Learning to use classic vintage Honeywell Chronotherm thermostat

Chris_L

Here is a link to the manual for this thermostat or one like it:

https://thermostat.guide/honeywell/honeywell-t8090a-heating-or-cooling-thermostat-owners-manual/

Jamie Hall

Congratulations on getting the old girl running, and having the patience to get it working right! but I was sort of hoping…

CoachBoilermaker

It really overshot this time.
Set it to 64F, yet boiler was still running at 66F.

36F outside

LR is 61F

Set thermostat to 64

0:00 - LR temp is 61

10:00 - LR temp is 61

20:00 - LR temp is 62

30:00 - LR temp is 63

35:00 - LR temp is 63

37:00 - LR temp is 64

40:00 - LR temp is 64

41:00 - LR temp is 65

45:00 - LR temp is 65

46:00 - LR temp is 66

48:00 - LR temp is 66 (Boiler shuts off)

55:00 - LR temp is 67

1:00:00 - LR temp is 68

1:10:00 - LR temp is 68

1:42:00 - LR temp is 68

2:00:00 - LR temp is 67

109A_5

You will have to experiment. Having the heat anticipator setting optimized to keep the LR at say a constant 64 degrees may not be the best heat anticipator setting to recover from a setback. During a recovery from a setback the radiator(s) probably get more saturated with heat than they do when just maintaining a steady temperature of 64 degrees, due to the boiler duty cycle (longer boiler run time during the recovery). With the radiators being more saturated it is more likely an overshoot will happen.

CoachBoilermaker

https://forum.heatinghelp.com/discussion/comment/1824442#Comment_1824442

Right, but the boiler was still running at 66, when tstat was set to 64. There is double overshoot. Better if boiler stopped at 64 where the tstat is set, and then rads can overshoot from there to 66-67

But the other problem is that boiler won't turn back on until it drops to like 62

ethicalpaul

Sounds like it might be time for one of those evil, inaccurate modern digital ones…mine definitely never is running when the house is already 2 degrees higher than the setting 😂

ChrisJ

@CoachBoilermaker

I had a mercury type T87 and I swapped it out for a modern themrostat. I did the same at the shop I work in, 4 times (4 different heaters).

Some people like them (old thermostats) for whatever reason and that's ok.

I don't feel like dealing with it. I did play with the anticipator etc and observed how it worked. I like mercury switches as much as the next guy, I even had shoes with them in them back in the early 90s, but I prefer a digital thermostat.

Right now my favorite thermostat is still the Honeywell Prestige and you can program it from your phone while watching the original Twilight Zone on tv like I did. It also only needs 2 wires.

109A_5

As was previously mentioned in one of the threads the thermostat may not be level (Mercury switch technology) and with the age of it, if it is level it may calibration. If it otherwise works good I could live with a 2 degree offset, so set at 62 to maintain 64. Up to you, if you scrap it remember Jamie wants it.

EdTheHeaterMan

Just spitballing here

Is the thermostat mounted securely to the wall?

Is the thermostat level?

Since there is a mercury bulb that controls the set point on that particular thermostat, you can get an inaccurate calibration if you tilt the thermostat. If you take a close look at this exaggerated photo of the thermostat mercury bulb you can clearly see that the set point on the blue and red lever is identical on all three thermostats. You can also clearly see the mercury switch is in the satisfied position on the lower two thermostats , However the top thermostat is so far out of level that the mercury is still calling for heat even though it should not be calling for heat if it were level.  

That is why it is important to make sure the thermostat is secured to the wall and that the thermostat is level.   It is clearly stated in the instructions

CoachBoilermaker

Thanks for the ideas, guys. I think the thermostat is accurate since it will turn on the boiler when I slide it to 65 (and the room temp is 64) It's the overshoot that needs calibration. I will test anticipator settings a bit more.

CoachBoilermaker

I just turned on the heat slider to 64 on the thermostat. Boiler turns on. Room heated to 64, and boiler still firing at 64, so I slid back the slider until the boiler turned off. Slider is now at 62, with the room at 64. Question becomes at what temp will the boiler start up again?

