Pressuretrol to Vaporstat with timer

what I did was to interrupt my thermostat circuit with the timer relay

I like the idea but do you have any sense of whether it is and how much your boiler may be overpowered? I think putting a vaporstat on my system (I don't know about yours) would be foolish because of the very short amount of time it takes for gauge pressure in boiler to drop to near zero after a cut-out of 2 psi. I mean it can't be much over 2 mins. My question in my case would be how low do I want to let absolute pressure go? If I had a vaporstat cutting out at 12 oz or something my boiler would go into negative gauge pressure pretty quickly. If vents were still closed I realize that negative gauge pressure in the boiler could still by supplying steam to radiators that go even lower gauge pressure from condensation but I wonder if it would work for me. Any timer would have to not be set long enough that any radiator vent opens.

I remember reading (Dan Halohan?) that the best place for the vaporstat was at the end of the steam main.

I think there are two factors to determine feasibilty of setting a timer so that you maximize efficiency. 1) You can't have any of the radiator or steam main vents open up (= timer setting was too long), and 2) In every steam main you have to have absolute pressure in the main be greater than the absolute pressure in the radiator, for each and every radiator, otherwise they stop getting steam (= timer setting too long). Steam goes from high pressure to low always. Another problem is that you may lose balance. I remember reading about a system with two radiators connected to each other and one wasn't heating. The guy even removed the vent completely during a cycle to try to get some of the steam to go to the one that wasn't heating. No go. That one had atmospheric pressure still at the vent but the one that was heating had a negative pressure relative to atmospheric. So the steam went there and only there.

It would almost take some sort of complicated pressure/temperature sensor data logger array to figure this out. I wonder how much gain there is. Interesting though.

The purpose of the timer is to have the boiler shut off on pressure (first cycle) when the control closes after a pressure drop the burner is delayed from firing so as to allow the radiation to give off heat. After this pause if the stat is still calling the boiler restarts and builds pressure and continues to rinse and repeat until the stat is satisfied with the burner running longer cycles and less short cycling.

@109A_5

That diagram looks familiar!!

My timer relay is a module like this: https://www.amazon.com/dp/B097PPBG4J/

And the low pressure switch I use to signal it is one of these (they have different ranges):

https://ebay.us/m/5a29Ph

BUT, the boiling point of water is a function of absolute pressure. So, as long as temperature never drops below Tsat, which is a function of pressure (which can be below atmospheric), it theoretically will never stop producing steam.

But as I mentioned previously, if pressure is greater in the boiler, which is greater than pressure in the steam main, which is greater than pressure in the radiator, steam will continue to flow from the steamchest through the main to the radiator. This "could" continue to persist even when the pressure in the boiler is sub atmospheric, as long as the vents remain closed. The enthalpy of saturated steam will continue to be delivered to the radiator but at a diminishing mass flow rate and at a diminished value (due to lower pressure) and without using any more fuel. Then when these conditions no longer are met, that is when you would want the timer to time out, starting the burners back up, with steaming commencing almost immediately due to the lowered Tsat. But how do it (the timer) know lol?

I mean, you don't even know at this point that your boiler is oversized. "Maybe" it isn't going over 1 psi ever because it is perfectly sized except for maybe a polar vortex or something like another ice age and then it might not keep up lol. The other thing is, this is kind of bass ackwards at this point to be trying to scrape the last shred of efficiency out with this scheme when you are getting water hammer on cold days and long cycles. That's a major inefficiency caused by steam hitting pools of condensate. That produces very wet steam when that happens. I would fix that first.

I was interpreting that as saying that the pressuretrol never hits it's cut-out setting of 1.5 psi because the boiler can't put out enough btu to hit that pressure.

It would help if we could get it cleared up. Maybe I got it wrong……

So if the boiler is not cycling on pressure why are you considering a timer that will not do a thing?

The timer system is for a boiler short cycling on pressure.

"after observing for three hours sitting in front of the boiler."

I finally found someone more obsessed with their steam boiler than me.

The thermodynamics of what pressure a boiler connected to a condensing system are insanely simple. For the sae of giggles, we will assume that our boiler has a reasonably constant energy transfer rate from fire to water over the temperature range we will consider.

So. Our condensing system, on the other hand, does not. It's condensing rate is a rather gnarly function of the temperature of the condensation inside it; suffice it to say here that, again in reasonable temperature ranges, it is a more or less linear function of temperature difference between the outside air and the inside condensation point.

With me so far?

Now what happens in a sealed system? If the boiler is producing more heat output than the condensing system can use, the pressure and, much more importantly, the temperature of the phase change will rise.

In principle, if the system is truly sealed, one could run as small a heat input as desired, simply by lowering the pressure and thus the temperature and heat output. Problem. That only works if it is possible to remove all non-condensable gas from the system and keep it that way (think heat pump or air conditioner). A more practical solution is to set the lowest operating pressure at roughly atmospheric. Then, as the pressure and thus temperature rises, all the air is pushed out and you use automatic vents to close to allow the pressure to rise.

It would seem, perhaps, that the obvious solution to an oversize boiler — or even an intentionally large boiler — would be to simply allow the pressure to rise until the radiation was hot enough to absorb all that heat. And, in fact,, that would work. It is somewhat undesirable, however, in that the temperature or the radiation could get impressively high (if the boiler were double the required size, you'd be looking at radiators running in the 400 F range. Some folks might object…) The minor losses — pipework and the like — would also increase, of course. So would the simple matter of fittings and pipework which could handle the pressure and temperatures.

