I Want to ADD a 4-wire GVD Vent Damper to Thermopile-powered DHW Hot Water Heater

Steamhead,

I won't open the gas part of the Honeywell valve, at most only the cover, but trying to avoid even that. I am trying to find a wire external to the Honeywell that I can cut/switch to prevent burner firing while not interfering with the pilot/thermopile. I am not intending to POWER the damper off the thermopile's millivolts. I noted that the 4-inch Damper will be externally powered and externally controlled (using 24VAC power) e.g., from the Boilers thermostats. I only want to add a switch (e.g., End Switch on the Damper or a relay) to DELAY the firing of the burner in the DHW. I am not going to be tapping any power away from the DHW thermopile or the gas valve. I need to find a wire/way to stop/delay the firing of the DHW burner while I also avoid interfering with the pilot/thermopile. Actually, one of the purpose for this modification is to reduce the PEAK DEMAND for combustion AIR for the five gas appliances in a basement (2 Utica gas boilers, up to two gas DHW (currently one), and one gas clothes dryer). The idea is to only allow one (or at most two) of these gas burning appliances to burn gas (AIR demand) at same time. (and add an 8 inch fresh air supply (also closed/opened using a powered damper in synch with the gas appliances).

Hi 109A_5

Thank you for your insights.

Quote: "Anyway familiarize yourself with DS1820, DS18B20 and LM34, LM35 temperature sensors, there probably are others too." These are thermal sensors optimized for "Arduino". I do not need to add a thermal sensor, nor an arduino for anything in this case. If I can find a wire outside the Honeywell valve box that I can cut/switch to delay the firing of the burner in the DHW, the DHW damper will be controlled by 24VAC signal derived from the other gas appliances having a higher priority.

Quote: "Did you kind of ignore the 28 pin microcontroller on the PC board?" I do not want to modify the PCB board or its operation. I will ignore the 28 pin microcontroller on the PC board if possible. Treating the Board as a Black Box, I just want to find a wire outside the honeywell valve box that I can cut/switch to delay the firing of the burner in the DHW.

Quote: "I would hope the temperature sensing device and the valve solenoids are at least supervised." I am not sure what "supervised" means here. I am worried that the 3-wires from the thermal sensor are not set up such that I can just CUT/switch-off any of them and get the result that I want (delay the onset of fire on the DHW burner). For example, if that "28 pin microcontroller" actively verifies a minimum/data signal from the thermal sensor, and switching off any (of 3) wires triggers a FAULT condition that is not automatically reset unattended, then I can't do this switching as simply as I wanted to.

Quote: "The Red and White wires, probably are from the Power Pile" I agree that the Red/White wires seem to be the main milivolts power supply from the thermopile, and thus not usable for my external-control purpose.

I need to find a wire/way to stop/delay the firing of the DHW burner while I also avoid interfering with the pilot/thermopile. Actually, one of the purpose for this modification is to reduce the PEAK DEMAND for combustion AIR for the five gas appliances in a basement (2 Utica gas boilers, up to two gas DHW (currently one), and one gas clothes dryer). The idea is to only allow one (or at most two) of these gas burning appliances to burn gas (AIR demand) at same time. (and add an 8 inch fresh air supply (also closed/opened using a powered damper in synch with the gas appliances).

Jamie,

Quote [

As I understand it, your basic and fundamental objective appears to be to ensure that, first, only a select group of gas fired appliances is running at any given time, to minimize combustion air demand in your space and, second, to reduce the heat loss from the open flue for this particular appliance. Is this correct?] That is correct. Especially considering that the gas clothes dryer has an exhaust fan that creates a negative pressure in the basement (boilers, DHW space) during its hours of operation. Replacing existing DHW with a power-vented DHW would currently aggravate the problem, unless it was provided with its own dedicated fresh air intake from outdoors. It is not practicable to provide four separate PVC fresh air intakes (and four holes in the exterior wall). In order to add insulation and make the basement "tighter" airwise, and to allow us to close the open window/door currently providing fresh air for gas combustion, and to thus avoid risk f freezing pipes in basement, we need to cut hole in wall and affirmatively provide a dedicated fresh air intake pipe (with powered baffle). With consideration of existing plumbing, we have only have a space for adding one 8 inch tall round or rectangular fresh air intake (one hole in 8-inch block wall). The idea is to time-multiplex that single fresh air source using a 24VAC damper and a mesh of 24VAC relays (or perhaps a programable circuit as your suggested) to detect and delay the fire-ON status of each of the gas burning appliances. I have not decided whether the power-vented gas dryer will have priority over the gas boilers. But I have decided that each of the two currently-independent Utica Gas Boilers will lock the other out (First come first served, with latch-off of the other) and both boilers will have priority over the one (or two) DHW burners.

