Wiring relay to cut heat call during DHW (combi boiler)
Comments
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Yes, works perfectly (left that way all night in 20 degree weather, and we aren't dead :D). Agreed on the delay and that was always the plan to integrate it once it all worked. Thanks again!
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Can't just break the heat call (not the fan call or blower tap) to the air handler?
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Read the previous comments. that is what the A/H contacts on the Navien are for. But there is a Catch 22 problem. You need the heat call to make the circulators start so the circulator end switch will fire the boiler. using an aquastat will do the same thing…. Catch 22 again
Edward Young Retired
After you make that expensive repair and you still have the same problem, What will you check next?
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I mean the heat call that connects to the air handler. Basically, turn on the pump with the heat call & let the aquastat turn on W1/W2 at the air handler. I can't see why that wouldn't work. Fan on to heating speed with warm water, fan off when the water cools. Isn't that what we're trying to accomplish? Might take some tweaking to get the on & off points just right, might need to think about the location of the sensor bulb, but am I missing anything?
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Agree that would work if it was heat only, but that thermostat also operates air conditioning, so a "Heat Relay" would need to be added to the air handler to have the thermostat do the circulator thing you suggest. That is basically what I did on this diagram from 2024
but this needed to switch from HP to Hydro and the delay from that switch to the coil actually getting hot was over 4 minutes of cold air blowing.
Since the two TAM7 is AC only and the DHW demand is what is stopping the pumps from running, AND the Navien has these A/H contacts that are supposed to account for this, AND this worked on the previous boiler that the new Navien Combi replaced, then why not try the same thing again. That is when we all found out that there was a design flaw in the Navien that works fine on a single air handler but when you add a relay to split the A/H signal to serve 2 air handlers, that is when we all found the problems.
Edward Young Retired
After you make that expensive repair and you still have the same problem, What will you check next?
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Where do the wires marked "upstairs" and "downstairs" go? From W from the thermostat/zone controller to W1/W2 on the air handler?
Hydro air is the only heat? The thermostat/zone panel heat call turns on the boiler, the boiler turns on the heating circ pump(s), which may be overridden by a domestic call? Not a heat pump, no heat strips?
It is strange that the AH contacts on the boiler aren't reliable (& that would be the preferable way I think), but:
Thermostat/zone panel W → aquastat → air handler W1/W2, temp probe attached to a hairpin bend of the hydro coil (or even jammed into the fins for best response). Pump comes on, coil gets hot, blower turns on. Pump goes off for any reason, coil gets cool, blower turns off. Water temp comes back up, blower comes back on. No race condition, no timer needed. It's possible that the timer & relay method would be cheaper, & I am somewhat arguing for my way because it's my way—but it's also better, because it directly addresses the problem of low discharge air by controlling to water temp vs indirectly addressing it with a timer.
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@ratio, I didn't understand the problem until I took this wiring diagram and added a flow chart just below it. That is when I saw the Catch 22 problem that the OP was trying to tell us about. I was not understanding the problem until I mapped it out.
So trying to use what the OP already had on hand, that worked with the old boiler, was not working with the new boiler because there must be a momentary dropping out of the A/H and the DHW PRI contacts during the transition from the space heating call for heat and the loss of space heating due to DHW priority.
Something else to consider is, when the DHW is on priority, if you power the circulator pumps bypassing the boiler demand/priority circuitry, then the space heating pumps would continue to operate and perhaps cause someone to take a cold shower. That is because the burners will run at the max 240 input for DHW but the pumps will not stop if the boiler's circulator logic is bypassed. Much of that 240 input may be used up by those circulators. So that must be taken into consideration when designing the perfect control logic for this location.
EDIT: Personally I would not have used a Combi of any type for this application. I would get better performance with a Mod Con heating only boiler and an indirect water heater for this application. But that is not the equipment that is on site. I try to spend as little extra $$$ as possible and use what is already there. This is where the difficulty arises... Designing a fix instead of designing a better plan from scratch.
I actually have this 2 air handler with hydro coil in my previous home that my son now lives in. the most recent upgrade added some radiant floor heat and a variable speed air handler for the second floor system. the first floor system will be upgraded within the next few years and the indirect has been there for over 20 years and will probably last another 10 to 20 years. Indirects are the best water heaters in my opinion.
Edward Young Retired
After you make that expensive repair and you still have the same problem, What will you check next?
