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t-stat overshooting

You need a thermostat with more than just a basic anticipator; you need a thermostat that can continually learn how your system runs and compensate for both overshoot and run time. The Honeywell RTH7400D has this feature. It learns how your system works and compensates for overshoot and run time automatically. I have it running my steam system within two degrees and one degree in the room that it’s located in. All the other programmable thermostats I’ve tried would overshoot as much as 10 degrees. The flywheel effect with a steam system is more than your basic anticipator can handle on a basic T-87 or programmable thermostat.

Comments

  • Timco
    Timco Member Posts: 3,040


    I have a couple different t-stats in the house, both programmable. Figured this would save money, but both let the temp get 2-4* hotter than the stat is set for. Any advise on brand / model to save the $$$ I am spending on overshooting the desired temp? HW system with CI rads...

    Tim
    Just a guy running some pipes.
  • Scott Larabee
    Scott Larabee Member Posts: 28


    Tim,

    I have the same problem with my cast iron rads! I just have an old T-87, and now matter how I set the anticipator, it overshoots! The fact is that the rads are high mass, and once they are hot, they will give off heat until they cool down! I've been thinking of installing an outdoor reset, my thinking on this is that the rads will be running cooler, and therefore won't have as much heat to give off after the thermostat is satisfied!

    Scott
  • Have you guys

    done heat-losses on your homes, and compared them to your boiler sizes?

    Same thing happened to me- boiler was oversized and so was the circ. I down-fired it and went to a smaller circ, and that helped.

    The Honeywell digital programmable stats with "smart recovery" will help too, since they can adapt some to the system's characteristics.

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  • Scott Larabee
    Scott Larabee Member Posts: 28


    I haven't taken the time to do a heat loss on my house, I'm too busy taking care of the customers! I believe that if I do a heat loss, I will find that my house is over radiated, and the boiler is too big! On the occasion that we have seen -20* here, and it used to be a not so uncommon occurance, I could set the heat on 72* and have it cycle!

    The boiler piping is a huge mess! The main system is a monoflo, but there have been a couple of loops added, as well as an indirect. I rent the house from my Father, so anything I do he has to approve. I would really like to repipe the boiler, and straighten out the rats nest. I'm sure the circulator is oversized as well. I think, given that the house is over radiated, I would benefit from lower system temp. in the main zone as well. The seperate loops are properly sized, not installed well, but sized right. Of course, after doing the heat loss and determining that the boiler is too big, a MPO would look real nice in my basement!

    This house also has no insulation in the walls, and only 4" in the ceiling. Newer windows, but old wood doors. House is around 100 years old, and drafty as can be. There's more work.
    Ok, there goes my summer! LOL

    Scott
  • Timco
    Timco Member Posts: 3,040


    My heatloss is dead-on for my boiler. WM CGA gold cat I CI no frills boiler. 70K BTU in a 1500 sq' 1910 bungalow. System is converted gravity so large pipes, 2-pipe setup with older 15-42 grund 3-speed circs. I will try outdoor reset and the t-stats you mention to see if that helps, but when set to 73, I get to 77 at the highest after the call for heat is done. If I set back further, it gets colder than I want it before the heat call, of course....

    Tim
    Just a guy running some pipes.
  • Mike T., Swampeast MO
    Mike T., Swampeast MO Member Posts: 6,928


    Are your secondary circulators running constantly or do they go on-and-off with a call for heat from the thermostat(s)?

    If they're running constantly, you might be able to slightly reduce overshoot by having them run only when the thermostats are calling for heat.

    Some degree of overshoot is nearly guaranteed when you are recovering from setback. The thermostat thinks that the supply of heat stops--quite rapidly--once it is satisifed. If your supply temp is sufficient for fairly rapid setback recovery, then those massive pieces of cast iron will continue to give off heat LONG after the thermostat stops calling--the more moderate the weather the more the overshoot.

    Some form of reset (simple mechanical should be fine) seems to be warranted.

    Do you already have some sort of low return temp protection for the boiler?

  • Al Roethlisberger
    Al Roethlisberger Member Posts: 194
    How about a Honeywell T8602D


    I too am looking for a thermostat that would work well with my old hot water and cast iron radiator system.

    I have the same issues as most do with these, and was looking at the Honeywell T8602D thermostat.

    Does the Adaptive Intelligent Recovery feature really seem to work well and is it automatic as the name implies?

    Thanks,
    Al

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  • Bill W@Honeywell
    Bill W@Honeywell Member Posts: 164
    The answer is Yes...

    But the T8602 is no longer available, it is obsoltete. The replacement is the VisionPro TH8110U1003. Recovery works by gradually ramping up the setback temperature so that the occupied space is at the comfort temperature when the period begins. Say you set back to 58 at night(sleep). Depending on the home's heat loss, the Adaptive recovery will start the climb back to your comfort temp of 68 by calculating how long it will take to reach the comfort temp. It does this in slow increments, but at 6:30 AM when the "wake" period starts, you will be at 68 degrees. The older stats without this feature will just start the equipment out of setback at 6:30, with no regard to comfort.
  • Timco
    Timco Member Posts: 3,040


    Bill,

    Will these units help with overshooting by themselves, or would an outdoor reset do more to help with these t-stats you mention?

