Transformer on old Taco SR506-EXP too small for smart thermostats

m11k

I have an older Taco SR506-EXP (2005-era) switching relay, with 4 active zones (all for baseboard heating, DHW doesn't use this relay). I'm replacing my old thermostats with Ecobees. I've run new 18/5 wire to all of the thermostats. The issue is that the SR506-EXP only has a 15VA transformer on it. The ecobees need 3.5VA per thermostats, so 4 of them means 14VA. I have a couple of options to move forward and I'm not sure which is the best. I haven't had good luck getting any useful information from Taco customer support (via email).

My options:

* Replace the old switching relay. I cannot for the life of me find the VA rating of the new SR506-EXP-4 (or the SR504-EXP-4, which would be sufficient since I only have 4 zones). I've contacted TACO support but have only gotten back links to the online documentation, which *does not* specify the transformer rating in VA. I see that the Tekmar/Watts 306P does claim to have a 40VA transformer, which should be enough to power the 4 ecobees. I would rather buy another Taco relay, but not if the transformer would still be insufficient. This has another advantage of allowing me to turn on the pump exercise mode, which should help keep my 15 year old circulator pumps operating.

* Add a second transformer to power the thermostats. Since I've run 18/5 wire and I'm not using air conditioning, I can use 4 wires to run the thermostats: Connecting Rc and C to the new transformer to provide power, and connecting Rh and W to the SR506. This is somewhat logistically messier for me because in order to add a new transformer, I need to pull out the hammer drill to mount some more plywood to the basement wall, add a 4x4 electrical box, wire up the new transformer and add an inline fuse for protection.

* Add a second transformer and disable/bypass the existing transformer on the SR506. This would power the SR506 using a new 40VA transformer instead of the built-in 15VA. I don't know if this is possible. The specs for the old SR506-EXP do state that it supports "24 Volt Power Input or Output Terminal" (https://fwwebbimage.fwwebb.com/ProductInfo/101-052.pdf) however I cannot find any information on how to use the 24VAC screw terminals as power input instead of output. The only advantage of this approach is that it would allow me to use the traditional 3 wires instead of 4 to power up the thermostats (maybe less confusing for a future homeowner when replacing the thermostats).

* Add a second transformer and connect the new thermostats using isolation relays. This would be another way to use three wires instead of 4 to power the thermostats. I can use an isolation relay like the Emerson 90-380, but I would need 4 of these. I'm not a huge fan of this idea because I shouldn't need it (the Taco SR506 *is* effectively an isolation relay), and each relay has a coil current of 125mA, so 4 of them would add 600mA, or about 12W to the power draw of the system, just for the 4 relays. That's about the same amount of power as the ecobee's themselves need.

What do you think I should do? The cost of either of these options isn't really a big factor. I've spent far more than the cost of any of these parts in time just googling for suggestions and information.

Thanks,
Mike

mattmia2

You could do the separate transformer and get the kind that mounts to a knockout and mount it to an existing box and knockout on the boiler.

A schematic of your existing zone control would tell all. Probably can disconnect the internal transformer and add an external one to the in/out terminals.

Some of the newer Taco zone controls can take a second transformer, it just plugs in inside the enclosure, this is probably why the controller has no power rating.

EdTheHeaterMan

You could remove the 15 VA transformer from the existing SR50x relay and replace it with a 40 VA. Add a 3 amp fuse while you are at it. There are so many different models and configurations of that part, I'm sure you can find something that will work. Just snip the existing 15 VA transformer primary and secondary wires from the solder connections on the printed circuit board. Connect the primary of the new transformer to L1 and Neutral. the terminals in the red circle here
Then wire the secondary to these terminals in this picture indicated in red.

That is the simplest, Least amount of additional rewiring.

It is important to get the R and the C and the W wired correctly. Here is a Taco Tech support guy with info on the correct way to do that: https://forum.heatinghelp.com/discussion/comment/1706103#Comment_1706103

Hope this helps.

m11k

Thanks for the suggestion @EdTheHeaterMan. I was a little concerned about sufficiently snipping the transformer legs and insulating them to prevent any possibility of arcing.

I wonder if that is even necessary. I also found this post: https://forum.heatinghelp.com/discussion/comment/1645512/#Comment_1645512. That is referring to the SR506-EXP-4 (whereas I have the SR506-EXP) so maybe it doesn't apply, but the diagram attached to that post from Taco would seem to imply that I can connect a second transformer to Rh and C of the new thermostats, and use Rh and W to connect to the inputs on the SR604, without needing to disconnect the old transformer. This would use the new transformer to drive the thermostat, and the old transformer to drive the relays. I was worried that I shouldn't have the two transformers connected electrically in any way, but maybe this is okay. From my 20 year old memory of electrical engineering classes in college, I think it should be okay if the two transformers share one leg. In theory it is only if they share both legs that any difference in voltage would be a problem.

