Hypothetical: Turn up the thermostat, boiler turns on, and then immediately turn it back down?

CoachBoilermaker

Oil/steam system.

What would happen?

ethicalpaul

the boiler says “stupid homeowners” and shuts off in frustration.

EdTheHeaterMan

Electricity would pass thru the thermostat and continue to the system and operate any burner that is not interrupted by some other safety device like a LWCO or other limit. After the burner starts to operate, if you turn the thermostat lower so there is no call for heat, then the open electrical circuit caused by the new temperature setting would no longer be completed and therefore the burner control would stop operating due to the lack of electric power.Do you have such a oil heated boiler? Could you do a scientific test and record the results and report back? We would all be interested in your findings. That way it won't be Hypothetical anymore.

And since you have mentioned in previous posts that you have experience as a mechanic of sorts. I would ask you a similar HYPOTHETICAL question about turning on something and then turning it off right away. For example, if you were to set the head light switch on an automobile to on, then soon after that turn the switch off, what would happen to the head light beam. Would it stop traveling at the speed of light thru the atmosphere and out into outer space? And if that switch were on a Semi Truck and you did the same thing, would there be any difference?

Was this answer detailed enough? …or was @ethicalpaul's answer sufficient?

HVACNUT

Are you mad at your boiler?

ratio

It sounds to me like the OP is wondering if there's any kind of minimum on time with the boiler.

That's a real thing, but I'm not aware of a residential boiler that has that function.

A better car analogy might be starting the engine and putting it into drive, then putting it back into park without taking your foot off the brake, and shutting the engine back off. Not going to hurt if it happens to occur, but not something that you want to do all the time.

CoachBoilermaker

Yes, I was wondering if there would be a delay.

Related to the delay in cycling on/off with a +/- 3 degree tolerance so boiler does not go on/off repeatedly at 67.9 and 68.1 What is this effect called ?

Jamie Hall

https://forum.heatinghelp.com/discussion/comment/1816882#Comment_1816882

Swing or in control engineering, dead band. If the controlled device is on/off, you have to have it.

ethicalpaul

Yes, I was wondering if there would be a delay.

OK, respectfully, that's the question to ask!

In even modern steam boilers, the firing controls are all driven by electromechanical inputs. The "brain" of the boiler I would argue is the ignition module which does have digital circuitry and logic in it, but that all seems to be used for ignition proofing (if that is the right term). If the thermostat says "we're done" (by simply opening a relay or contact) then the gas valve is immediately cut off.

There was an external control that had a lot more inputs, timing, and logic called Ecosteam that I understand a forum regular from the past created and sold to a few customers including some still active members.

Jamie Hall

The whole question of control design and theory is an absolutely fascinating one — to those with a certain turn of mind! There are inevitable tradeoffs in any control scheme between accuracy in maintaining the target parameters, reliability, and cost (and complexity), and balancing those in a way acceptable to the user is… difficult.

The heating industry on the residential/small business level has been curiously slow about implementing more "advanced" controls; largely, I think (opinions may differ) because the users don't see, in general, any real need for them and they object to the cost and complexity — and regulators or other authority types have not seen, until recently, any need to force them to comply.

It is perhaps more instructive to look at the automobile industry, as there one can easily see these tradeoffs. Or perhaps it is easier for me, since I've always loved working on machines and cars. But I do have a usable timeline. Consider these machines, all sitting in the yard as I type. First, a 1951 Ferguson tractor. Second, a 1980 Chevy C10 truck. Third, a 1994 Chevy K2500. Fouth, a 2015 Chevy Cruze diesel car. The first has a manual transmission and a manual choke. There are a whopping total of 4 adjustments needed to get it running perfectly (which it still does). The second is more advanced: there is an automatic choke and a fancy solenoid which regulates idle speed. Still, that only adds one adjustment. The third has a computer which controls fuel delivery and transmission shifting. Better economy and, when everything is working right, overall easier management. If something isn't working right — you don't get out the screwdriver, you get out your wallet while you wait for the tow truck. The fourth is a joy to drive when it's working properly, which is most of the time. There are, at last count, 15 different computers in it and 3 separate serial bus networks. I've not counted how many sensors and actuators there are. Probably a couple of hundred. Nothing is controlled mechanically (normally — there is a fall back mechanical control for the brakes and the steering, but you'd better be strong). When something goes wrong, it's not repairable. Just replace it. Get out your bank account — youe wallet won't do.