I think I have invented the simplest solution. I will remove the PCB board from the Honeywell gas valve, and put a plastic straw (actually should be a cylindrical scrap of electrical wire insulation such as PVC. Or. wrap several times in a thick Teflon tape PTFE (Teflon)) over (only) the steel PIN for the burner valve on the Honeywell milivolt gas valve, thus preventing electrical contact with the metal fingers (electrodes) of the PCB board outlet. This thin insulator ("straw") prevents the 5 volts? flow of current from the PCB controller to the mechanical gas valve itself during a Call based on the 3-wire thermal sensor output. Then, I will replace the PCB board and then connect or solder a wire to the exposed tip of the steel PIN (of burner valve). I will also connect or solder a wire to the metal fingers (electrodes) of the PCB board outlet (of burner valve). At this stage, I can revert the valve to its nominal operation by twisting together the other ends of the two wires (completing the original circuit through the added wire extensions). Now, with the two wires, I can cut/switch the activation voltage sent to the burner valve (only) during a call for heat. Thus, I can prevent or delay the firing of the burner without interfering with the operation of the pilot, the thermopile, the thermal sensor, or any other aspect of the DHW. [Does the DHW have a burner "flame sensor" function other than the pilot thermopile?] I can wire these two wires through the END Switch of the added 24VAC 4-inch Damper (or through a 24VAC-coil relay operated by concurrence End Switches of the 4-inch Flue Damper and a 8-inch Fresh-Air Damper. I can also externally control the activation of the 4-inch Flue Damper and a 8-inch Fresh-Air Damper using a mesh of 24VAC-coil Relays (DoubleThrow available as Double-Pole, Triple-Pole, and Quad-Pole for a few dollars each on AliExpress). I can latch-off the start of the gas Dryer (has an AC-powered mechanical timer) using a 24VAC relay (turning off 120VAC to the Dryer) pending completion of the Utica Boiler calls. I can latch-off each of the two Utica Burners and the DHW(s) pending completion of the gas Dryer (inherently power vented). At some point, this might be best implemented as a Arduino-controlled with multi-relays, but I believe most of it can be prototyped or implemented using 24VAC relays (e.g., double-throw, quad pole to make it easier)

Earlier I tried to post:

Jamie,

Quote[use the powered damper end switch as the signal to the equipment]. I am trying to externally control (delay) the firing of the DHW burner (based on status of the other gas appliances in same room, and status of the added fresh air intake damper). If the DHW was designed like a typical milivolt gas furnace, this would be easy, because the wire to the burner's gas valve is explicit (e.g., passes through the burner TCO and safety switches) and easily accessible. The problem here is that in the DHW Honeywell valve setup the millivolt "wire" between the control PCB and the burner gas valve does not exist. Rather, there is an electrode PIN (plug) that pushes into through the PCB board (outlet). There is no accessible "wire" to cut/switch for my purpose of delay the fire-ON of the burner. It would be necessary to modify the Honeywell, pulling the PCB off the 3 pins, to make space to install some 3 new wires in between the 3 valve pins (plugs) and the 3 PCB receptacle holes (outlets). This is possible, but adding wires and connectors to the pins would require sourcing unusual shaped connectors and might be vulnerable to various failure scenarios. But ultimately, this might be the safest and most practicable solution.

