Overshoot after setback in 2 pipe steam system

wmarsh

For background, my system is a circa 1934 Hoffman Equipped System (originally vapor-vac)
You guys helped me a lot some years ago to get it running.
https://forum.heatinghelp.com/discussion/83257/hoffman-boiler-return-trap
Both loops seem balanced, and all radiators heat almost simultaneously.

It has always bugged me that when recovering from setback, my boiler short cycles a few times (cuts out on pressure -- set to 10 oz. on vaporstat), and it then overshoots.
For instance, if I am trying to recover to 68, it always goes up to 70.

Maybe I am stupid, but it never occurred to me to use another set point on the programable thermostat to compensate for this.
So I'm going to try recovering to 66 instead, knowing it will overshoot to 68.
Then program the thermostat to 1/2 hour later turn itself to 68 (which should already be satisfied) and hold there.

Has anyone else tried this?

acwagner

Is this a nighttime or daytime setback? How big of a setback are you doing?

An alternative is to use the thermostat to add in some dwell time and slowly increase the temperature setting over time. A degree an hour or something, if that works for your situation.

The goal is to allow time for the radiators to do their thing.

I think your approach will work too. Best way is to try--it's simple to undo.

Jamie Hall

I'd use your approach. Simple. Reversible. For what it's worth (here I go again) I've found that electronic thermostats tend to do that. They claim to be able to anticipate properly, but on long runs like coming out of a setback... not so much. They're really really dumb when it comes to high thermal mass radiation.

ChrisJ

I recall doing something like that a few years back when I was using a setback.

I'd raise it and stop 2 degrees cooler than I wanted and wait an hour and then raise it again etc. That might work, or 1 degree cooler. Mess around with it a bit.

The Ecosteam does incredibly well with recoveries and 99% of the time does not overshoot at all even when I've increased the temp by 10 degrees. It also has a feature where if I hit a set pressure it shuts the system down to rest and dissipate some heat for a set amount of time, in my case 10 minutes. Sadly, actual production of the unit never happened.

PMJ

@wmarsh ,

I agree with the suggestions above.

One way or another to prevent overshooting the single speed boiler must be "paced" to spread the run time out to be net btu/hour closer to what is required to satisfy the demand. If you are limited to working with a commercial thermostat then another possibility for the setback overshoot is to use the wake,leave,return segments to step the recovery over hours. I spread my 3 degree setback over 6 hours this way and no one even notices.

To do better with overshoot generally in a 2 pipe system and never see any pressure beyond a few inches of water a simple delay on/ delay off timer can be installed in series with the tstat to cycle the boiler on and off during all calls for heat. The timer can be adjusted such that the "ON" time will be enough per hour to surely heat on design day but never run long enough in a single stretch to possibly build any pressure...ever. When you combine all three things, setback, delayed recovery, and ongoing pacing without pressure, the heat is more even and the gas bill lower. Are these things worth the trouble of installing a less than $100 timer? Individual choice.

To do even better than that you need a control on the level of an Ecosteam which is a PLC based control with more capability. I am sad too that the market never embraced it. Fine solution to all of these issues. I think the market would be in less of a hurry to dump steam altogether had better control capability been integrated as it should have been long ago.

wmarsh

My understanding is Ecosteam required the use of natural gas. Given that its not available here, and propane is expensive, I'm intending to stick with oil.

People asked -- I offset from 68 to 62 at night.

I'm encouraged others think this idea might work. I'm trying it but won't really know until we are in the heating season.

ChrisJ

wmarsh said:

My understanding is Ecosteam required the use of natural gas. Given that its not available here, and propane is expensive, I'm intending to stick with oil.

People asked -- I offset from 68 to 62 at night.

I'm encouraged others think this idea might work. I'm trying it but won't really know until we are in the heating season.

No, the Ecosteam didn't care what fuel you used.
In fact, It's creator was originally using an oil fired boiler.

JakeCK

I used to step my thermostat during set back and recovery too. Otherwise it was like kicking a giant pendulum and it swinging back and forth until it finds its new equilibrium. I finally decided it was better to just set it and forget it. That said another option is to use a thermostat that is connected via zwave, zigbee, or even wifi or bluetooth. The tstat wouldn't be the brains but rather just the switch that the home automation would control the heat through.

Here is a simple automation on a open source platform called Home Assistant running on a raspberryPi. In this example it is recovering from a setback of maybe 68 degrees up to 70 and then finally 72 over an hour starting at 2:30pm and ending at 3:30pm. The day of the week has not been taken into account but could be. Obviously how large of a delay between the two setpoints would depend on the systems heating curve.

- id: '1571167347524'
alias: Sample Thermostat setback1
trigger:
- at: '14:30:00'
platform: time
action:
- data:
entity_id: climate.2gig_technologies_ct101_thermostat_iris_heating_1_2
hvac_mode: heat
temperature: 70
service: climate.set_temperature
- id: '1571167440337'
alias: Sample Thermostat setback2
trigger:
- at: '15:30:00'
platform: time
action:
- data:
entity_id: climate.2gig_technologies_ct101_thermostat_iris_heating_1_2
hvac_mode: heat
temperature: 72
service: climate.set_temperature

clammy

I ve found over the years that setting it and forgetting it leads to the least amount of over shoot ,short cycling and in some cases quieter systems because the pipes don’t have all that time to cool down and when she fires up it’s doesn’t t have to re heat cold system piping. Also w auto feeder not set correctly flooding in some cases w slow returns and large piping system that produce a lot of condensate on start and often will take on water when coming out of set backs .Especially on older systems that really weren’t designed w set backs in mind .On another note unless the house is well insulated it some times require long boiler run times to bring out of set back and in a lot of cases it runs more then it would have required to maintain temperature without the set back . And then there’s the short cycling due to the fact that your now trying heat the space up and every thing if filled w steam slowly disputing and the boiler is just firing on and off listening to the pressuretroll oi vey maybe a very simple time delay relay in the pressuretroll circuit could lessen the short cycling a delay on make type that’s commonly used in hvac business Peace and good luck clammy

SuperJ

Might seem counterintuitive or controversial, but I would move the thermostat a tiny bit closer to a rad so it will pick up the heat faster. It will end the call for heat sooner.

I don't mean right next to a rad, just close enough to see a quicker response once the rad is up to temp and radiating strongly, it sounds like there is heat energy building up in the system that stat isn't aware of so it runs to long.

An thermostat with a wireless remote sensor would be a good way to experiment.

wmarsh

Thanks for all the comments. This seems to be working to minimize the short cycling. My next door neighbor also has steam and says its working for him to. I could move the thermostat closer to a radiator -- not that motivated as I am content now -- doing so also would put it right in the draft from the front door, and I really don't want people going in and out turning the heat on.