The tradeoff? Number one is not all that easy to drive and control. Number four simple —hop in and go. Number one is simple, reliable, and repairable. Number four isn't.

I could say the same about airplanes — a Piper Cherokee is all mechanical. You'd better know how to fly. A Boeing has some computer controls (which give problems when they aren't working right) but still has enough mechanical backup to be flyable — if you know how to fly. An Airbus has no mechanical backup. It's easy to fly, and will keep you out of trouble if you goof up — so long as the magic works. When it stops, which it does once in a while, that's it. You are now a passenger on the way to the crash. Take your pick… tradeoffs.

End of ramble.

CoachBoilermaker

Great thoughts. @Jamie Hall is a rare breed who grasps old technology like carbs vs. FI and also knows what CANBUS is! Impressive.

My question arose due to wondering the accuracy of my vintage thermostat. It's currently set at 60F. House indoor thermometer says 65F. I wanted to slowly slide the thermostat up to the known 65F and see how accurate it is by seeing where the boiler kicks on.

EdTheHeaterMan

https://forum.heatinghelp.com/discussion/comment/1816912#Comment_1816912

I wanted to slowly slide the thermostat up to the known 65F and see how accurate it is

You know, that can be adjusted also… in 1976, when I went to oil burner school, they called that calibration. We had a very thin flat 3/16" wrench that would grab the adjusting nut while you held the dial with the numbers with one hand and rotated the actual adjuster with the wrench with the other hand.

The instructor, Mr Spence, was about 85 years old (I might be exaggerating a little) offered this story. When coal was king in the heating business, it was not uncommon to hand fire the boiler with a little extra coal so you did not need to make as many trips to the basement. As a result many homes were comfortable at 80° to 85° on a regular cold winter day. When the oil burner folks came along to convert those coal boilers, the salesman told the customer that all they needed to do was set the thermostat to 70° or 72° to be comfortable

Of course when the job was finished, and the thermostat was set to 72° the home was nowhere near comfortable compared to the previous 80°+ temperatures they were used to. So the service mechanic would make a service call to see what the problem was and saw that the thermostat and the burner were operating properly. But the customer insisted that the house was too cold. So the service tech would set the thermostat to 80° and solved the problem.

The customer however was so nervous about setting the thermostat so dangerously high that they turned it back down to 72° where the salesman recommended. (you don't want to set it too high, it might blow up) After several nuisance service visits the service manager would do a psychological calibration. They would calibrate the thermostat so it was set at 80° when the dial was set at 72°. but they also had to adjust the thermometer on the thermostat so it also looked like it was 72° when it was actually 80° in the home. That is when the nuisance service calls stopped.

The house was comfortable and the customer was no longer nervous any more. Win, Win… until there was a new homeowner that wanted a new thermostat because the old one on the wall was way off…

The stuff those dead men did back in the day

HVACNUT

https://forum.heatinghelp.com/discussion/comment/1816882#Comment_1816882

Some thermostats do offer programming for minimum run times and minimum off times.

CoachBoilermaker

https://forum.heatinghelp.com/discussion/comment/1816915#Comment_1816915

https://www.youtube.com/watch?v=4xgx4k83zzc

fentonc

I've been augmenting my home 3-zone hydronic setup with a PLC that sits between the 3 thermostats and the zone valves. I tried out a 'zone synchronization' setup last year that did indeed increase the efficiency, but caused enough annoyance for the family ("I turned the thermostat up, why didn't the heat kick on!?") that I ditched it this year, but I did implement a post-run thermal purge scheme that I have high hopes for this year.

Mucking with the controls to try to find the right balance of efficiency and ease-of-use is lots of fun, especially when you realize that the 'simple' controls can result in miserably low efficiency if hooked up to a badly designed/oversized system. I also worked out a modulation scheme for cast-iron boilers, but that might have to wait until next season!

bjohnhy

Many thermostats have a swing value in the range of 1.0 to 2.0 degrees F. So the system won't cycle on at 67.9 and off at 68.1.

An example would be the system turns on at 67.0 and off at 69.0 when set thermostat is set to 68. I believe this would be an example swing value of 2 degrees.

Some thermostats allow for changing the swing value (no smart thermostats I know of).

