Click here to Find a Contractor in your area.
In fairness to all, we don't discuss pricing on the Wall. Thanks for your cooperation.
Need to contact us? Visit

24v delay relay

ttekushanttekushan Member Posts: 901
I get mine from Mouser electronics, though DigiKey should have them also. You can shop on line from either's website.


  • BostonBoston Member Posts: 107

    I want to add a simple 24v time delay relay to my boiler. Anyone know where I can pick one up and what the proper wiring would be? I think I remember reading a thread were someone added one to their boiler.

    My boiler is short cycling from pressure after all the radiators are hot. It could kick on and off 5 -7 times before the thermostat is satisfied. But if I could get the boiler to wait a few minuets before it kicks back on I think I could reduce the cycling by more then half.

    Seem like a very simple fix, am I missing something?

  • time delay

    that cycling is a normal part of modern boiler don't want it to continue firing after it reaches its low cut-out pressure.
    maybe you need a better thermostat/location which will turn the boiler off when the selected room temperature has been satisfied.
    how was this boiler sized?
    maybe a hi-lo burner would keep the pressure in the sweet spot without cycling the burner.--nbc
  • BostonBoston Member Posts: 107


    I didn’t think they made a modulating gas burner for steam boilers? The installer did a complete heat loss and calculated the boiler based on those findings. It is slightly oversized because next year we are going to add a hot water tap to heat an addition.
    After much work and tweaking the system works great, but after all the radiators are hot, it tends to cycle on and off. I put the thermostat in the warmest room (the room with the largest radiator) to help reduce the over shoot, but because the system cycles so quickly, it doesn’t give the thermostat a chance to register the heat.

    I thought if I can get the system to wait 2 -3 min before cycling back on, it will give the radiators a chance to get that heat into the room.
  • modulating burner

    some steam boilers can have a hi-lo burner. i may try one on my atmospheric peerless.
    some experts think that the mixture will be out of adj. on either the hi or the lo.
    it is difficult to find the perfect spot for the thermostat.i chose a cold area for mine and set it lower, but it's still not as even as i like.on my visionpro thermostat there is no anticipator-only a "cycles per hour" selector[set to 1 cph] which i think is trying to do the same thing as your time delay.-nbc
  • BostonBoston Member Posts: 107

    I have the visionPro 800 as well. From what I understand the anticipator functionality is now part of the thermostats internal programming, I’m not sure it works as well as intended. My system still has a tendency to overshoot the temperature by a degree or two (not nessisary a bad thing when you have a wife and child that are always cold). I have the CPH set to one, but since the thermostat doesn’t recognize the boiler is cycling on pressure, I think it still considers it part of the first cycle… even though it has cycled several times.

  • Here's an item that might be of help to you.

    It has a adjustable delay of up to 10 minutes. It also delays AFTER the cycle. Download the installation diagram PDF It has instructions for wiring it up.

    Your "problem" seems to be very common. Ideally we all would have a perfectly balanced steam system and this cycling wouldn't occur but in reality that doesn't happen that often.(A lot of us are experiencing the same thing you are) You need to check to see what is causing the boiler to cut off. If it is a low water cut off there are work arounds to solve that. A pressure cut off is harder to do.

    I've sort of been playing with the idea using of a thermostat with adjustable cycles per hour. I haven't figured out yet the principle on which they work so don't know if they would solve the short cycling situation.

    I also see that someone today has posted a question involving hour meters. An hour meter might be a good thing to try as you could figure out your burn times (oil use) to see what settings/adjustments get the the best economy while still preserving the house temperature you want. I'm rather curious about the pros and cons of evening/day setbacks - Do you save more fuel making using a large setback (15+ degrees)or use more fuel making the setback up vs is better to just use a setback of say 5 degrees? The short cycling and burn time economy are my next projects.
    Let us know what works out for you.
  • David NadleDavid Nadle Member Posts: 624
    I used this one

    If your VisionPRO is wired to run off the 24V, and you cut out on pressure, it's possible the power is being cut to the thermostat as well. Something to look into. If so, the fix is to isolate the tstat with a relay.

    The VisionPRO uses a P+I feedback algorithm in place of an anticipator. There is a setting to make the control "less agressive" which may help prevent overshoot. CPH is a limiting value; I do not believe it will turn off the stat early.