In view of your insightful comments (all of you) I have done some research into the components within the Honeywell gas valve including a PCB (with microcontroller) and a 3-wire thermal sensor.I have made the following conclusions.
The 3-wire thermal sensor is positioned in the bulb in the back of the Honeywell valve. Thus, there is only one thermal sensor (location) on the DHW tank. I can access the three wires by removing the plastic cover from the Honeywell. Optimally (instead of cutting wires) inserting a wire-tap harness with two connectors, in between the plug on the cable of the thermal sensor and the matching PCB outlet
The 3-wire thermal sensor is an active sensor having a power, a ground (center pin) and a data line. The 5-blink error signal described on the cover of the valve is triggered by the MPU's detection of some defective operation of the 3-wire thermal sensor. This error detection is necessary to provide reliability to the 4-blink error feature upon overheating (which triggers a "lockout" of the valve).If a 5-blink (thermal sensor) error does not trigger shutdown of the pilot (or lockout), then I can easily cut/switch one of the three wires of the thermal sensor (using the End Switch in the Damper) to delay firing of the burner.But on the other hand, if during 5-blink error the MPU will turn off the pilot (upon cut/switch-off of any wire in the 3-wire thermal sensor) then I will need to find a more sophisticated way to delay the firing of the burner. A more sophisticated way to delay the firing of the burner may be to obtain a second identical 3-wire thermal sensor (and continuously "heat" it further up higher near the top on the side of the steel DHW tank) and switchably deploy it as a "decoy" sensor to convince the MPU that there is no malfunction in the thermal sensor. Then, provide at least a Single-Pole,Double-Throw rapidly switch to connect the decoy thermal sensor to the MPU (and disconnect the original thermal sensor from the MPU), to avoid the 5-blink error sequence. Depending upon the temperature that triggers a 4-blink overheating lockout, and depending upon how the MPU senses and verifies the data from the thermal sensor, this might work to prevent a 5-blink error. [But, if the whole tank is depleted of hot water up to the top, the MPU will fire the burner, which might happen while the damper is closed. But the open conical hood at the top of the DHW will allow combustion gas to flow by convection, preventing any acute danger of carbon monoxide development in the flame? I have videos of blocked chimneys.]. In my opinion, it would be better to "bit bang" an arduino to simulate the nominal output of a thermal sensor at high temperature, to provide a more reliable decoy thermal sensor signal to the MPU. This would require oscilloscope inspection of the nominal signal from the thermal sensor, and I am disinclined to attempt this.

Larry,

Quote: "adding a fan to your dampered air inlet? That would give you much more combustion air."

I personally do not know how a positive pressure in the furnace-room/basement would affect the three (will be four) convection-operated gas appliances there. It would likely accelerate the exit of air and gas fumes up the chimney. I would not want to force air through these devices faster than it is intended. But, given that there is a open gap and a cone at top of each gas appliance before start of flue, presumably there would be no direct "differential pressure" from bottom (intake) to top (exit) within each gas appliance. But, my plan is to feed fresh air through a 8-inch fresh air duct to feed one-at-a-time the gas burners having 4-inch and 6-inch flues. So, it is not currently apparent that a fan will be necessary to import the fresh air.

109A_5,

Quote: [You plan on living there the rest of your life ? The problem with building something odd for a residential scenario, is when you are gone at the first no heat or no hot water service call received for that address all that gobbledygook is going to be ripped out or disabled. The techs can't repair the equipment now who's going to maintain what you want to build ? … I feel sorry for the folks that live there when it is very cold out with heating equipment is locked out when they should be running."

These are good points to consider. I might be living there in my Eighties and forget how I wired this up, and the boiler or DHW has to be replaced by someone else. I think that to mitigate this issue, I will add TAGs to the wires that provide clear instructions HOW TO REVERT THIS [DHW, BOILER] TO INDEPENDENT OPERATION. The wires to be connected/disconnected will be labeled DHW1_1, DHW1_2, and wiring instructions to revert the connections to original independent configuration will be provided at the scene of the crime. Additionally, an operational/explanatory manual will be written for the setup as a whole. Also, each relay will have its function described on a label.

Also, the relay mesh (or control circuit) should be implemented such that the failure or removal of any one gas appliance from the room does not hinder nominal operation of the circuit for control of the remaining appliances.