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Ok, I see what I was missing: the TAM7 controls the circ internally. For some reason, I was thinking that they were controlled directly from the stat through the 502. The question that comes to mind is what benefit from the hydro board in the TAM7? What would happen to the TAM7 if the pump command came directly from the stat/TrueZONE panel? I'm reasonably certain there are no valves internal to the hydro coil, so would the AH care if that coil got hot before it turned it's fan on?
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Works great… no negative effect on the heat calls that I can tell. And yes, final step is to wire in the PTD102 :)
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Unfortunately it would have been a ton more work due to how my hack installers set it up (admittedly not THAT bad, but limited in options… they were hacks in plenty of other ways though lol). I could have re-done it all so that the Navien wasn't at the end of the call tree, but it was easier to just wire in the relay. The hard part is always getting the Navien happy :D.
I'm not sure why, but I never got the factory aquastat that came w/ the TAM7 to reliably cut a heat call by itself. For whatever reason (configuration of the A/H, maybe always stayed above 100 degrees, or something else).
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Well since you found the secret magic Navien DIP switch I wonder if the original way with the ST82D would have worked. Since using the PTD102 and the DHW contacts is doing the same basic thing just in a opposite way.
The original way the closed A/H contacts constantly trigger the ST82D (closing the NO contacts) and when the DHW call happened the ST82D extended the thermostats call (if present) for a short duration so the air handler did not drop out with a short DHW calls.
With the PTD102 it delays the relay energizing with short DHW calls so the air handler does not drop out with a short DHW calls. PTD102 does have the advantage of being able to adjust the time delay.
National - U.S. Gas Boiler 45+ Years Old
Steam 300 SQ. FT. - EDR 347
One Pipe System0 -
I'm sure it would have… I just kept the new relay because it has a cool LED which made it even easier to test timing (and hey… it just looks cool 😁). It is also not as clunky and has screw terminals instead of requiring me to crimp female spades which I friggin hate. Finally, it was half the price for two of them, and came w/ a cool mounting bracket that allows you to just pop them on. I'm very happy w/ this model.
The original way worked but I think I was more lucky than good to find a combination that allowed it to. The ST w/ the time delay board required constant pull in which was noisy and buzzy and probably also not good for the relay. I knew the newer Navien would allow me to do it "right" which I eventually got to work once we found the secret sauce. The PTD allows a "normally closed" to do what I originally wanted to do for the old one but never could - open when the relay was energized. In reality, if I had just used the same relay and PTD on the old Navien, I probably could have got it to work better, but that was eight years ago and I was a relative rookie. Hindsight eh?
Happy to say I wired it all up nicely, set the PTD to the 2nd notch which is about 1 min 20 sec, tested, and buttoned up. Works a treat and now I can move on to the next job. Rewiring the basement 😂. Sincere thanks for all the awesome help!
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I think the hydro board in the TAM7 is just an option (or comes automatically w/ the slide in coil) and gives you the flexibility to use it if it benefits your configuration. If your boiler cannot run the circulators you can send the stats to the control board on the TAM7 and it will take care of the rest, more of an all-in-one solution. Problem is, the Navien has also evolved in a similar way and this current model has a lot more functionality than the old one. As to your question on whether it would care if the coil got hot or not, It's hard to tell… it's always been commanding the circulators so have never tried it otherwise, but my guess is if something else did, as long as the main board got a heat call it would just blow the fan regardless of whether the coil got hot or not. As I mentioned it does have a factory aquastat but I've never seen an appreciable difference using it or not. It might be because W1-W2 are all jumpered together to maximize fan / duct pressure to make the longer runs (they didn't mastic the trunks either… because they are hacks).
I have found that a lot of the functionality of the TAM differs based on how it "thinks" it's set up via the various DIP switches and jumpers. I have fiddled with fan speeds and tried to tweak, but never found a setup better than what I already had. The manuals are just "ok"… I really wish they made a guide to tweaking your TAM 7. I will say pros have a love or hate relationship with them, and I have never found one who could talk perfectly to all its functions. I'd love to pick the brains of Trane but they won't talk to homeowners.
There were many ways (and many better ways) I'm sure that this could have been set up, but all would have required a lot of reworking what already worked. As Ed correctly mentioned, I knew the relay did what I wanted it to do before, and figured it would do so again for the current version.
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Great to hear it is now better than it was. 1 min 20 sec is about the same as the default 80 seconds of the ST82D.
National - U.S. Gas Boiler 45+ Years Old
Steam 300 SQ. FT. - EDR 347
One Pipe System1
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