    Tim
    Just a guy running some pipes.
  • Timco
    Timco Member Posts: 3,040


    Not constant circ...only on heat call, and currently no reset. I will be installing a 260 this week. This should help with the overshooting as well as save $$$ on warmer days. Am running @ 140 today, but I put the a-stat there myself as it is really unseasonably warm weather.

    Tim
    Just a guy running some pipes.
  • Bill W@Honeywell
    Bill W@Honeywell Member Posts: 164
    Reset is a great tool...

    "cruise control for your house", if your budget allows for it. Our AQ2000 series is a good option, if you want a simple, effective reset system. Check it out a www.customer.honeywell.com. No stat or reset system will make up for an oversized boiler, though.
  • Timco
    Timco Member Posts: 3,040


    My boiler is properly sized, but lacks the ODR and decent stats is all. Would you recommend the stat described above with a ODR, or one or the other...

    Tim
    Just a guy running some pipes.
  • Bill W@Honeywell
    Bill W@Honeywell Member Posts: 164
    The AQ2000...

    uses it's own communicating t'stats, 1 per zone. All programming is done on the reset module. The zone module will handle up to 4 zone valves OR 4 zone pumps. You can't mix pumps and valves on the same module.
  • zeke
    zeke Member Posts: 223


    As pointed out,overshoot with an oversized boiler plus oversized radiation with high thermal capacity cannot be cured by tstat and reset alone.

    What you really need is a reset system that covers the full range of outdoor temperature conditions that require heating;the standard reset system is limited since on warmer days the required water temperature would be too low for the boiler.

    To solve this, I would install an indirect tank heated as a zone from the boiler and dedicate its use (not for domestic hot water since its reset controlled temp will get too low on warmer days ) for such days that the reset temperature is too low for the boiler.

    You would need a 3 way valve to switch over at the the outdoor temperature crossover point.
  • Timco
    Timco Member Posts: 3,040


    Maybe this will do it. I bought a tN4 setup, with 2) 542 stats, a 420 reset module and a 336 zone manager. From what I learned, this will indoor & outdoor reset each zone independantly, and will do well for me when I change to a mod-con and indirect. The wiring is the easy part, programming looks to be much harder. I looked for the Hywell AQ 2000 but Johnstone looked at me like a deer in the headlights...

    Tim
    Just a guy running some pipes.
  • Plumb Bob
    Plumb Bob Member Posts: 97


    The OP reports overheating during normal operation, so Bill@Honeywell recommends a stat that does good recovery from setback! Setback has nothing to do with the problem, and no electronic stat does anticipation.

    Others are talking about matching the boiler to the heatloss, but that has little to do with the problem either! Every boiler is always oversized in spring and fall.

    The house is overheating because of a combination of overradiation and 180F water in mild weather conditions. Outdoor reset will help the problem. Until you get outdoor reset installed, you'll get some relief by turning the boiler aquastat down as far as it will go (140F or so). Cooler water = less overshoot, but if you hit a very cold spell, you'll have to turn the aquastat up manually.

    With your current stat, increasing the number of cycles per hour will help also. Ignore what it says about how water being 3 cyles/her and forced air being 6. Those are mindless formulas. Set the number of cycles higher than it is now. Of course this means shorter-cycling.

    Outdoor reset won't help as much as it would if you had a full range of water temperatures, which requires bypass piping.

    If it is any consolation, many former gravity homes are overradiated, and many of them run on bang-bang (180F) heating using cast-iron boilers, which is what you're reporting.
  • Timco
    Timco Member Posts: 3,040


    This new setup I am installing will calc the heat rise rate, or 'anticipate' each zone independantly so the room with the glass will get hotter water that the main house zone based on the rise in temp, indoor temp and outdoor temp. It should really kick butt, and has fully programable t-stats as well. It has an extended unoccupied feature, and will run the mod / con as well as the indirect with simple wiring when I change over. My rads & boiler are very well matched to the home's heatloss and I am still overshooting even with 140* water as I already set the temp back myself. These stats creep up towards the desired temp as it was explained. This boiler was installed in Feb 2 yrs ago with no heat in about 5 hours, or it would be a better system.

    Tim
    Just a guy running some pipes.
  • Bill W@Honeywell
    Bill W@Honeywell Member Posts: 164
    The AQ2000

    has setback as an OPTION. I proposed this control as an outdoor air temp reset. That's like cruise control, using just the heat you need. The "flywheel" effect of overradiated homes is tough to tame, but reset is a good way to keep it under control.
  • John Cockerill
    John Cockerill Member Posts: 94
    boiler size


    try ramping back your aquastat 5 degrees a day until comfortable. Do not go below 140 F.

    See do it your self page; www.Exqheat.com
  • Timco
    Timco Member Posts: 3,040


    One last time, my boiler is NOT oversized, and I am not a do it yourselfer, but the water was far above current conditions. The tN4 system really has done the trick, and I now do not overshoot and am making 140-150* water rather than 180. This is a real state of the art system and has really cool features. I highly recommend it!!!!! BTW, you can find any info you would ever need right here on this site...

    Tim
    Just a guy running some pipes.
This discussion has been closed.