Minimum run time setting on thermostat

RuffinoVino

HVACNUT- yes the boiler shuts down, the thermostat heat icon goes away and I'm assuming for( the exact 4 mins every time it does this) the off time the boiler pressure is going down then it restarts. 

Yes I'm using Ecobee with the sensors that are averaging the set point.

RuffinoVino

Pecmsg I think you need to get someone in to see where your losing all that heat instead of replying with nonsense to posts.

EdTheHeaterMan

pecmsg said:

Define your optional boiler?

My optional boiler would be the one on a cruise ship. Then I would be enjoying the sun and fun on the Lido Deck while the crew handles the day to day boiler operations.

Or did you mean optimal?

HVACNUT

RuffinoVino said:

HVACNUT- yes the boiler shuts down, the thermostat heat icon goes away and I'm assuming for( the exact 4 mins every time it does this) the off time the boiler pressure is going down then it restarts. 

Yes I'm using Ecobee with the sensors that are averaging the set point.

Something's not wired right if your losing the icon when the circuit opens on pressure. I remember a post about this happening with the LWCO on a Weil McLain steamer. Even if the burner circuit opens on pressure, the thermostat shouldn't lose the demand. It might need an isolation relay. How long does it take for the boiler to make 1.5 psi on a cold day? As @pecmsg correctly stated, if the boiler isn't even reaching steady state, then that's what we call short cycling. And its not good. And on a design day, the boilers max output should exactly match the calculated heat loss by running constantly. Without ever opening on pressure. 

RuffinoVino

On a cold day if going from 64-67 from overnight after maybe 1 hour,.I could be off by 15 mins give or take but take around that and it will shut off for exactly 4 mins then restart. Not even sure why that exactly 4 mins each time is. Ideas on that to?

EdTheHeaterMan

So to put things in perspective, The oversized boiler versus the proper size boiler.

Let's say that @RuffinoVino has the identical home to @pecmsg.
Let's say that @RuffinoVino has a 150,000 BTU boiler in his home and the boiler runs for 20 minutes and is off for 40 minutes when the outside temperature is +10°F outside, So that is equal to burning 50,000 BTUh of fuel ever 1 hour.

Now @pecmsg has a 45,000 BTU boiler in his identical house. In 60 minutes the boiler does not stop operating when the outside temperature is +10°F outside. So that is equal to 45,000 BTUh. Since the boiler in @pecmsg can not burn more than 45,000 BTU per hour then when his boiler runs 24/7 it is more efficient than @RuffinoVino boiler that cycles on and off.

Both thermostats are set at 68° and both homes are comfortable.

So by having a boiler that is too big is not always a good thing and the cycle minimum time for the oversized boiler is always going to be a problem. Setting the minimum time to 4 minutes and you should get where you need to be.

This is just an example, and I do not know if one of the boilers is 3 times bigger that you need and the other boiler is exactly the correct size. Chances are the you do not live in identical homes either.

EBEBRATT-Ed

@RuffinoVino

It is true that a properly sized boiler will run about 100% of the time when operating at design conditions. That said boilers are not "made" to fit every houses exact heat loss so we end up with some or a lot of boilers being oversized depending on the installer. Keep in mind we only hit design conditions about 1% of the time.

Thermostats like Ecobee & Nest's are not really designed for steam. But they can be made to work I guess.

As far as minimum run time, if it has to have a minimum I would set it for 3 min.

RuffinoVino

Thanks. Are there any good smart thermostats for steam boilers?

Jamie Hall

RuffinoVino said:

Your telling me a optimal home boiler is running non stop for 24 hours a day??? Seriously, based on your previous comments and this one I think with all due respect your clueless.

That is correct. That would be the optimum. However, the optimum can only be achieved if the load -- the heat loss from the structure-- and the power applied -- the net output of the boiler -- can be matched exactly. In by far the vast majority of the applications this is simply not feasible, as should be obvious. Could it be done? With a heating system which can modulate from zero net output to at least the maximum design demand, and suitable controls, yes.

Without getting too far off into the weeds with control theory -- a fascinating subject, but complex -- the supply modulation -- the net output from the boiler -- can be managed in two ways: varying the firing rate of the boiler (or the duty cycle of the resistance elements, for an electric resistance system) or by allowing the boiler to only run at intervals, the length of which vs. the total time (start to start) of a cycle will determine the effective net output. The former is referred to broadly as amplitude modulation, the latter as pulse width modulation. Both are acceptable; both have design tradeoffs. Technically speaking, most home heating systems used to use the latter entirely, but newer hot water (and some forced air) systems use the former within a fairly broad range. The problem with either one is selecting the correct control parameter or parameters and implementing them.

Most steam systems actually have two match problems. The boiler is usually rather poorly matched to the rest of the system -- the radiation -- and is almost always oversized, sometimes rather seriously. Ideally (there we go again) the boiler net output would exactly match the condensing capacity of the radiation. If that is the case the boiler pressure would never rise above that required to overcome the friction losses in the piping -- typically a few ounces. In most cases, however -- as in your system -- the boiler net output is greater, and eventually the system will reach the point where all the condensing capacity is operating at full rating. At this point, the boiler pressure will start to rise again -- uselessly -- and the boiler should shut down on pressure until the radiation can catch up. This is actually a form of pulse width modulation which matches the average net available boiler output to the condensing capacity. The off time of the duty cycle should be as short as possible, to minimise losses involved in stopping and starting the boiler. This is most easily done by actually sensing the pressure and turning the boiler on and off as needed (best done with a sensitive control, such as a vaporstat; note that this control is not an overpressure safety device, which is also needed). Now the second match problem is matching the boiler and radiation net output -- as a system -- to the demand of the structure. There are a number of ways to do this, but the simplest -- and generally most satisfactory -- is to use an indoor thermostat which directly senses the desired parameter, which is space temperature. Here you run into another control problem, however. The system does not start to provide heat to the structure immediately on startup, nor does it stop immediately on boiler shutdown. In the bad old days this was compensated for by a simple thermodynamic device called an anticipator, which altered the behaviour of the thermostat to compensate for this. Correctly set -- which is a trial and error procedure -- this can produce astonishingly close control of the space temperature (typically within half a degree without undue fuss). Many modern digital thermostats can also do this electronically, but not as accurately. The Ecobee happens to be one of the better designs in that regard, provided it doesn't have to cope with setbacks. It is almost impossible without true computer control (which they don't actually have) to manage a setback recovery without a dregee or two of overshoot (annoyingly, the older units with anticipators could do it).

Some people -- best described as dedicated hobbyists -- have had considerable success in designing tighter control of the desired output parameter (space temperature) using dedicated computers -- usually some flavour of Raspberry PI machines -- and multiple outdoor inputs (normally free air temperature, wind speed, and effective insolation as a minimum) and indoor temperature sensors together, of course, with custom programming.

I could go into this much more thoroughly if you wanted, but I daresay that you really don't want a dissertation on control theory?

EdTheHeaterMan

@Jamie Hall I could not have said it better myself.

Seriously, That is something I could never have said, Let alone understand. But some of those big words are impressive! Thanks for sharing!

ratio

Jamie Hall said:

I could go into this much more thoroughly if you wanted, but I daresay that you really don't want a dissertation on control theory?

I for one certainly would, but perhaps in a separate thread?

Pretty please?