  • BostonBoston Member Posts: 107

    Thanks David,

    This will work, how did you end up wiring it? I looked through all the programming setting in the visionpro and the only setting I found was the CPH? Do you happen to know the code to make it less aggressive?


  • Edit: Use David's instructions. Sorry - I forgot that the pencil modification we did on the installation drawing isn't on the downloaded one.
    Getting it from your local Grainger is far easier than the source I used. (I'd wish I had known that)

    David- Any thoughts on the Vison Pro? Any idea how the cycles per hour system works? How is the delay system (ICM module)working for you?

    The ideal thing would be a modulated burner so like a pan of water on a stove you could turn on High and then when the water was boiling, turn the heat down to the point where it is just remained boiling.
  • David NadleDavid Nadle Member Posts: 624

    [EDIT #2: Moving the relay maybe didn't fix the problem.]

    [EDIT: I originally installed the relay after the vaporstat but before the thermostat. The ICM203 timer resets if a switch after it opens, so it was delaying start every time the thermostat cut in. Moving the relay after the thermostat fixed the problem. I've edited the instructions below to reflect this change.]

    The relay goes in series immediately following the thermostat. Turn off the boiler, then disconnect the wires from TT. Remember which color went on which terminal. Turn on the boiler and measure the voltage from each tstat terminal to the transformer common. One terminal should have 24V, the other zero. Turn the boiler off again. Reconnect whichever tstat wire was on the hot terminal, then connect the other tstat wire to the 'input' terminal on the relay. Finally, run a wire from the 'load' terminal on the relay to the other tstat terminal. Turn the boiler back on.

    I have a length of DIN3 rail screwed into the side of the boiler for easy mounting of all my little experiments. You can bolt the relay to a DIN3 clip and pop it on or off.

    For the heat temperature control you could try code 0680 and set it to 3. It should normally be 2.

    I recommend you make one change at a time, and wait 2 days or so to gauge the effects. It's possible that by stopping the late short cycling with a few minutes' delay you will also cure the overshoot problem.
  • David NadleDavid Nadle Member Posts: 624
    UPDATE: possible improvement to the wiring instructions

    It looks like if you wire the relay so it's after the pressure switch but before the tstat, and the tstat opens, it screws up the timer. Next time the tstat closes you have to wait the delay. That's no good. The relay probably needs constant connection to the load.

    I will place the relay after the tstat and report back. This should fix the problem because everything after the tstat is normally closed, and the interval between tstat open and close should be longer than the delay.
  • BostonBoston Member Posts: 107

    I changed the setting for code 680 from 2 to 3 and it seems to make a small difference. I noticed that the boiler now shuts down earlier and the overshoot has been dropped by 1 degree. It did not change the fact that my boiler is short cycling on pressure.

    David, how did it go with placing the relay after the Tstat?
  • Al HendricksonAl Hendrickson Member Posts: 2
    Maybe not a good idea.

    If you delay the burner too long from coming back on every time it hits the high pressure limit, the boiler will stop making steam, the vents will open and the system will fill with air again. The next time the boiler turns on, the air will need to vent again. Instead of just cycling the burner, you'd be short cycling the entire system.

    I think it would be more efficient to keep the system full of steam until the thermostat is satisfied . . . even if it means cycling the burner a few times.

  • Al- This has bothered me too! Sort of like boiling a pot of water and then moving it back and forth between the stove and the frig. I'm trying to decide in my mind whether the cooling off of the mains due to the intake of cooler air through the vents, is offset by the saving(?)of fuel. I need to first figure a way to time when the main vents open after the heat cutoff. I've been wondering about taking off my main vents (Gorton #2) and insulating them with spray foam so they wouldn't open as quickly(?)though I need to cut one apart first to determine what exactly the mechanism is ( is it a vacuum breaker or just bimetal or both?) and therefore whether insulating them would be in any way beneficial.

    This whole situation seems to be similar to boiling a pan of water on a stove. You turn the heat on high to boil the water quickly and then when it boils you turn the heat down to just maintain the boiling point. The idea thing would be to have a fully adjustable burner with multiple setting like you have with a stove top burner. If an adjustable or 2 stage burner isn't practical, two separate burners High & low with changeable nozzles. I've also been wondering if one could setup some sort of heat sink, like a thick ceramic plate with holes in it, between the flame and the boiler so that the residual heat from the plate would provide continued heat after the flame shut off, sort of like coals from a wood fire. Unfortunately modifications like this aren't really practical (or safe) for homeowners.

