Steam Boiler Calculator

Interesting set of comments. There is one key element about steam heating: it is incredibly forgiving. We often make a fairly big production out of getting the header "just right" or making sure the insulation — if any — is properly accounted for and all of that. But, at the end of the day, it turns out that steam systems — whether one pipe or two, or "normal pressure" or "vapour" work most of the time with careful craftsmanship and attention to detail.

I have no beef with @SteamNow 's efforts here — in fact, I applaud them. And, of course, his comment that it must be possible to apply more rigorous engineering to them is quite correct. That said, however, there is a classic engineering problem here: there is a very large number of variables, and some of them have a very large impact on exactly how the system will operate on a given day. That wouldn't be too bad, except that… most of these variables are very poorly quantified (e.g. the heat output of a radiator on a given day) and may have rather large uncertainties (plus or minus 10% would be quite reasonable for some of them). So while the output of a mathematical set of calculations such as is proposed may appear to have great precision, the accuracy may be rather poor. Not through any fault of the computations, but simply because even the known data are poorly quantified.

This makes the engineering of a heating system quite unlike some other engineering disciplines — some branches of mechanical or aeronautical or electrical come to mind — where accuracies on the better than one percent level are to be expected routinely, this is much more like geotechnical or most of civil engineering, where coming within 10 percent of the real world on a given day is cause for considerable celebration.

I do have one perhaps minor disagreement with one statement, however: it is stated "It should be possible to calculate how long the boiler needs to run to get to pressure and how long it will be off before it turns back on." If by "get to pressure" is meant "get to an equilibrium operating pressure on start up of a cycle", then one can expect to calculate it, subject to the comments I made above; both the ambient environment and the preexisting conditions of the system will have a major effect on the time and must be taken into account (just an example for Cedric, my most-studied system: on a cold day, from a genuinely cold start, it will take about 10 minutes to reach running stable pressure (four ounces gauge), but on the same cold day, but when the system has running at capacity in the previous 15 minutes, it will take around one minute). The second half of that quote states "how long it will be off before it turns back on", which implies that the system will turn off — which simply isn't true of a well adjusted system. Such a system will operate at the running stable pressure indefinitely until the control device — usually a thermostat — is satisfied. On the other hand, on systems which are not so well adjusted and which cycle on pressure before the thermostat is satisfied, the time lapse between when the pressure control turns off the boiler and when it turns back on will be governed — or should be governed — by the characteristics of the pressure control device and the particular burner controls (Cedric again, which is not perfectly adjusted): in the situation where he does cycle off on pressure while the thermostat is still calling, which occurs at between 45 minutes and an hour into a cycle, depending on ambient factors, the pressure drops to "cutin" pressure (two ounces gauge) within 10 seconds, but the burner post purge/prepurge cycle controls and is one minute.

To follow the cycle point further a bit (sorry!), many if not most steam boilers have a constant firing rate. Thus the only way in which they can modulate to match the load (the radiation) is by cycling on and off. Provided that the "cutout" pressure is reasonable (no ore than twice the running pressure — Cedric again, 7 ounces gauge) the duty cycle is a direct result of the mismatch between the power output of the boiler and the power absorbing capacity of the system — and the latter is, as I noted above, variable, but if the both are known to some level of accuracy, the duty cycle may, of course, be calculated quite easily.

Great analysis. I have always found this to be true as well. It is best practice to just get out of The Steam's Way! Mad Dog

I believe the precise adjustment of boiler output is dependant on thermostat activity over time. Start up has to be longer to heat the system piping and radiators from cold to warm. Attaching a laptop to monitor behavior of systerm for a day or two allows for data collection which will indicate what software variable factor ranges can be adjusted for the particular building. Once set you forget it. This compensates for design inaccuracies, oversized boilers and state of insulation. Important of course is to assure that radiators or properly vented or trapped.

adjusting fire time hour to hour from thermostat activity will keep fuel use to a minimum.

You helped me with this 25 years ago. Thank you

Yah. Steam head.

Was I involved with this? Can't remember……………

They say the memory is the second thing to go as we age- I've forgotten what the first one was…………..

Could be him! I'll mention this when I see him.