Experts:

I'm a little confused about the best way to wire a Honeywell L7248 with a Taco SR506 relay. I would love your guidance, please.

I intend to get a correctly sized (net ~75k mbh) oil-fired boiler in my Massachusetts home shortly. This will replace an absurdly large postwar coal/oil unit. I've done a manual J, etc. My house has hyrdonic zones with wall-embedded convectors. It also has what I call "opportunistic" solar DHW, which is oil-backed for a stretch of cloudy days. So I do need DHW priority, even if I only rarely use it.

The boilers I've decided upon (Pensotti, Biasi) typically ship with a Honeywell L7248 triple aquastat. Among other great features, this allows a high (i.e., safety) limit and low limit (minus the variable differential... effectively the "cold" setting of cold-start), but it also gives the ZR/ZC 120v remote relay functionality, which I'll need to have six zones on a Taco relay controller. So I expect the Honeywell family will work with my Taco SR506 (a first generation, non-EXP variant) w/ Taco-side DHW priority for my indirect DHW tank.

I'm inclined to do it the following way, which Taco calls "how to correct the wiring":

http://www.taco-hvac.com/uploads/Fil...ry/100-9.0.pdf (bottom of page 3).

...and I'll use the cold-start "no tankless coil priority" approach. I think this allows for seasonal outdoor reset (on the Honeywell side), DHW priority (i.e., the Taco SR506 holds back its five non-DHW zones when its own DHW calls), plus protection from circulating inappropriately cold water into both the DHW tank (i.e, chilling my showers in July) and into the cast iron boiler (i.e., cracking my boiler core in December), because the Honeywell holds back "ZC."

Questions:

• Do I have this right? I don't think I want the more basic X/X or T/T options, unless I add a small primary loop and 4-way mixing valve. That seems extravagant, with these controllers already right there. The other real option, I think, is to wire DHW directly off the L7248. But then I think I lose some of the functionality of the Honeywell controller (like anti-short-cycle, I think), with rather limited benefit in my case.
  
  
• Is the ZC-to-ZC 120v 12ga wire color yellow in the NEC (USA)? Can't figure out the color. Seems like it should be, but nobody calls it out that way in their controllers' product docs. Maybe everybody just uses black, since it's always handy?

I made a great .pdf of the whole arrangement, which I'd be happy to have posted. It's very pretty, but I don't know if it's accurate. Hence my question.

Thanks for your expertise and guidance!

 

If you choose the cold start option on the aquastat and all heating zones are in the SR relay you do not need to do anything different. Wire the zones to the relay, wire X-X to the boiler aquastat T-T and that is it. I hope you got the Taco SR control with priority.

 

Thanks for the prompt reply! That would definitely work as designed in the dead of winter. But I fear that wiring scheme might be less-than-ideal in the shoulder seasons. If I don't use the zone request (ZR) function, and just treat the aquastat as a simple TT instead, then there's nothing (i.e., ZC) holding back the circulators until the boiler comes up to temp. In August, that would likely send 5 minutes of 65 degree water (the boiler's water jacket) circulating into my DHW tank while somebody takes a shower and it calls on a cloudy day.

Then, in the winter, there's the well-known issue of a big temperature differential for boiler return, possibly "shocking" the cast iron.

Am I understanding that wrong? I apologize. This isn't my area of expertise. I'm just a guy with a house. Hence the name.

Thanks again!

 

Your boiler water will be room temp. Never the less, on a DHW call, the cool water in the boiler will be sent to the hot water tank and be warmed by the hot water in the tank. All this while the boiler is firing warming the water in the loop. The loop is small and will heat quickly. That small bit of cool water circulation is not going to translate into cold water in your DHW tank.

 

Holding the pump off is not a good idea. People look at condensate issues but that is only part of the problem.
Getting the boiler warm and hitting it with cold water is not a good idea either.....thermal stress.
As Dro stated the water will be warmed in the indirect and will come back and not be a problem. Remember a cast iron boiler will condense on every new call for heat. We just minimize the time it condenses. An indirect is a very short time and not a problem.