  • David NadleDavid Nadle Member Posts: 624
    Good question Al

    Boston: I edited my instructions above to place the relay after the tstat. It's working properly now.

    Al, when my boiler hits the pressure limit and cuts out, which is only happening after a 30+ minute run, the pressure drops to about -2 oz. (vacuum) within 20 seconds and is back at zero within 40 seconds. The damper shuts and then the whole thing starts up again. Igniter, damper open, etc. After 40 seconds of burn the vaporstat cuts out again. It's an 80-90 second cycle. Intuitively this just seems wrong. Yes, maybe I'm sometimes putting an extra cycle on the vents that were holding, but I see that as a good tradeoff.

    My theory is that there is thermal inertia in the radiators and there is some delay that can be found which will prevent any further cycles more than 50% of the time when a pressure limit is hit. The pressure is supposed to hold long enough to create that delay (and be adjustable with the cut-in setting) but on my system and others' it just isn't working.
  • Al HendricksonAl Hendrickson Member Posts: 2

    I have given this considerable thought myself. I have a Burnham I6 boiler in my house with intermittent pilot and a subtractive pressuretrol. It's now set at 2 PSI cut-out and the minimum 1.5 PSI differential. I experimented a while and finally got it set up so my system doesn't suck air between burner cycles, but it's close.

    When the boiler hits 2 PSI it shuts off the burner. The iron in the boiler retains a lot of heat and the water continues to boil for a couple of minutes. When the pressure drops to .5 PSI the ignition sequence starts. It takes about 30 seconds to fire up. Just before the pressure hits zero, the boiler starts steaming again and the pressure starts to climb.

    I know that using lower pressure settings is generally more efficient . . . but without a two-stage burner and given the considerable delay in re-starting the burner, I believe my (probably somewhat oversized) boiler is operating about as efficiently as I can get it. Installing a vaporstat would probably only reduce the on time and increase the cycling. A standing pilot would allow for an almost instant re-start, but that would hurt the overall efficiency as well.

    I inquired about retrofitting a 2-stage regulator and a vaporstat to initiate low-fire on my I6, but the local boiler expert advised against it, saying that at a lower gas pressure, the combustion mixture would be off and efficiency would actually drop.

  • David NadleDavid Nadle Member Posts: 624
    Bad analogy

    It's not like moving the pot between stove and fridge. After shutoff the system continues giving off its heat to the rooms, it's not wasted by heating up a "fridge."

    Considering that the delay will sometimes prevent any further cycles before the residual heat satisfies the tstat, I think the efficiency is a wash.
  • Al HendricksonAl Hendrickson Member Posts: 2

    So what is more efficient in the absence of modulating the flame? . . . my Pressurtrol allowing the boiler to produce a little higher pressure and letting the burner run longer, or your Vaporstat operating at a lower pressure, but making the burner cycle more frequently?

    Obviously, neither is ideal.

    At least your system will hold vacuum for a little while, excluding air and, in effect, increasing the pressure diferential.
  • David NadleDavid Nadle Member Posts: 624

    What makes hi-lo so much better than on-off? On-off is 100% modulation! I'm being facetious but I think it's a valid point.

    It's not the vaporstat that's making the burner cycle more frequently. If I had a pressuretrol and cut out at 2 psi I'd still lose pressure in 30 seconds. I don't have a good explanation why. If I knew how to change it I wouldn't use the relay.

    My steam system is supposed to cycle. Yours too, probably. It's not designed to run 100% of the time. 1 psi steam is 216 deg., 2 psi steam is 219 deg. The extra pressure and burn time is not making your radiators much hotter.

    If I didn't have various electromechanical parts on the boiler to be concerned about I'd say, cycle away! I think the relay is protecting those components, so far without sacrificing comfort. I'm not convinced that efficiency is suffering either. I think we'll need to do the math on that one. There's too many assumptions being made by both of us.

  • Al HendricksonAl Hendrickson Member Posts: 2

    If you can modulate the flame between "pressure slowly climbing" and "pressure slowly falling" you can maintain a really low steam pressure without sucking air into the system.

    It also stands to reason that with a lower flame, the the hot gasses would stay in the boiler longer, proportionally more heat would go into the water, your stack temperatures would be lower, and overall efficiency would be better.