Thank you for the insights.

As for constructing a "boiler room" partition walls around the two gas boilers (and the two DHW), that would not solve the original problem of not having a safe dedicated Fresh Air source in the basement as a whole. And, would hinder maintenance/inspection of the boilers and DHW. At this time, sheets of sheetrock are deployed in front as moveable partitions to prevent unintentional touching or access to the boilers (by persons transiting to the gas clothes dryer and washing machine)

109A_5

Quote: [if you open a supervised circuit you get an error. 7 Flashes: Gas control valve failure. Replacement is required]

Good point.

Depending on the sophistication of the method of verifying the burner solenoid circuit, it should be possible to overcome this "open" circuit error risk by using a double-throw RELAY such that the PCB thinks that it still connected to solenoid coil (burner) but is instead connected to a DECOY (coil or resistor of sufficient equivalent impedance/resistance). If the "supervisor" method is simply to measure a steady-state CURRENT (ON resistance = OFF resistance) through the valve then a simple resistor of the appropriate Ohms should be sufficient to prevent the "7 Flashes: Gas control valve failure" error? The static Resistance through the burner coil solenoid can be directly measured through the two PINs (ground at bottom).

This video states that a burner valve solenoid (measured between the PINS) should be 10 Ohms (not more than 13 Ohms). Thus, a 10 Ohm resistor presumably would be an appropriate and sufficient DECOY current path.

https://www.youtube.com/watch?v=ekEwRgPxdWM

A matching resistor equivalent can then be wired into the NO (normally open) position of the Relay, to act as a DECOY to satisfy the supervisor function of the PCB microcontroller while the "millivolt" power is actually disconnected (through the NC pole of the Relay) from the PIN of the burner solenoid. (The PCB board has two coils and two capacitors on it. So I infer it has at least one BOOST circuit to boost the thermopile milivolts up to around 5 volts sufficient to operate the microcontroller and/or the valve solenoids).
Thus, the same intended Delay-the-fire function can be obtained using an additional 24VAC-coil SPDT relay (controlled by the END Switch of the added 4-inch Damper and the END Switch of the 8inch Fresh Air Damper) and a Decoy (e.g., resistor).

The next question is, (how) could there be a distinct "Flame Sense Error" detector other than the thermopile itself. Can a thermopile be used to detect a pilot flame differently from a pilot-plus-burner flame? I see no additional wires to implement a distinct burner "flame sensor". The thermopile wire is connected through the TCO (rollout sensor) at the bottom. A Flame Sense Error may be limited to detecting a low voltage upon the thermopile wire (and thus would be activated by a turn-OFF of the TCO during a transient flame rollout caused by atmosphere pressure inversion). Thus, a Flame Sense Error should not result in a lockout requiring manual reset.

P.S.

Anecdotally, the 7 Flashes Gas control valve failure error in a "Honeywell" valve does NOT compel "replacement" of the PCB/Valve. But, such error may cause turn-off of the pilot valve and thus require manual-restart of the pilot flame.

https://www.reddit.com/r/Plumbing/comments/kq3xzs/water_heater_7_blinks_gas_control_or_valve_failure/

And see, reset instructions for a similar Honeywell (Resideo) WV8840C1513 gas valve.

How I reset my Honeywell (Resideo) Home Water Heater Gas Control Valve When It Failed With 7-Flashes Error Code.-Failure indicated "Gas Control or Valve Failure" -Valve: Honeywell (Resideo) WV8840C1513-Water Heater

"https://www.youtube.com/watch?v=7ZrApWtz4_0"

https://www.youtube.com/watch?si=pJZUyGgU9ThSsYSU&v=5f6UFwRlXr8&feature=youtu.be

Quote: [Page 22, 23 looks like the temperature sensor is just two common 10K @ 77 degree Fahrenheit thermistors.]