Jamie Hall

Some older thermostats have an anticipator, which is basically a tiny heater inside the thermostat The objective is to heat the thermostat faster than space is heating, so that — when using high mass radiation, such as cast iron radiators, the extra heat they give off after the boiler stops just brings the space to the desired temperature. They — and the thermostat — then cool down faster, too. They can be a bit finicky to adjust, but if they are adjusted correctly they result in holding the space temperature within astonishingly narrow ranges — often less than a degree (a "swing" of less than half a degree).

Digitals can't do that, but some of the better ones have an electronic substitute which "learns" how much that extra heat is and cuts off the thermostat early to compensate.

CoachBoilermaker

https://forum.heatinghelp.com/discussion/comment/1816863#Comment_1816863

Actually, running a car engine for short periods in cold weather can be harmful because it doesn’t allow the engine to reach its optimal operating temperature. Here’s why that matters:

1. Condensation Buildup: When an engine runs for a short time, it doesn’t get hot enough to evaporate the condensation that naturally accumulates inside. This water can mix with oil, creating that yellowish, milky "mayo" under the oil cap. While a small amount of this gunk can be normal, consistently finding it could indicate moisture buildup due to frequent short trips in cold conditions.
   
2. Oil Contamination: The engine oil doesn’t reach full operating temperature on short trips, so it can’t properly burn off fuel, water, and other contaminants. These contaminants dilute the oil, reducing its effectiveness and potentially leading to sludge formation over time.

CoachBoilermaker

https://forum.heatinghelp.com/discussion/comment/1817017#Comment_1817017

So, what is the default swing value in digital modern thermostats?

Are you saying they cycle on and off at 67.9 and 68.1 ??

ethicalpaul

No, he's saying you (and the thermostat) want to prevent the system from doing that.

The thermostat manufacturers often hide the actual swing value from us for some reason, but the important thing is that "forced hot air" is typically the smallest, and "steam" if it has it is the largest.

Probably all of us here know that running engines for short periods is bad…I think you posted that from some kind of AI answer. I'm not sure why!

EdTheHeaterMan

https://forum.heatinghelp.com/discussion/comment/1817827#Comment_1817827

But what about the headlight beam? Hypothetically those photons travel out into outer space forever, unless Newton’s laws and Einstein’s theoretical police department pulls them over and give them a ticket for driving at the speed of light.

ratio

Depends on the thermostat, but it shouldn't be that tight, the equipment can't respond that fast. 1-4° would be more like it, but some (notably Honeywell) use a "cycles per hour" instead of a deadband.

109A_5

Dead band, Swing or Hysteresis. The old style heat anticipator could dynamically bias the thermostat's mechanical hysteresis, when set up correctly it worked nice. IMO "Cycles Per Hour" is a software engineers poor attempt to mimic the old style heat anticipator's functionality. For best comfort Cycles Per Hour needs to be more dynamic and it seems some are working towards that goal.

Some AI for Paul.

epmiller

Slightly off topic but does anybody else here dislike the algorithm in the current Residio (Honeywell branded) thermostats that claim to have no deadband? Cycles-per-hour is the only way to control them and I dislike them with high-mass radiators. I love the mounting/wiring base though. I've had to upgrade some for a customer and it was a snap.

ethicalpaul

I think they are lying. Their cycles per hour setting I believe is a misdirection. I don't think it matters much, pick 1 or 2 for steam and use what you like.

IMO Cycles Per Hour is a software engineers poor attempt to mimic the old style heat anticipator's functionality

Don't blame the software engineers, they are told what to do

Jamie Hall

At some risk of sounding like a stuck record (what's a record, Grandpa?) — neither cycles per hour nor a fixed deadband/swing is ideal for close temperature control. Neither truly (or at all) takes into account the inherent lags in both bringing the heat on or in turning it off, never mind subtleties such as actual heat loss rate at the moment (which affects potential overshoot and undershoot) and a few other variables. Not, as I've said in another thread, that a digital thermostat can't be built and programmed to do that satisfactorily, provided it has some adjustments available, but that it simply isn't worth it in a plug and play world.

109A_5

Some Industrial temperature controllers use PID (Proportional – Integral – Derivative) to enhance performance. May be an interesting experiment for a residential heating environment, if you wanted to spend the money.

ratio

The Honeywell 9000 series has PID control…

NB: I am not a Honeywell shill. I just use the 8000 series a lot, and I dislike the T series very much.