Still interested in knowing the VA of the new SR506-EXP-4, since I wouldn't be opposed to upgrading this 15 year old board anyway.

Also including some pics of the current setup (thermostats are not yet connected).

Tim McElwain

Ed the HeaterMan I am not sure if TACO would authorize altering there relay

EdTheHeaterMan

Tim McElwain said:

Ed the HeaterMan I am not sure if TACO would authorize altering there relay

Technically you are correct. But it seams the warranty period is over and I have repaired these older relays in the past in this manner. A 40VA transformer on the service truck gets the heat up and running on a cold weekend night when the supply houses are all closed.

If it works for a repair, then it will work for a thermostat upgrade!

I always add a 3 amp fuse to the R post or wire on the secondary of those replacement transformers, they help keep the smoke inside those transformer windings.

SteveSan

@m11k I'm not sure with whom were emailing with and I apologize that your question wasn't answered. Yes, you can add an external transformer to add the 4 power robbing t-stats. Please see the wiring diagram attach and be sure to land the R-W on the correct TT for the t-stats. Normally it doesn't matter if only a 2 wire connection but when adding a com it needs to be W to left T and R to the right T. If you need to speak to Tech Support you can always call during business hours Mon-Fri 8am-5pm EST 401-942-8000.

@EdTheHeaterMan Thanks for sharing that post, I did chat with that gentleman and haven't heard back from him, so no news is good news!

m11k

Thanks @SteveSan for confirming how I can add a second transformer to the existing setup. I didn't know about the left vs right terminal, so that is very helpful!

Would you be able to check on the transformer size for the new SR506-EXP-4 and SR504-EXP-4? I did specifically ask Taco support but they just pointed me to these docs, which don't actually state the transformer VA:

http://www.taco-hvac.com/tmp/SR506-EXP-4IOM.pdf
http://www.taco-hvac.com/tmp/SR-SubmittalDataSheet_101-052_120820.pdf

Maybe the official stance is that smart thermostats shouldn't be powered from the SR506-EXP-4 and you should always use an external transformer regardless of built-in transformer VA?

Thanks,
Mike

SteveSan

@m11k The new SR504-EXP-4 has a 15va and SR506-EXP-4 has a 20va.

The rule of thumb with Nest is anything more than 2 you will need an external transformer on anyone of our SR controls.

Maybe the official stance is that smart thermostats shouldn't be powered from the SR506-EXP-4 and you should always use an external transformer regardless of built-in transformer VA?

I would say that would depend on the brand of t-stat used. Not all smart t-stats will require an external transformer. Hope this helps Mike.

Joe Mattiello

Hi M11K
I read through your tread about the SR506 which is equipped with 15VA transformer. Have you talked to Nest about their recommendations for powering their Thermostat? they may have best practice schematic. If you were to basterdize Taco control, you would do what's mentioned here which is to cut the transformer legs feeding the control, and use the 24volt terminals which are designed as 24 volts out, or in. I would check with Nest before you risk damaging a perfectly good control. Thanks for using taco products. Additional information forTaco controls is available by calling Tech support at 401-942-8000

hot_rod

Sometimes when you try to modify a circuit board, the solder traces on the backside are designed for a specific current. It may not be wise or safe to wire in a larger transformer to a board designed for a 15 Va for example

mattmia2

hot_rod said:

Sometimes when you try to modify a circuit board, the solder traces on the backside are designed for a specific current. It may not be wise or safe to wire in a larger transformer to a board designed for a 15 Va for example

That is true if you are dealing with a lot of power, but 40va at 24v is under 2 a so unless the traces are hair thin it won't be a problem.

EBEBRATT-Ed

Separate transformers and relays will always work. Why take a chance on modifying a control?

HomerJSmith

I think that the transformer on the Taco and the aux transformer are in parallel the way the SteveSan's diagram shows. The question I have is the proper phasing of the two transformers. That would be an important consideration. I would want to confirm the phasing.

mattmia2

EBEBRATT-Ed said:

Separate transformers and relays will always work. Why take a chance on modifying a control?

that control is basically separate relays and a transformer in a convenient box...

EdTheHeaterMan

Remember when you could buy a blank board with a ton of holes on it from Radio Shack and make your own ...anything! ...with some resistors relays and other parts from that big wall of electronic parts on the right side of the store.

mattmia2

radios hack was expensive, i went to the electronics supply houses. now you have to mail order from the catalog supply houses

HomerJSmith

Ed, yup! Those were great times.

m11k

Just an update for the thread. I did end up adding a secondary transformer, although to be honest I'm not 100% sure it was absolutely necessary. I have set up 3 ecobees (I'm still waiting on a fourth). I used my multimeter to measure the current draw from the 3 ecobees, and it peaks around 120ma when I first turn the system on, which at 24V is 2.9VA. At idle the 3 ecobees draw about 80ma, or about 1.9VA. I don't know if they draw any more when are they in heating mode and when they activate the relay. My 40VA transformer is definitely overkill for driving this 2-3VA load.