Previously, because of the 3-wires of the thermal sensor (and the microcontroller on the PCB) I concluded that the 3-wire thermal sensor is and "active" device sending a "data" signal. But now 109A_5 has shown evidence that the thermal sensor is an analog 2-terminal circuit element that has a passive resistance that varies inversely (almost linearly, but deterministically and monontonically) with increase of temperature. (I do not know what material on Earth has a resistance that decreases with an increase of temperature, but there it is)

This means that selecting an ordinary Resistor having a resistance on the chart provided at page 22 ( http://waterheatertimer.org/pdf/Bradford-white-service-manual-44943-D.pdf ) can be used as a DECOY (at the NO normally-open terminal of a Double-Throw relay) to signal a high-enough temperature (e.g. 150 degrees?) to delay the onset of FIRE-ON at the burner valve of the Honeywell. [In that case, this provides a second option for using a Relay to delay onset of Fire-ON. Because Double-Throw relays can be obtained with Four Poles, both DECOYs resistances can be deployed simultaneously if there is any advantage for doing so. ] This Decoy resistance is swtiched in for the purpose of avoiding the 5-blink error while tricking the microprocessor into believing that the water temperature is already high enough to not require opening the burner valve (and not too high to trigger a 4 blink high temperature lockout).

The 3-wire thermal sensor is positioned in the bulb in the back of the Honeywell valve. Thus, there is only one thermal sensor (location) on the DHW tank. I can access the three wires by removing the plastic cover from the Honeywell. Optimally (instead of cutting wires) inserting a wire-tap harness with two connectors, in between the plug on the cable of the thermal sensor and the matching PCB outlet

The 5-blink error signal described on the cover of the valve is triggered by the MPU's detection of some defective operation of the 3-wire thermal sensor. This error detection is necessary to provide reliability to the 4-blink error feature upon overheating (which triggers a "lockout" of the valve).

If a 5-blink (thermal sensor) error does not trigger shutdown of the pilot (or lockout), then I can easily Relay-switch among two of the three wires (e.g., the center pin and another pin as show at page 20) of the thermal sensor wires (using the End Switch in the Damper) through the DECOY RESISTOR, to delay firing of the burner.

Because during 5-blink error the MPU will turn off the pilot (upon cut/switch-off of any wire in the 3-wire thermal sensor) (and just cutting/switching the wire of the thermal sensor will produce an infinite resistance that will trigger high temperature 4-blink error lockout) it is necessary to use a double-throw relay and precisely tuned resistor to delay the firing of the burner. Depending upon the temperature that triggers a 4-blink overheating lockout, and depending upon how the MPU senses and verifies the data from the thermal sensor, this probably will work to prevent a 5-blink error. If a resistor is selected to prevent 5-blink error, it probably must be coordinated matched) with the temperature setting on the temperature dial on Honeywell (which can be inadvertently changed). This resistance-dial matching requirement is a possible risk of failure (e.g. failure to disable the burner valve, potential to continuously enable the burner valve). Thus, cutting/switching the burner valve solenoid voltage at the PIN may still be necessary.

109A_5

Quote:"The [Honeywell valve PCB boards] are using IRDA serial protocol as physical layer" and a Texas Instruments microcontroller.

You must be an electrical engineer to know all about that serial communication stuff.

I noticed that the Texas Instruments MPU has only ONE Analog-to-Digital converter. So, if I locate the external pin for that one ADC, I may be able to:

follow the solder trace to wherever it is deployed to measure [resistance through a voltage divider] [voltage at the thermopile plug] etc. In order to measure two different features, the ADC would have to be externally multiplexed (time shared) on the PCB. [implies the use of at least one binary signal at an output pin of the MPU and a selector device, e.g., Q, on the PCB] It may be possible to infer what quantities the ADC has been deployed to measure. Thus, it might be confirmed that the microprocessor is using methods [X, Y or Z] to supervise error features [3, 4, 5 flashes etc.] in a time-segmented manner. In that case, the probability of success [in using DECOY-substituting Double-Throw relays] can be increased.

P.S.

If one wanted to completely convert the Honeywell PCB board into a component of a remote monitoring system, they might connect another microprocessor to the various pins of the TI MPU, to:

1) detect and count the flash signals output to the LED by the TI MPU, thus externally monitoring the status of the DHW, etc.

