Thermostat setup question w/ Aquastat L8124A,C and Taco SR501-4

dorph

Hi.

I need help with two unrelated problems in my oil boiler relay setup. Thanks in advance for all that read through!

My setup: I have an oil burner, that runs 3 heat zones, and 1 hot water tank.

Problem #1: The hot water tank was an addition made ~2 years ago. The guy who installed it, added a Taco SR501-4 switching relay, and connected it to the oil burner's main relay, a Triple Aquastat type L8124A,C. More specifically, he connected N/C and N/O terminals on the Taco relay to the T/T thermostat terminals of the L8124A. And the Taco's thermostat terminals go to the water tank's thermostat.

That seems correct. However, he added this dual-wire connection without moving my existing 1st floor heat zone off those L8124A T/T terminals. So now the L8124A T/T terminals each have 2 connections. One to my 1st floor heat zone, and one to the Taco relay (that runs the water tank). This I believe is the culprit to a problem I've experienced for the last 2 years (since the water tank was added), which is-- when the water tank thermostat kicks on, I get heat on the 1st floor even though there is no call for heat from the 1st-floor thermostat. I originally thought it was a failed pressure valve. Had my oil company check them, and even replace one but that did nothing. Until now, I've resorted to shutting off the zone's pipe valve over the summer. Now that I've discovered this wiring, I believe this is just wrong. It seems clear that the water tank thermostat will trigger the primary zone's circulator every time it runs.

Am I right? Should he have isolated them with a 2-zone? Or used something else? Is there a way to make it work correctly with what I've got? Or if I call them up, what should I ask for? Is this installed to code?

Problem #2: The reason I discovered the wiring issue described above is because I've replaced my 1st floor thermostat with a Nest thermostat and looked around in the basement to figure out how to get Common to it. Luckily for me, the cable between the thermostat and the furnace already has 3 wires, but only 2 are in use (in the L8124A T/T terminals). I just need to figure out how to get Common. This is a heat-only setup-- no A/C or other systems to pull C from and I'd rather not pull more wires or run a separate transformer near the thermostat if possible.

I've come to learn from this site and others that the L8124A doesn't allow you to get 24V Common easily, but the Taco SR501-4 used by the water tank can. I was wondering if there is a way to switch the two (tank and 1st floor zone) to gain C for the 1st floor thermostat (assuming the water tank thermostat doesn't need it... how would I figure that out?), and if so, how would I go about this?

Aquastat L8124A,C:



Taco 501-4:

STEVEusaPA

For starters if each zone has its own circulator, and you have flow checks for all 4 zones (and they are working properly) you should have no flow to the other zones.
But the wiring is wrong.
If you switch to a Taco 504, your wiring can be simplified. You can run X-X from the 504 to T-T on the aquastat, make DHW a priority if needed, and you’ll have a common for your Nest thermostat.

dorph

Thanks for the quick response steve.

STEVEusaPA said:

For starters if each zone has its own circulator, and you have flow checks for all 4 zones (and they are working properly) you should have no flow to the other zones.

Correct me if I'm wrong, but if the 1st floor thermostat is wired to the same terminals as the water boiler thermostat, won't those 2 circulators both run in parallel all the time? Since they are both calling for heat concurrently?

STEVEusaPA said:

But the wiring is wrong.
If you switch to a Taco 504, your wiring can be simplified. You can run X-X from the 504 to T-T on the aquastat, make DHW a priority if needed, and you’ll have a common for your Nest thermostat.

I figured switching to a 2 or 4 zone relay would be the "cleanest" solution to both my problems, but this will run me at least $200, which is more than I spent on the Nest thermostat. I was hoping to fix it with what I have. Or at worst, add a Nest Power Connector (which I think is similar to a Venstar add-a-wire), but I don't know if that will work, will it? Do you know if that could be added to the L8124A even though it has no proper C terminal?