 

Thanks again for the great replies! Very helpful, and very thoughtful. I made this schematic (yes, really, from scratch) in Visio, and wanted to discuss it with my installer:

It's here if you want to download it full-screen.


I thought this was the compromise between best practices from Honeywell and Taco.

But it sounds like y'all don't like "ZC" in general, on a low mass boiler?

Thanks!

 

Let's see if I can make some sense of this for ya...

First, the ZR and ZC on the HW a'stat aren't 'magic'. They really don't do anything that will benefit you by using them. They simply allow alternate methods of controlling zones, and in your case you don't need to use them at all because you are zoning with the 506 panel. So, basically, fuggedaboudem.

Not exactly... the 506 will control the priority. If you set priority ON there, and a call comes from the indirect, the other circs will not run.

Ummmm... no. The LOW limit setting is used for a WARM START boiler. If you have no 'thankless coil' in your boiler you want the LOW setting turned OFF. Using the LOW limit setting will keep the boiler WARM 24/7, and you definitely do NOT want that. You want the boiler to be able to go stone COLD in the absence of a heat or indirect call. THAT's what COLD START means.

The way you want to wire this is take the X X on the 506 directly to the T T on the HW a'stat (you think it's a 7248? let's hope it's a 7224). The only function of the 7224 a'stat is to fire the burner when called by the 506, and to provide high limit and differential functions to that burner call.

But now you've got an issue with the ODR control. I haven't studied the HW ODR that plugs into the 7224 at any length, so I might be talkin' out my a55, but I don't believe it has an input to tell the ODR that the current call for heat is coming from the indirect, so the ODR can't be over-ridden and your indirect calls will be subject to the water temp which the ODR calculates. This will slow the recovery time of the indirect.

Do you already own the 506 panel? If not, then you want to explore the use of the -EXP model of this panel which provides a 'plug and play' port to install the PC702 ODR module. This module would know that there is a DHW call because it's interfaced directly to your zone control panel.

It's not that we don't "like" the ZR and ZC controls, it's that their use in this application is inappropriate.

 

The ZR and ZC terminals are used when the boiler has a domestic hot water coil in it. They are used when there is more than one heating zone. It stops the other zone pumps on the aquastat low limit so the coil does not run cold.

 

Thank you! Makes sense. So if I use the T-T connections on the Honeywell as the only connection to the Taco, does that preclude connecting Honeywell's outdoor reset on 1-2-3?

 

I believe it does... UNLESS this module allows a means to communicate to it that there is a call for the indirect water heater... and it will over-ride the calculated temp... that's what I meant in my previous post about there being an 'issue'.

What is the model number info on the HW ODR unit? I know I can research it myself, but I'm lazy... give me the model number and I'll take a look-see.

By the way, only the very newest 7224 HW a'stat can use that ODR, and if you get the 7248 with your boiler it might not work anyway. Manufacturers stock up on these controls so when you get a new boiler, you might get a control that's been sitting on the shelf for a while and it might not be the most current version... beware of that!

 

OK, here's a PDF for the module:

Honeywell ODR module

And it says in part:

Outdoor Reset
The Outdoor Reset feature adjusts the target boiler temperature
to a point below its local high limit setting and above the boiler's
condensation temperature by using the EnviraCOM
communication bus to directly adjust the Aquastat set-point.
Should a call for Domestic Hot Water be detected, the boiler
temperature is commanded to return to the High Limit setting,
ensuring a hot water supply is available. When the Domestic Hot
Water demand is met, the Outdoor Reset feature is once again
enabled. See Table 1 and Fig. 3.

====================

and the way the module knows that there is a DHW call is via a configuration in the aquastat that uses the ZR terminal as a signal input. This will mean that you will need to rig something up to cause this to happen...

I'll be back later ... gotta git...

OK, I'm back...

Here's a snip from the L7224U a'stat PDF file:
7224 install manual

ZR-Domestic Hot Water (DHW)Request

The ZR terminal can be selected to service an indirect
water heater heat request. This parameter is set via the 3
digit display (see Adjusting Settings section of this
document). A heat request via the ZR terminal will have
priority over all other features such as the Anti Short-
Cycle feature or those enabled by the Outdoor Reset
Module (See form #69-2335 for more information).