Jamie Hall

Place your bets now, ladies and gentlemen! 62? Depends a little on how fast the anticipator cools off…

ethicalpaul

I’ll bet 62

ChrisJ

Alright I'll join in.

I'll Price Is Right it and take 61.5

ethicalpaul

Price Is Righting it would be 1 degree

ChrisJ

https://forum.heatinghelp.com/discussion/comment/1824918#Comment_1824918

Usually.

In this case I'm just undercutting everyone that has already bid.

Still a messed up move on the game.

CoachBoilermaker

Currently blue=58 and red=62.

Went to bed set at blue. Woke up to house at 60F

Toggled to red when I got home from work.

Boiler ran until 64F, residual heat got to 66F

I then toggle to blue for the night.

I'll post the answer this weekend when I need a 2nd boiler cycle for the day.

ChrisJ

https://forum.heatinghelp.com/discussion/comment/1824975#Comment_1824975

ethicalpaul

Well one thing to say for this themostat—it is saving you fuel

EdTheHeaterMan

I still say that the person that decided that it was a good idea to put numbers that match temperatures on a thermostat should have been tortured to death with ice cubes and candles. All the aggravation that we heating and cooling professionals have put up with since that happened because a little old lady calls the office with a no heat call only to find that the thermostat was set at 72° and the home only reached 71°.

The proper thermostat would have: Colder «« A B C D E » » Warmer as the only indication on it. The instruction would read. if you are too cold move the dial closer to warmer by one letter. if you are too hot move the dial to colder by one letter. How many nuisance service calls would have been eliminated this way?

I guarantee you this discussion would never have happened to go on for 3 pages.

Oh, by the way. I will take 16.5°C which is actually 61.7°F.

ChrisJ

https://forum.heatinghelp.com/discussion/comment/1824975#Comment_1824975

https://www.supplyhouse.com/Honeywell-Home-TH3110U2008-T3-Pro-Non-Programmable-Thermostat-1H-1C

This $35 thermostat will make all your thermostat issues go away.

It looks like you may even be able to set it to 2 CPH if you're using a steam system which is much nicer than 1 CPH.

CoachBoilermaker

Currently blue=58 and red=62.

Came home and house was 61F. I toggled to red and boiler fired up. So, if set to 62, it goes on if 61F

Boiler runs until 64 and room hits 66.

After cooling back down to 61, boiler kicked on.

Set at 62F results in a range of 61F to 66F. Is this normal for these old thermostats?

109A_5

I'd say no, however if the heat anticipator is not set correctly it won't work as good as it could or should. I'd monitor its behavior over the weekend.

HVACNUT

Is the bubble in the center of the level? Is there dust on the mercury bulb? Is there a draft coming through the hole where the wires are? The mystery continues. Stay tuned!

CoachBoilermaker

Here is another bigger concern. I woke up and the house was at 58F (due to blue setback at 58). Now, I switch to red toggle 62F (62F = boiler runs until 64F), and after 30 mins of boiler running, the rads are full, yet the house is still only at 59. Boiler running, but should ideally turn off now, since rads full, but instead the quiet vents start "breathing". Wasting oil. Should I manually turn off the boiler? b/c otherwise boiler will run with full rads for the next hour until the room hits 64F. Boiler does not turn off with full rads. One rad started hammering. One inlet valve starts to leak/hiss. Pressuretrol is a 1&1, but I assume it hasn't worked in decades.

EDIT: Once room hit 64F, and boiler still running, and one or two rads were hissing and hammering, I slid the red slider down until boiler stopped running.

Jamie Hall

What you are describing in the latest post there, @CoachBoilermaker , is a pressure control issue — not a thermostat issue. Get that pressuretrol off there, clean out (or better, replace with brass) the pigtail, mount a low pressure gauge on the same pigtail (a T, couple of elbows, a few nipples) and see what's really happening.