other resistances in in place of the 1K resistor and varying the resistance until you find a value which keeps the thermostat happy and still convinces the SR501 that it is seeing a genuine open or closed circuit. To avoid having to connect and disconnect a tray full of resistors, you could connect a substitution box or potentiometer with enough capacity (several watts) until you find the sweet spot and then cut in a fixed resistor of that capacity — or assume that, with time, the sweet spot will drift as components age and just leave the pot. in there to fiddle down the road.
Br. Jamie, osb
Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England0 -
Hello Steve,
I gave Taco technical support a call and they indicated that they have never seen this problem before. Problem started with existing unit and remains the same with new unit so something else is going on.
Thanks!
Jack
0 -
I had done those checks before purchasing the new unit and had convinced myself it was the relay box. When disconnecting the t-state wire from the SR501 it continue to run the pump, red light remained on. However I will try the t-state jumper right at the box to be sure, without the resistor.
0 -
Yes, I thought the resistor was the magic bullet as initially after installing the SR501 it wasn't shutting off properly (remained on even with one thermostat wire remove at the SR501). After installing resistor it seemed to function properly, for a day or so anyway. Same problem with two different thermostats btw.
0 -
Is your problem solved?
0 -
what boiler and thermostats do you have
0 -
" red light remained on"
When there is 110Vac power to the SR501 the red light will be on. When the pump turns on for a call to heat the green light will turn on, I believe.
Check your wiring and the polarity of the connection to the sr501 and pump. The pump"s hot lead is switched by the 501 relay not the neutral. Both the pump and the TT connection should be wired to the NO (Normally Open) connection on the 501 relays.
Does the boiler run with the pump when you have this condition? You can check the terminal box on the pump and make sure that wiring is correct. There is a typical wiring diagram, this just shows the NO and power to the pump connections.
OR
0 -
Did you test the control ONLY before condemning it? If you did, you may not have condemned the control, and you would not own two working controls that have the same problem. The attached wires/thermostat/pump/burner?!?
I try not to fire the parts cannon until I know where the problem actually is.
This illustration is an example of some old wiring inside walls, wrapped around pipes and just old nasty insulation that is falling off from age. If you test the control at location A you will eliminate the thermostat only as the problem, but you may not find any of the problems that are hidden in the walls. If you test at location B you eliminate any nasty wiring AND the thermostat. You would find that the SR501 control was just fine.
Edward Young Retired
After you make that expensive repair and you still have the same problem, What will you check next?
2 -
No way two SR501 controls are bad. There's a short somewhere.
1 -
@EdTheHeaterMan I like that illustration above and think it should be posted somewhere on this forum for instructional purposes. I don't think the physical wiring is often considered when troubleshooting. It can be one of the harder parts to replace though. Good job here.
2 -
I have a place where I look for my old wiring diagrams. I will post it there so I can find it in the future
Edward Young Retired
After you make that expensive repair and you still have the same problem, What will you check next?
1
Categories
- All Categories
- 86.3K THE MAIN WALL
- 3.1K A-C, Heat Pumps & Refrigeration
- 53 Biomass
- 422 Carbon Monoxide Awareness
- 90 Chimneys & Flues
- 2K Domestic Hot Water
- 5.4K Gas Heating
- 99 Geothermal
- 156 Indoor-Air Quality
- 3.4K Oil Heating
- 63 Pipe Deterioration
- 916 Plumbing
- 6K Radiant Heating
- 381 Solar
- 14.9K Strictly Steam
- 3.3K Thermostats and Controls
- 53 Water Quality
- 41 Industry Classes
- 47 Job Opportunities
- 17 Recall Announcements