----------------------------------------------------------------

Take a look at figure 9 [ no, not 9, but TEN (10)]in the PDF manual... it shows that what you need to do is configure the aquastat to monitor the ZR terminal and run a wire from the indirect circulator to the indirect...

so, yes, you can use the honeywell ODR module if you wire it like that.

 

[please don't link our forum to others. read the rules please - NJT]

 

Folks (and especially NJ Trooper):

Thanks so much for your help, and for researching this with me.

First, I fear we have a small miscommunication or - more likely - I'm failing to grasp what you are telling me.

Figure 9 [I goofed, meant 10, edited in other messages - NJT] in the L7224U Oil Electronic Aquastat Controller document shows local circulation from the Honeywell triple a'stat for a single zone, whereras I'm using a (remote) Taco SR506 with six zones. My setup includes 120V circulators w/ priority coming directly off the Taco SR506, and with a low-voltage t'stat's/a'stat (only) to the Taco SR506. In my configuration, there is no local 120V circulator off the Honeywell controller, nor does the a'stat connect there.

But I certainly appreciate the clarification on the ODR. My goal would be to have the following parameters:

• High (safety) limit of 210°F
• "Cold start" (not tankless) boiler configuration
• An appropriately-sized boiler, so it will tend to have a warm water jacket in the winter, but come up quickly for DHW in the summer (if needed)
• ODR Minimum Outdoor Temperature (i.e., ODR curve parameter) of -5°F
• ODR Minimum Boiler Temperatue (i.e., ODR curve parameter) of approx 135° for oil
• ODR Maximum Outdoor Temperatue (i.e., ODR curve parameter) of approx. 45°
  

Somewhat annoyingly, Honeywell refers to some of these settable parameters by different names within the same Outdoor Reset Module installation document (see Table 1 vs. the paragraphs that preceed it, for example). But moving on...

It still seems like I need to come off the Taco multi-zone relay and go someplace other than TT on the Honeywell, in order to leave 1-2-3 available for the Honeywell ODR Module connections. And that, in turn, means connecting to the Honeywell's 120V terminals. And that, in turn, means using ZR/ZC.

Not a big deal, perhaps. I think it may just be a matter of using my schematic, but setting the duu (that's Honeywell-LED-speak for DW) parameter to "off", so there's no holding back of the circulators, except for DHW priority within the Taco SR506.

BTW, NJ Trooper, should this whole thread be moved to the T'stat controls area? When I posted it, I thought it was going to be more 120v boiler-centric, but now it seems to be as much about the behavior of the t'stats/a'stats.

Thanks again.

 

I just want to mention again, lest we forget, that this discussion hinges entirely on the fact that the a'stat provided with a new boiler is new enough to include these features. The ODR that connects via the EnviraCom wiring is a 'relatively' new offering (maybe within the past year or so).

Ooops... I goofed. I was kinda rushed at the time I posted that info and got the figure number wrong and didn't explain fully... It should be FIGURE 10. What that drawing illustrates is the use of the ZR terminal to signal to the aquastat and the attached ODR that there is a call for heat from the indirect coming in, and that the ODR settings should be over-ridden.

I need to look at your drawing again, but as I recall it in memory, what you would do is take the HOT 120VAC to your indirect pump from the 506 panel directly to the ZR terminal.

The 506 X X to a'stat T T would signal that heat was needed, and the signal from the 506 to the indirect pump would also be routed to the ZR terminal and signal that it was an indirect call.

You would want this setting at 180°.

Making sure again that the a'stat you get with the boiler is new enough to support the feature whereby you can turn OFF the Low Limit operation.

Not quite sure what you mean here... the Outdoor temp setting will surely be wide enough to accomodate your location. There shouldn't be any issues with that at all.

No promises... but if I get some time this busy weekend I will see if I can edit your drawing to give you a better idea of my thoughts.