All of which information could then be exported using an inexpensive ESP32 arduino via WiFi or Bluetooth to a local computer and/or or over internet. (or a LORA network)

Someone also might be able to salvage a Honeywell PCB board from a leaking DHW and deploy the PCB and its attached sensors for use in operating/monitoring other DIY apparatus, such as the guy here elsewhere who wanted to heat water in pipes using firewood.

P.P.S.

Concerning your invitation to explore the internals of the microprocessor through the serial port, I have seen youtube videos of experts doing similar things with commercial products. I don't believe that endeavor is strictly necessary for results in this in my project. If someone were to accomplish this exploration of the MPU, they should extract the MPU runtime firmware and use GenerativeAI to decrypt the Machine Code to detect and explain the algorithms being employed using the single ADC. This assumes that the GenAI model has been finetuned to learn the binary/hex Assembler Machine Code instruction set internal to that TI MPU, and also the internal address location values for the ADC etcetera.

The 24VAC RELAYS that I contemplate using to implement the Decoy-Swapping and control circuits are, for example, the Double-Throw 4-pole models:

HH64P (AC) at 24VAC Relays PLUS matching "HH64P Base" (at 10 AMPs)

available here.

https://www.aliexpress.us/item/3256806188043194.html?spm=a2g0o.detail.pcDetailTopMoreOtherSeller.1.2abd8OcN8OcN5n&gps-id=pcDetailTopMoreOtherSeller&scm=1007.14452.396806.0&scm_id=1007.14452.396806.0&scm-url=1007.14452.396806.0&pvid=2de5a988-fa16-4271-8988-bba368c67ec5&_t=gps-id:pcDetailTopMoreOtherSeller,scm-url:1007.14452.396806.0,pvid:2de5a988-fa16-4271-8988-bba368c67ec5,tpp_buckets:668%232846%238109%231935&pdp_npi=4%40dis%21USD%2112.61%215.07%21%21%2191.46%2136.77%21%402103209b17384399518136140e41b2%2112000036944364578%21rec%21US%21%21ABXZ&utparam-url=scene%3ApcDetailTopMoreOtherSeller%7Cquery_from%3A&search_p4p_id=202502011159118537766260083574659199_0

or here

https://www.aliexpress.us/item/3256805523330270.html?spm=a2g0o.cart.0.0.d20638da8GSOuZ&mp=1&_gl=111msb0j_gcl_awR0NMLjE3MzgzMDcwMzAuQ2owS0NRaUE0LXk4QmhDM0FSSXNBSG1qQ19HdW4zck9NdFdWbU11OXZ5ek8ydlZ3d1BmNUhGNHFlWXpWZ24wSno1bUo0cnRrOUFrV1l1c2FBa0UxRUFMd193Y0I._gcl_dcR0NMLjE3MzgzMDcwMzAuQ2owS0NRaUE0LXk4QmhDM0FSSXNBSG1qQ19HdW4zck9NdFdWbU11OXZ5ek8ydlZ3d1BmNUhGNHFlWXpWZ24wSno1bUo0cnRrOUFrV1l1c2FBa0UxRUFMd193Y0I._gcl_auMTE2NTk4NTI5OS4xNzM2NDY2Mzg4_gaMTI5MDgyMTg4NDA4ODE2NC4xNzM2NDY2Mzc5MzA2_ga_VED1YSGNC7*MTczODQzODIyMS4yNS4xLjE3Mzg0NDA3MzcuNDMuMC4w&gatewayAdapt=glo2usa

These are not very expensive (at quantity 10) and the 4-poles (four parallel 10 Amp switches as the fanout for each 24VAC relay coil) will increase design options and flexibility and probably reduce the total number of relay (coils) that would need to be employed and powered to make the control logic do what it needs to do. (Note I think that some vendors on AliExpress specify and offer different HH64P 5 AMP Bases and 10 AMP bases at different prices)

Unused poles or throws might also be wired directly to Arduino (E.g., ESP32) board pins to provide a system status reporting feature via WiFi, Bluetooth and/or internet.