    I doubt we're talking about more than a couple of percent efficiency though.

    I need a temperature-controlled laboratory, some thermocouples, a datalogger, a mass flow meter, and lots of spare time to answer this one. Got any?

  • I'm more in favor of the boiler just "coasting" along philosophy rather than the start/stop.

    I'm trying a delay (5 min) on the boiler. Unfortunately it's my mother's house and so I don't have a chance to monitor it closely other than when I pay the fuel bill. (next project is an hour meter) I mainly put the delay in to stop my mother complaining all the time about the starting and stopping.

    David - How long a delay are you using? BTW- I really appreciate your input on this subject. The really neat thing about this website is that we can share ideas and all learn a lot.

    This short cycling "problem" appears to be pretty common.. A lot of people seem to have oversized boilers or like in my case, our boiler is now oversized as we have shut down the heating in unused rooms to try to save fuel. While Al mentioned that it might be only " a couple of percent efficiency", the one thing I have learned, in tinkering around with my steam system, is that even a small "tweak" in the right direction can save you a lot on costs over the whole heating season.
  • David NadleDavid Nadle Member Posts: 624

    My boiler is not oversized. Its rating is actually 1 ft^2 less than the connected EDR. The boiler does "coast along," operating at a plateau pressure of ~6 oz for at least 30 minutes before it starts to increase. Most of the time the tstat will be satisfied before then.

    On cold days, or if I used setback (I don't), I can expect to hit the cut out pressure. That's when the short cycling begins. I respect Al's opinion but I think it's just as plausible that elimination of late short cycles improves average efficiency instead of hurting it.

    I picked 5 minutes out of a hat too. I think longer is better, provided that comfort does not suffer, like undershooting the tstat.

    I share your thoughts about this site and the people on it. They're the best.
  • chuckNJchuckNJ Member Posts: 38

    If your boiler size is sufficiently close to the load size (and the system otherwise operating well) such that it builds preasure only when all the rads are full (all air vents closed), you can manage the time to reach that point (and the resultant cycling) by slowing down the venting on one or a couple of selected rads. The radiator in the room with the stat is an obvious candidate, and the adjacent rooms that may affect the stat should be considered as well. There may be other points in your system where a slower rad could either be tolerated or even desirable in the total balancing. The common situation of radiators that have been rendered oversize by building envelope improvements may actually facilitate this strategy as you can get enough heat out of them for the improved structure even as they fill more slowly. How long does it typically take to heat your house when recovering from setback? Look far ways to stretch out the venting somewhere in the system to get out to this time before you build presure. This has worked well for me. Venting ALL the rads as fast as you can may not be working the system in the most holistic manner.
  • David NadleDavid Nadle Member Posts: 624

    I would say I'm already where you're at. It takes 30-40 minutes for the last vent to close. The only problem with this is you're burning fuel the whole time. The ideal situation is to time it so all the rads fill at the same time on a cold day, as fast as possible, and cut out at the lowest pressure you can. I'd be happier if I could fill the rads in 15 minutes, cut out, and wait a bit to see if I need to cycle again.
  • David NadleDavid Nadle Member Posts: 624
    UPDATE: spoke too soon?

    Well, it looks I spoke too soon on this ICM203 relay. I thought moving it after the tstat had fixed things but I just happened to hear the tstat click on and went downstairs to find the boiler still off. It does come on eventually. I'll do some tests this weekend and see if I can pin down what's happening.
  • mikemike Member Posts: 675
    that 203 won't do

    what your attempting to do bec as soon as you lose your 24v, and that's what's happening, all bets are off. those 203s are generally used for a/c compressors. and to avoid a separate stat relay, if you can wait until tuesday, i'll get the info for you
  • David NadleDavid Nadle Member Posts: 624
    No it won't

    Thanks for your interest Mike. I agree the 203 isn't working out. I'm going to go back to my original idea, which was to isolate the vaporstat with a delay on make relay. I attached a schematic of the current and new circuits.

    The idea is that if the vaporstat cuts out while the thermostat is still calling for heat, the delay on make will hold off a fast cut in by the vaporstat, but the delay will have long expired by the time the tstat calls again so there will be no delay on startup.

    Love to see what you have for us on Tuesday.
This discussion has been closed.


It looks like you're new here. If you want to get involved, click one of these buttons!