The details of your 506 wiring seem to be mostly correct, save for the wiring to the a'stat.

It's fine here... if you posted it there, I'm pretty sure Jay would move it back! He's awesome like that! (hi Jay!)

 

Didn't take long...

note: 'duu' in aquastat must be turned ON.



Many browsers you can right click and 'view image' to make larger.

 

By the way, there is also a "Domestic Hot Water" module available that also connects to the EnviraCom terminals, but this module would be used only if you were zoning with valves because there is no provision to run a DHW PUMP with this module. See fig 12 in the a'stat PDF for a bit more info on this, and here is the PDF for the DHW module:

http://customer.honeywell.com/techli...it/69-2343.pdf

I'm adding this as general info for other readers, it's not something you need on your setup.

 

Wow, that's great! Thank you again.

Surprising that your diagram isn't just sloshing around the internet already. It's all the dominant control gear (Taco circulators, Honeywell triple aquastat, a three-pass boiler, etc.). Not "better" necessarily, but pretty much available at any supply house. You'd think this diagram would be under every rock.

So thanks for the quick modifications. Awsome!

A few follow-up questions:

• Do you just screw the (farthest to the left) X/T connection on top of T/3 on the Honeywell, even though the ODR Module is screwed down there already? In other words, screw two connectors on the same terminal? It's low-voltage in both cases, right? I'll have to check the docs again.

• On Zone 6 (DHW) off the SR506, it's 120v 12g solid copper. So it's not cool to double-screw onto the same post, so that would necessitate a 6" black whip, I assume, and then...

• We dropped my other original question altogether: is that X/T 120v wire yellow in NEC? That controller-to-controller switched connector should probably be yellow off the whip, correct?
  

Thanks so much!

 

A couple of things we glossed over...

Okay, I'm just used to setting-back manually through the year (no ODR today) in a 150° to 210° range on my old 150k mbh coal/oil unit from the 1950s. Very tedious. I follow a spreadsheet. But I'm down to like 1/3 of my original consumption over the past decade. I've found my embedded convectors work rather nicely at around 185°, but I don't need them to work all that efficiently in the shoulder season, when they're just taking the edge off.


I'm just talking to contractors now (it's challenging to finding one who doesn't want to do the "just make it a little bigger than the last boiler" approach). Since it's net-new, I can specify pretty much any a'stat I want. I will indeed watch for old vendor stock. Thank you.

Conversely, that's not negotiable. There are just too many dependencies in my basement (how to route pipes, how to drain, etc.). That part is cast in stone, long since paid for.

Thanks again!

 

Yes, that would work fine. If you were as 'ar' as I am, you might be tempted to use crimp on spade or ring lugs, and that would be fine too. Yes, all LV wiring there.

Those are compression type connectors, and you are correct, not a good idea to try to jam two wires in. Probably wouldn't fit both anyway. I call those whips 'pigtails', and that's what I would do. I wouldn't concern myself with the NEC wire colors, I would use BLACK for the wire to the ZR on the a'stat. I don't think the wire to the ZR need be 12 ga since it's not carrying any current, it's a signal wire only, so theoretically you could use something smaller for that one.

If you go with the HW ODR module, it's not a problem. You would only need the EXP model if you were using the TACO ODR module as you would need the port to plug it into. Your existing model will work just fine.

I meant to mention this previously... some of the modern boilers, Burnham MPO as a particular example, allow return water temps down to something in the range of 100°, so depending on the delta T across your emitters, you may get away with a lower supply temp. You might even be able to go down to say the 120-125 range.

Did you say all the emitters in the home are convector cabinet type? What size piping? (trying to get a feel for the water volume in your system)

 

Each of two living floors is about a ninety foot 1" Cu (type L) circulated loop, with an average of seven 1/2" venturi take-offs for room emiters (which are high surface area, low mass... think a finned baseboard in a box). For water mass purposes, the basement is actually a much larger thermal mass (including water, cast iron, and concrete), but the basement is warmer and rarely used. By "rarely," I mean I haven't let it call for heat in about three years, and it doesn't drop below 64°.