Ctoilman

It's wired wrong, the 8124a aquastat needs to use the ZR terminal that will see 120 volt power when the hot water tank SR501 relay is activated.  It's an easy fix, the cost is a length of 14 gauge wire (get rid of the low voltage stuff he wired into the 8124a).  

dorph

Ctoilman said:

It's wired wrong, the 8124a aquastat needs to use the ZR terminal that will see 120 volt power when the hot water tank SR501 relay is activated.  It's an easy fix, the cost is a length of 14 gauge wire (get rid of the low voltage stuff he wired into the 8124a).  

Can you be more specific? I'd like to try it but I'm a real novice with this.

Update: I should add that the ZR/ZC terminals are already in use going to another relay (one of the other 2 heat zones). Does that matter? Or can you hook up multiple sets of wires to ZR/ZC?

Also, do you think there is an easy fix for my Nest common wire problem?

Ctoilman

You can use multiple hookups on the ZR wire.  This is how the ZR functions: whenever this gets 120 volt power applied to it, it tells the boiler aquastat to run until high limit is reached WITHOUT running the boiler circulator.  ZC terminal is 'alive only when boiler temp is equal to or greater than low limit.  I rarely use the ZC terminal.
On how to get a C wire to your Nest....hmm, not sure actually.  You sure you really want a Nest anyway?  They are WAY over-rated, too expensive, and honestly, way too complicated.  Sell it on Ebay and buy something nice instead, do a WIFI enabled t-stat if you want something hi-tech.

STEVEusaPA

Whether you need/use the Nest or not, the 4 zone switching relay solves all your wiring problems. You've already saved that much doing it yourself, as long as you wire correctly and don't smoke anything.
But I agree, unless it's a vacation home or something you actually need to monitor, ditch the nest.
I have 6 thermostats for radiant heat, 2 for AC. Haven't changed the temperature in years. Unless it's to change batteries, the only one I touch is setting the master bath radiant to 'Away' in the summer because it's running off a slab sensor. I like the slab cool in the summer. Plus it's on ODR, so the circ will run, but the boiler will never fire.

dorph

Reason for the nest is that I already have nests in my other 2 heat zones (was already installed in the house before I moved in), and I'd like to bring this last zone up to date (I use the nest features and would like to keep them consistent). I also got it practically free ($10) from a energy saving program in my state.

Switching to a 4-zone relay sounds like a good idea, I'm just a little hesitent to make all these wiring changes on my own. I was hoping to just reverse the hot water tank zone currently on the taco 501 with the 1st floor zone (currently on the L8124A) and leave everything else as is. Would that work? I'm pretty sure the 501 will give me proper C for the nest, but I'm not certain the water tank will run fine straight off the L8124A. Will it? Any concerns with this plan?

Thanks.

Ctoilman

Only one set of t-stat wires goes to the 8124, whether its the indirect tank or a heat zone.  Wiring instructions are everywhere on the internet.

dorph

So I think I'll try to reverse the two setups. I'd like to move the DHW tank and its TT to the 8124, and the Nest-powered 1st floor zone to the Taco 501-4. One thing I want to make sure is that priority stays on the DHW. Will that be the case if I have the DHW Tstat connected to the 8124 TT terminals as well (as well as the DHW circ connected to the 8124 circterminals) ?

Then for the Nest/Taco relay, do I just connect the Taco circ terminal to the 8124 ZR terminal? That will take care of pulling hot water in, without the need to connect into the 8124 TT terminal at all.

Does that seem right?

STEVEusaPA

Nope

EdTheHeaterMan

@Dorph I have a question. What is the other relay you are using for the other zones?

I may be able to design something with your existing controls. ALSO...there may be an additional saving by allowing the boiler to become a cold start appliance. Your L8124 control will maintain temperature even if there is no call for hot water.

EdTheHeaterMan

Here is the wiring diagram I spoke of. I'm not sure of the 2 zone relay you have but there is only one change. Remove the ZC wire between the L8124 and the 2 zone relay.

This will get your boiler to be a cold start.

In the future, if you need
to replace the L8124, use an L7224U control. It is less expensive and you can turn off the Tankless coil feature easily so the boiler does not need to maintain temperature.