So, since we operate in the real world of heating here, rather than the somewhat more interesting (perhaps) world of high temperature high pressure boilers, a far simpler solution is to modulate the power of the boiler to what the radiation can handle. Which is the whole purpose of the pressure control device— vapourstat of pressuretrol or what have you. (kindly note that if it is used this way, it is NOT a safety device. That needs to be a separate device, set higher). And, to avoid efficiency loss from starting a cold boiler, one wants to keep the off time as short as possible, assuming that the heat source is either on or off. Some gas boilers can, in principle, use amplitude — firing rate — modulation. I'm not sure why they haven't appeared for steam, but I expect that the expense and complexity and small market makes it unattractive. Instead, we use pulse modulation. To keep the efficiency up we use or should use as short an off period as possible, allowing the on time to make the overall duty cycle correct for the desired power output.

Oh well…

I would not give up on it. Once you get things corrected and dialed in steam is nice. I think it is very nice to have a quiet simple heating system. For the most part I don't even know when it is operating, the house is just warm.

On the Honeywell 8000, which you have, you can set the number of heating cycles per hour. No timer needed till you repair pipework. Just reduce the number of allowable heating cycles and the thermostat wont call the boiler to fire. Initial cold fire will bring home to temp, or pressure, following fire to maintain would be limited by the thermostat.

@Jamie Hall I knew it wasn't as simple as you are saying because heat of vaporization goes down with increasing pressure, whereas Tsat does increase. Then I started wondering about my situation where my cut-out was (used to be but now drifted to 2.0psi) 1.5 psi and my cut-in was effectively (by the time flue damper re-opened) 0.25 psi. Here's the difference that it makes when you ignore hfg and just base it upon temperature, with a typical radiator and ambient room temp. of 68 F.

Oh well…..

At 0.25 psig (14.95 psia):

• Tsat ≈ 212.85°F
• hfg ≈ 969.81 BTU/lb
• ΔT ≈ 144.85°F

At 1.5 psig (16.2 psia):

• Tsat ≈ 216.93°F
• hfg ≈ 967.33 BTU/lb
• ΔT ≈ 148.93°F

Without correction for hfg (i.e., based solely on the temperature-driven external heat transfer):

• The heat transfer rate is ~3.69% higher at 1.5 psig than at 0.25 psig.

With correction for hfg (i.e., scaling the above by the ratio of hfg values, as sometimes applied in simplified sizing estimates despite hfg not directly affecting external heat transfer):

• The heat transfer rate is ~3.42% higher at 1.5 psig than at 0.25 psig.

These values account for the slight non-linearity in convection (proportional to ΔT^{1.25}) and radiation (proportional to T_sat^4 - T_air^4 in absolute temperature). Condensate accumulation is assumed negligible in this theoretical scenario.

@Captain Who is quite correct here. Key is his speculation that steam could continue to flow from the boiler to the radiators with boiler off and the system sub atmospheric. And, more importantly, the time for the next firing is when this flow has or is near to stopping. This is precisely how I have run my system for many years. No need to speculate about this any more. I can confirm it.

There is little if any appreciation here for the significance of maintaining continuous flow to radiators at all times, likely due to lack of first hand experience with it. It is something an open vented system cannot do. A system running evenly spaced burns closed up and sub atmospheric in between them will win on comfort and efficiency over an open vented system cycling on pressure walking away. I have run both ways and it isn't close.

It is 5 degrees F outside right now and my system is doing 12 minute burns spaced by 18 minute waits. I've had 24 burns in the last 12 hours and the tstat was satisfied 7 times for a total duration of 105 minutes. So the system is calling for heat 85% of the time and firing 40% of the time. The boiler will return to full boil within a minute of fire after being off for 18 because the system will be at -7"hg at that point. What happens during these waits is a plus for net efficiency and certainly not a minus.

@Captain Who , my system control is on a PLC platform and my thermostat is just a standard digital one which provides the call for heat and nothing else. The firing of the boiler is controlled by monitoring the amount of steam and the amount of natural vacuum in the system. I use one remote temperature sensor and have an analog pressure sensor on the boiler header. The longer calls are the more even the heat is by definition so that is the goal. So there are multiple burn and wait periods in each call for heat - more as demand increases. What I know for sure is that if all calls end with the boiler running the heat is not anywhere close to as even as it could be. It is a two pipe system which is easier than one pipe by a lot from the venting perspective to facilitate running sub atmospheric between firings. I operate easily with one solenoid vent. The maximum pressure during the very short periods at the end of burns when the vent is open and leaked in air is pushed back out is never above 3 ounces. Peak naturally generated vacuum between cycles 5-7"Hg.

Happy to discuss in more detail anytime if you are interested. PM probably works best.

My system was originally just like that too. The boiler ran on high no matter what the demand was until the thermostat was satisfied. It was not even heat or close, and no perfectly sized boiler could make it so under all conditions. Some form modulation is absolutely required. For goodness sake - the original coal boiler had it. How is it possible that anyone thought that modulation could just be eliminated with the switch to intermittent fire?

Also, what happens when the boiler is off really is significant. Letting air right back in kills the steam flow immediately because the void in the radiators created by the collapsing steam is immediately filled with air from the space and not continuing steam from the mains. And, having to push all that air right back out every time really does cost more, it really does. The net difference in both comfort and cost is not a small one….really.