Since the R terminals on the SR506 I assume are all connected together on the same bus, I just connected the red leg of the new transformer to one of the unused terminals, and then jumpered all of the common legs together up to the thermostats. I've attached a photo. The power from the second transformer is the pair entering on the right. I also picked up a PC610 exercise timer card to cycle the circulator pumps on periodically when not in use (30s/2w).

Thanks all for the help!

pedmec

are those VA ratings with the system in a call for heat?

mattmia2

They may draw significantly more if the battery is dead and it is bulk charging the battery.

m11k

I did another measurement with the thermostat calling for heat, and didn't see any measurable increase in current draw.

AFAIK, the Ecobee 3 lite thermostats don't have any batteries. They are strictly 24VAC powered, and do not have the ability to do "power robbing" like nest thermostats do. I couldn't find a reference on Ecobee's site, but I did find this: https://www.thermostatforums.com/showthread.php?564-Power-Stealing-Thermostats

I did also find this post on Ecobee's site which specifically calls out the SR506 for having a transformer which is too small for the Ecobee, so I guess better safe than sorry. https://support.ecobee.com/s/articles/My-ecobee-thermostat-keeps-losing-power-and-displaying-a-calibrating-message-Why

ChrisMcc

I just found this thread and have a similar question.  I have three taco controllers, rlteo are just SR-506 and one is an SR-504EXP, all about 8 years old and all have 15VA transformers.

I have 12 zones wired on these and have been converting to ecobees.  I have purchased and mounted an external 75VA transformer and wondered if I can use it on all three controllers in the way described here?  It looks like no modifications of the existing controller we're needed but not sure about tying three together on the same transformer.  @SteveSan, does this work?

Thanks,
Chris

SteveSan

@ChrisMcc I reached out to one of my controls guy and he suggested going to two 75va transformers. If you are adding 12 Ecobees, what is the load from all 12? One might not be enough to handle the 12.

If you have any questions, please give Taco Technical Services a call during normal business hours Mon-Fri 8am-5pm EST 401-942-8000

ChrisMcc

@SteveSan, Ecobee says that the max power load from any of their devices is < 3.5VA @24V. Field measurements (mine and others) indicate this is rarely, if ever, approached. I suspect on some of the higher end devices that have speakers for built in music and digital assistants like Alexa, that might draw more. At 12 of them I calculate 42 VA max load, so 75VA seemed to be enough to me. Is it OK from Taco's perspective to use the same transformer across multiple controllers? These seem like fairly older controllers, 8+ years old.

ChrisMcc

I also realized that I'm *only* powering 11 Ecobees from here. One is powered off the AC system which is a separate Unico transformer. The old Ecobee 3s could power off either but the 4s and beyond have to power of the Rc input. So if you have a separate Rc and Rh input, you need the common from the Rc - I went round on that (seemed a bad feature to remove) but have that sorted. So I'm under 40VA load I would say, quite conservatively.

SteveSan

What is the model number of the controller? Does it have a metal or plastic cover?

ChrisMcc

Two of the controllers are labeled SR506, the third is labeled SR504-EXP.  All have green plastic covers.  I've checked the transformers and all are 15VA.

SteveSan

Chris, below is from Dave Sweet who designed our controls and is the PM for the controls department.

"Probably from an agency approval basses this would not be allowed, but in the field many things are possible"…

"We advertise that if the transformer fails, then you put 24 VAC power in the 24 VAC connection on SR’s. So with that said, if external transformer is used, the internal transformer needs to be disconnected to prevent bucking of transformers".

ChrisMcc

Thanks @SteveSan!  Was that different than what @m11k did?  I thought he didn't disconnect the internal, though that makes a lot of sense.

ChrisMcc

I took a closer look inside one of the sr506, it says it's an sr506-4, BTW.  I noticed that the transformer is super easy to disconnect, easy to swap in power from the external.  It did get me wondering what the board is desifor, as someone else pointed out in this thread 

I'm thinking I'll put a 1amp inline fuse which would limit draw to any controller to 25 VA.  Even if I had 6 ecobees on one controller the theoretical max is 21VA.  Is that an acceptable approach?

Thanks, 
Chris 

SteveSan

He had the older control and only using a couple of t-stats. On the older controls when adding a t-stat with a com the Red & White wiring to the TT t-stat terminals matter which one you land them on.