Best way to handle air temperature overshoot after boiler runs heat cycle

eurban · 3:07PM

Hello,

We have a newish Navien combi boiler and particularly during very cold weather the temperatures is each zone tends to overshoot what the T-stats are set at by 2-4 degrees. I don't mean that one room in the zone is too hot but rather all the rooms in the zone get too warm and the reading on the T-stat goes above what it is set to.

We have large radiators throughout the house and the kitchen addition is radiant floor and is handled by a mixing valve and it is the only area that doesn't seem to over shoot.

We have an outdoor reset sensor and the system is active.

Our t-stats are simple non programable Honeywells.

My instinct is that we have an over abundance of radiator heating / thermal mass capacity.

What is the best way to improve this overshoot. Should I simply lower the max and min temps of the supply water and then use the "custom" outdoor reset curve? Right now I think the curve is set on "radiator" and max temp is around 180 and min is around 120.

Thanks for any suggestions!

Eric

hot_rod · 3:09PM

turn the thermostat down 2 degrees

eurban · 3:12PM

Won't the yo-yoing of the temp still occur even at a lower temp setting on the T-stat? My goal is to have the temp be a bit more consistent.

yellowdog · 3:13PM

Close the gap on your thermostat as tight as it will go to reduce run time for each call.

EdTheHeaterMan · 3:16PM

Yes the Yo-yoing will be the same. just at a lower temperature. But it won't be over your desired temperature. It will be just right!

Edit:

I have to ask, what is the temperature on the thermostat set at? (the desired temperature)

What is the highest temperature the room gets at that setting?

What is the lowest temperature the room gets at that setting?

This is to put some context into your query.

eurban · 3:20PM

What does "close the gap" on the T-stat mean? Here's some pictures of my T-stats.

yellowdog · 3:23PM

It means to get into the programming of the stat and see if you can make the differential tighter so the temp when it turns on is closer to the temp it turns off.

eurban · 3:27PM

Right now I have our T-stats set at 69 for the upstairs and down stairs main zones. It's 23F outside right now. The temperature upstairs reads 74 and the downstairs reads 73. The lowest the rooms get is 69 as set on the T-stats.

eurban · 3:29PM

So with these Honeywells is there any way to make the on off temp differential tighter?

EdTheHeaterMan · 3:41PM

You only have one adjustment on your thermostat that may help you. In the setup menu: function #5 Heating Cycle Rate

If it is currently on 3 (for hot water systems) then change it to 5.

If it is currently on 5 then change it to 7.

Screenshot 2025-01-09 at 10.30.55 AM.png

KC_Jones · 3:44PM

Why is no one recommending they tune their outdoor reset system?

KarlW · 3:54PM

If you are feeling adventurous, you could also go to a smart thermostat like Nest or Ecobee, but you almost certainly have a two-wire system making for a more complicated install. They absolutely can and will adjust using things like hysteresis and other algorithms.

But I'd try @EdTheHeaterMan's advice first unless you want to upgrade for smarter-home sake.

EdTheHeaterMan · 3:55PM

https://forum.heatinghelp.com/discussion/comment/1833195#Comment_1833195

It sounds like your thermostat is located on a cold wall.

I had a customer that owned a diner, the thermostat was always incorrectly reading the room temperature. It turns out that the kitchen exhaust fan was so powerful that it was difficult to open the front door with all that air pressure keeping the front door closed. So with all that exhaust pressure from the kitchen fan, outside air was leaking in every nook and cranny of the building including the hole where the thermostat wire came thru the wall. That caused the thermostat to be reading the outside air temperature that was being sucked into the building from between the stud walls.

To solve this I stuffed some insulation thru the hole in the wall where the wire was coming through and used mastic tape with really sticky glue to cover the hole and make a kind of gasket so outside air no longer came thru that opening. After that, the thermostat worked just fine.

Take the thermostat off the wall and use a cigarette or candle smoke to see if cold air is blowing in thru the wire opening in the wall

eurban · 4:02PM

Thanks for all the suggestions. The upstairs T-stat was set on the 3 and the down was on 5 for the heating cycle rate. I changed them both to 7 since the upstairs was overshot by even more than the downstairs. I will try that change for a while and report back.

Our old system system used these same T-stats but didn't have a mixing valve for the radiant floor. As a result we kept the supply water temps quite a bit lower (to protect the wood floors) and I don't remember the overshoot issue as being as significant. It would certainly take MUCH longer to satisfy the T-stat with the old setup. This is why I was thinking that turning the supply water temps down (and then using the custom outdoor reset setting) would reduce the overshoot.

hot_rod · 4:06PM

If you are comfortable with changing the ODR setpoints and curve you can do that also.

You want to set the high temperature for adequate heat on your coldest day. The low end for adequate heat on a mild day.

So either trial and error, or pick a mid point setting and adjust throughout the season. There really is no way to dial in an exact setting, it is customized to your system, home use, lifestyle, weather patterns etc.

Some ODRs have more adjustability than others, moving the curve is not available on all reset controls.

This shows how the curves can be adjusted. If you look at the 1 setting, for every degree outdoor temperature drops, SWT goes up 1 degree. Based on the type of emitters these are suggested starting points.

Screen Shot 2020-10-15 at 12.38.40 PM.png

eurban · 4:18PM

I'm not sure what is available for actual curve adjustment on our Navien NCB 240/130H but when I looked in the manual it seemed like what I should do was to change the maximum and minimum supply water temps and then choose "custom" for the outdoor reset curve. I can look at the manual a bit more carefully. Thanks!

hot_rod · 5:13PM

It comes down to how much you want to tinker with adjustments. The boiler may have step firing and other adjustments to prevent over-shooting, and maximize efficiency.

The thermostats may be easier to play with, first.

eurban · 6:18PM

BTW the T-stats for both floors are on interior walls close to the middle of the house. My basement has a radiator roughly in the middle of the house and is probably only a few degrees cooler than the 1st floor and the attic above the upstairs T-stat is finished and warm. I can check for weird airflow around the T-stats but I don't see how it could be cold air coming from behind the T-stat. My point and shoot thermometer confirms the interior wall / floor temps to be more or less the same as what the T-stats read.

I'm going to observe the results from the changes I made to the T-stats a few days and see if the overshoot is reduced.

Ill be back with a report / questions if needed. THANKS!!!!!!

yellowdog · 7:06PM

Your self described large radiators are the reason for your overshoot of room temperature. They are still emitting heat from their thermal mass after the stat shuts the zone down. The hotter boiler water from your outdoor reset when it gets colder out is why you notice it more when its colder outside.

Best way to handle air temperature overshoot after boiler runs heat cycle

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