Of course, if I yank out my absurd Mike Mulligan Steam Shovel boiler, I may need to heat the basement a little.

Having lived with this setup for a decade, the trick is to have each floor's primary loop be hot enough that the convectors actually convect (i.e., throw the heat into the room), but cold enough that they aren't heating the outside walls more than the room. In my experience, that's in the 175° to 190° range (on a design day). It can be any temperature when it's merely chilly outside, and cooler is better.

Again, this is just one guy's experience in one house. I'm not trying to suggest this will work for anybody else. Or anyplace else.

Also, I'm pretty sure that ZR/ZR link is 120v hot. I apologize for looking a gift horse in the mouth, but that one is probably a matter of safety. It's 120v hot, and it needs the appropriate gauge wire, with breathing room in the EMT.

Taco calls that out in their FAQs:

What are the ZC and ZR terminals on the SR506 and how are they used?

But I'd honestly never considered using the ZC/ZR arrangement in your diagram, and then just leaving ZC open. That's really the inspired part of the design. Thank you. Since there's sixty gallons in the DHW tank, and only four gallons in the boiler... it makes sense to just let it circ. until the (SR506) priority zone is happy.

I like it!

 

Yes... it is 120VAC hot, but it's not a heavy current carrying wire, just a signal wire. As long as the INSULATION on the wire is rated appropriately, you could get by with a smaller gauge for that. Just the same, it would mean that you would probably need to purchase that short piece of wire, so it would make most sense to just use a piece of the black that you probably already have. In fact, I'm actually not 100% sure that it's not carrying the current of the burner, so go with the 12 ga to be safe.

The GAUGE of the wire determines the CURRENT carrying capacity, the INSULATION on the wire is what determines the VOLTAGE at which you can safely use that wire. At my 'day job' I routinely specify 18 gauge wire for SIX HUNDRED VOLT circuits, but the insulation is way thicker than other wires!

I wish I could take credit for that... but it's Honeywell's design. They've provided a method to communicate to the ODR module and the aquastat that the current heat call is coming from the indirect.

A little bit about the ZR / ZC terminals... the Taco FAQ seemed a bit 'foggy' as far as explanations go. The 7224 / 7248 a'stats are different animals than the old 8124A, but let's look at one of those diagrams for the 'logic' as to what those terminals do. Notice here that on the 8124 a'stat, the ZR terminal absolutely DOES carry the current of the burner, I believe that the 7224/7248 uses that wire as input to a logic circuit (microprocessor) which decides what relays to open/close, and when. These old school 8124 have no electronics in them, but are useful for looking at functions.

The ZR is an INPUT to the a'stat, and should be thought of as a Zone Request.
The ZC is an OUTPUT from the a'stat, and should be thought of as Zone Control.

Look at figure 3 in this PDF file:
http://customer.honeywell.com/techli...it/95-6571.pdf

You can see that when 120VAC is applied to ZR it bypasses the K1 relay and powers the burner circuit directly with only the high limit in circuit to the burner. Notice also that the LOW LIMIT control will also bypass the K1 relay.

ZC terminal is an output to external zoning relays. It will be high (120VAC) any time that the low limit is satisfied.

Look at Figure 6, and note that ZC is wired as a 120VAC INPUT to the zone relays. This is done so that when the boiler hits low limit, any external zoning relays will be SHUT OFF, allowing the boiler to maintain it's low limit temp, and provide priority to the low limit circuit.

Take a look at what ZR is doing in figure 6. When a ZONE calls for heat, the zone relay raises ZR and powers the burner. This is done so that the zone can call for heat without running the circ for zone 1. It only powers up the burner. If the low limit on the a'stat is NOT satisfied, ZC will be low (0 Volts) and the zone relay will be powered OFF and can not call for heat.

Now, regardless of the fact that the 7224 is electronic and has a micro calling the shots, the basic function of the ZR and ZC terminals is the same... EXCEPT with the 7224 you have the ability to tell the micro that you want to use those terminals for some additional and different functions and the micro then communicates this to the ODR module via the EnviraCom wiring (network).