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Set-back t-stat cold in am

I have a Honeywell RTH7400 that is supposed to learn what time to heat the home in the morning. When the outdoor temp is real cold, as in 10 degrees, the morning temp at 6:30am is 65 and doesn’t hit 70 until 8am. Does this mean I need more radiation, is the t-stat defective, or what? It is properly configured for hydronic baseboard heat. I hate the idea of keeping it at 70 during the night to have it be comfortable in the am. The t-stat is fine when the night temp is above 25.

Steve from Denver, CO


  • NY_Rob
    NY_Rob Member Posts: 1,370
    What type of boiler do you have... legacy cast iron, mod-con, etc...

    What are your normal setback/recovery temps?
  • Zman
    Zman Member Posts: 7,607
    1 1/2 hours to make up 5 degrees seams reasonable enough.
    I am not convinced that short setbacks save any money at all. By the time you heat the air and more importantly the mass of the house, I believe that the money you saved is burned up in the morning. Think city driving vs highway driving.
    If you like the setbacks, why not just set it to start a 6 am?
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • STEVEusaPA
    STEVEusaPA Member Posts: 6,505
    edited January 2019
    Well if it's trying to learn you may need to give it more time. Is the thermostat sensing outdoor temperature?
    However 5° in 1.5 hours sounds better than average if your system is properly designed. In your scenario, the system would want to start recovery 1.5 hours before the time set on the thermostat.
    As you approach design temperature, the system should (in theory) run almost continuously.
    I only experienced long recoveries at my house before I had a generator and we would lose power. I have 5 zones and don't set any of them back. You could try a narrow setback to see if that helps with recovery.
    How long does it take the house to go from 70° down to 65°?
    If you want to continue this deep setback, you'll have to consider some options based on your entire system.
    Do you need the entire home to get down to 65°?
    What rooms do you want up to temp first (usually bathroom/kitchen)?
    Is you house zoned, can you make 1 or 2 zones priority so they heat first/faster?

    There was an error rendering this rich post.

    Steve Garson_2
  • Voyager
    Voyager Member Posts: 395
    I have several Honeywell “Smart Response” t-stats also and it isn’t obvious to me how their algorithm works or even if it does. I have 5 of these in our church (baseboard hydronic) and the church system responds very slowly to setbacks. It takes about an hour for each 1 degree rise when it is below 20 and below 0 it basically can’t recover. So, I reduce the setbacks in really cold weather (that is where the WiFi comes in handy) and start the recovery 24 hours in advance. Being a church, we only heat it two days a week so our setback and recovery periods are much longer.

    The slow recovery isn’t too surprising when you consider how heating systems are recommended to be designed today. Typically the “design temperature” is in the 0-10 range depending on where in the northern US you are located. If the system is truly designed for this temp, then it is putting out just enough heat to hold 70 degrees (typical inside design temp) when it is say 0 outside. When you set back to 65, your house loses heat more slowly, but the difference in heat loss between 70 and 65 is the only “excess” heat your boiler can produce to recover temperature. And as he inside temp approaches 70, the “excess” heat your boiler can produce approaches 0 so the recovery will get slower and slower as you approach 70 where, in theory, your boiler’s maximum heat output exactly matches the home’s heat loss. I don’t think this is often strictly true as the heat loss calculations and assumptions are generally conservative and typically you can’t match the boiler exactly to the heat load so you normally have a slight oversize in regard to maximum boiler output.

    If you designed the boiler and radiators to have enough excess capacity to rapidly recover from 65 to 70 when it is 0, then are will be grossly oversized and inefficient for 98% of the heating season. At first blush, it sounds to me like you have a properly designed system and you simply may have to lessen your setback in very cold weather and/or start your recovery an hour or so earlier when the temps get to 10 and below.
    Steve Garson_2
  • Jamie Hall
    Jamie Hall Member Posts: 23,675
    Or to put it a little more simply -- give the poor thing a break. It isn't as smart as you are. If it's cold out, and you want it to be 70 when you come down for coffee, acknowledge that it's going to have a hard time and tell it to start warming up earlier.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
    Steve Garson_2ethicalpaul
    EBEBRATT-Ed Member Posts: 15,849
    @Steve Garson_2

    Can't set it back that much with cold temps outside. Don't know where you are located but In Massachusetts where I am we design for 0 outdoor air.

    That means here a properly sized boiler will run close to 100& of the time when it is 0 out. And just because we design for 0 we still get -5 or-10 occasionally. When this happens the indoor temp will slip. normal indoor design temp is 70.

    Keep in mind when you set the temp back to 65 at night the air cools to 65, but so does the walls floors furniture etc....everything in the house cools down.

    Now when the heat tries to come up in the morning you have a room full of "65 degree ice cubes" all the contents are keeping the air cold until those objects warm up.

    The problem isn't your thermostat. You could have a problem with your system but if you are having it serviced and checked and cleaned as you should it's probably not that.

    You could investigate your boiler capacity, house heat loss and installed radiation if you want too.

    It may be possible to add radiation to make the system heat faster which could save fuel if you can run lower water temps when full load is not needed
  • Steve Garson_2
    Steve Garson_2 Member Posts: 712
    Thanks all.
    Steve from Denver, CO
  • pecmsg
    pecmsg Member Posts: 4,968

    Remember that allegedly Smart thermostat senses temperature only, if theirs any winds it will take longer to recover.

    When we get too single digits outside and the winds blowing i cancel the set back for that night poor system can take 4 hrs to recover!
  • mikeg2015
    mikeg2015 Member Posts: 1,194
    Just use a smaller setback.

    A system large enough to recover will be oversized and short cycle usnkng more fuel the other 95% of the time.

    But most costumers will,complain if a system can’t recover so comtractors obersize a little.

    A 60k 98% furnace if capable of a $300 natural gas bill. But you Find them mostly on homes with $100 bills.
  • unclejohn
    unclejohn Member Posts: 1,833
    You have to tell some stats that you have rad. heat or heat pump or forced air.