109A_5

Quote: "yes there is only one converter of one type of ADC but is it multiplexed between 8 channels, so access to it is via up to 8 pins"

Good, thank you. That was puzzling, as most of the error codes imply an analog measurement, and there did not appear to be any channel switching IC on the PCB. The channel switching is done internally within the microprocessor and various MPU pins are used to measure different parameters as voltages (at 8 different times). The same visual method of interpreting the PCB board can be applied given the identity of the 8 ADC sampling pins, to help determine the method of computing the "error" for each signal available on the PCB Board (e.g., thermal sensor (resistance), thermopile voltage, resistance through the burner coil, etc.)

The 8-time multiplex of the single ADC also means that there is an 8x-lower risk that during the Relay-switching interval (while the throw is flying from one pole to the other) there will be a measurement glitch to trigger a error code. In some cases this glitch risk could be further mitigated. Because there are two avenues to delay onset of burner FIRE-ON, presumably one of the two circuits will be transited to the Decoy resistor in a glitch-free manner at the moment that the relay throw flies. At worst, the PCB control circuit will (without causing a burner valve FIRE-ON) shut off the pilot and trigger a lockout requiring manual reset and restart of the PCB board (at least, in the above stated model of Honeywell valve PCB).

Quote:[I still think the engineering zeal should be on updating the building to safely provide the proper combustion air for all the heating equipment.]

There are two overriding concerns that require the feature of being able to externally turnoff/delay the DHW and to install a Flue Damper on all convection-operated gas burners. (Clothes Dryer operation pulls in Carbon Monoxide Gas down chimney) (carbon monoxide detector shut-off feature)

There already is enough "combustion air for all the heating equipment" because the house basement is old and drafty, and a door/window is intentionally left open to admit air in the summer and winter. But, this leaves the basement very cold, unheated, unused in winter, and one boiler pipe in baseboard in basement froze burst during a sharp cold snap this winter. Plans are for using foam around rim joist, closing basement-ceiling holes, and other upgrades to air-tight the basement, which will more require a dedicated fresh air supply. Because of winter cold, a damper on the new fresh air intake is necessary. A damper must be coordinated with the fire-ON of some or all of the gas burners. Concrete block must be broken to bring in fresh air, so this and space/existingplumbing, practically limits to one (time-multiplexed) dedicated air with a 8-inch-tall damper.

More compelling: When the Gas Dryer (or any dryer) fires, the internal fan creates suction in basement and blows warm humid air out of the basement. This carries the potential to draw undampered flue combustion gasses DOWN the chimney and into the basement. Thus, combustion gas from DHW can accumulate in basement for an hour during dryer operation. And, also the two Utica boilers, although they have 6-inch dampers, can have their combustion gas in chimney sucked down chimney and accumulate in basement during dryer operation. Plans for further Air-tighting the basement will increase the risk of this combustion fumes accumulation problem.

Especially given the clothes dryer and the plan to air-tight the basement, there is a risk of carbon monoxide accumulation because of four other gas burners (2 independent gas boilers, 2 independent gas DHW) (currently all convection-based because hard to PVC pipe fresh air and exhaust gas at this space). I have already purchased the FIRST ALERT brand RELAY that has NC and NO (Double Throw, Single Pole) activated when the wired 120VAC-powered CO Detector alarms. So, with this CO Detector switch on hand, it is optimal to at least have a way to DELAY or SHUT OFF the independent gas DHW in case of CO Alarm. Combining the FIRST ALERT relay with a four-pole Relay (from AliExpress) provides an easy way to SHUT OFF or delay-fire at least four separate gas appliances (2 independent gas boilers, 2 independent gas DHW) assuming that the standalone gas DHW has an external wire that can be cut/switched (obtaining an external wire that can be used DELAY fire, without triggering lockout and manual reset, is object of this modification). [Obviously, I can cut/switch the red/white wire at the bottom TCO (rollout switch) to TURN OFF the Pilot and burner of the DHW. But, this requires manual restart ]

In short, the risks described are common, and Honeywell and DHW-maker should have already provided the necessary external 5 volt input wire to enable switchable external control of delay-fire of the burner valve, and a 5Volt output wire for relay-activation of a flue damper and fresh-air damper.