Boiler pressure / venting

PTTG40

I have a 3 story home but only one riser to one radiator on the third level. Total cast iron radiators mostly pretty large about 10.
I have a 10 year old 280k btu Weill Mclain Unit that fires up and creates steam quickly. Radiators get hot quickly as well. There is a crossover trap at the end of the main line. Per advice I replaced the pressuretrol w a new vaporstat and pressure gauge along w a Gorton #2 air eliminator and a hoffman #75 on a tee above the boiler on the return line.

Question / problem is this: The system fires for about 20-25 min and stays perfectly at 0.5 psi.. After another 5-10 minutes it ticks up to 1.0psi and cuts out. Plumber set the cut-in to trigger back on at .4psi.
Why does the system finally build up so much pressure to hit 1.0 then it short cycles for another 10-15 minutes until the thermostat hit the right temp. and shuts it down ?
Should I get a vaporstat that has a max operating range up to 2 psi instead of 1psi ?
Do I need to add more air eliminators ?

Any assistance is much appreciated.

Thank you

Peter

Fred

It short cycles because your boiler is to large for the amount of radiation you have. Have your calculated the EDR of those 1 radiators? If not, you need to so we can understand how much the boiler is over sized. Changing the Vaporstat isn't going to change the sizing issue. I'm also guessing that you are using a night time set back?? That only compounds the problem because the boiler has to run longer to bring the house back up to temp. If you are using a set back, how much? Try leaving the tstat at a comfortable temp and no set back and see what happens.

PTTG40

I will do an EDR calc and get back to you. I leave the thermo at 62 degrees at night and kicks in up to 68 in the morning so that def could be the issue bc that is primarily when it happens. Just wasn't sure if there was anything connection wise to do to aid in keeping running at .5psi or so..

Fred

The best thing to do is leave it at 68 or, if you are setting it back for comfort, limit it to 2 or 3 degrees. If you are setting it back to save on fuel cost, you probably aren't saving much. The amount of time the boiler doesn't run, during the night is eaten up by the amount of time it takes to bring the house back up to temp, in the morning.

PMJ

Another approach to help some is to use the programmable thermostat to step the recovery back more slowly. I set "sleep" to 66 at 11 pm, "wake" to 67 at 6am so rads are warm first thing, "leave" to 68 at 8 am, and "return" to 69 at 10am. The slightly reduced temperature during the recovery period is not really noticed. And 66 is all I need to assure no hot rads for a few hours at bedtime which is preferred in my house.

I do this for comfort not savings though I'm quite sure it costs a little less than not doing it.

MilanD

PMJ has a good approach to step up the temp and avoid short cycling.

Someone correct me if I'm wrong, but on a well balanced system, short cycling is fine and quite economical. You also don't really have a problem with the pressure, and if anything, have a too high of a pressure. I am heating 10,000 sq. ft building, with 3 zones, on 7 oz of pressure.

There are 2 issues you are dealing with. 1. Thermostat calling for heat as the building is cold (which it should), and 2. Optimal operating system pressure. Always remember what Dan preaches: the less the pressure the better. So, separate the troubleshooting into 2 steps.

First, venting. You can never have enough vents. To balance where the steam goes is a function of where vents are and how big they are. Hoffman 75 main vent has a slightly less venting capacity than Gordon 2. So you can try switching between the two to figure it out. I fired the boiler with the main vent off, and timed how long it took for the steam to get to it once the boiler was cooking. With a vent installed, it should take as close to that as possible. This will all happen at close to 0 psi as possible, probably 1 or 2 oz of pressure, if that, to overcome the system friction. Dan's book "Greening the Stam Heat" has it all exaplained. This proper main vent will alow the main pipe to fill with steam first, which would then allow for all rads to fill with the steam at the same time. There are people who built an "antler", with a few gordon 2s attached to the original opening. This is usually on a 3/4 vent t, so more than 3 Gordon 2s won't do much as the 3/4 open pipe has the flow capacity slightly less that 3 Gordon 2s. This may be excessive as you probably also have some kind of F&T at the end of your wet return and it also acts as an air vent, esp. on a cold start. But I digress.

Now to rads. Let's assume your main vent is all good and as fast as it should be. The size of the radiator vents will now decide where the steam will try to go first, as will the diameter of the pipe, but nothing you can do about the pipe. As I explained above, as big of a vent on the main, the better, and the same is on the radiator: the higher the venting capacity, the quicker the steam will get to that radiator.

Now, look at the radiator that's the farthest from the boiler - as in - longest pipe run. That one will, by default, heat up last all else equal, and may also be on a smaller diameter pipe, sometimes, which further limits the steam speed. You can spead it up somewhat by using a "fast" vent - like that made-o-mist with a large orifice. If you have a rad that's closer to boiler that heats up significantly before others, you can put a smaller orificed vent (like hoffman 40 or again mad-o-mist with a smaller orifice). This will, in esence, slow down the air elimination from that radiator, and the steam will try to go somewhere else seeking the path of least resistance. And so on and so forth along the system. There is a bit of a trial and error with this, so play with different vents on different rads to speed up air elimination on rads farther from the boiler, slow it down on rads closer to the boiler, trying to get them all to come on as close as at the same time as possible. Hoffman 40s are slower, mad-o-mist can be adjusted with various size orifices. They are also more economical. Depending on what you now have, perhaps getting the sizes you don't have will be enough to balance the system, and keep the old ones for replacement down the road. Key is to get the farthest radiator to get warm as soon as possible, whilethe rest of the rads also get equally warm.

Now we are getting into details on the operating pressure. If at 1 psi your last radiator is still cold, your main venting may not be adequate, and/or your other rads may also be venting too slowly, as would be the case with that last radiator. Steam can't get to them as air is not getting out of the way fast enough. Bigger pressure increases friction as the steam travels down the pipe, and actually also slows down the speed of steam (because it is now also compressing it), so if at 1 psi your rads are not warm, it's the venting issue. Increase venting as explained above, smaller orifice vents closer to boiler, larger on a farther rad, or up the sizes on all the rads.

If the last rad is hot at 1 psi, and more importantly room has gotten comfortable, as did the other rooms and rads, see at how low of a pressure the steam arrives at that last radiator and and makes the room comfortable. Start the boiler, wait for the rad to get hot and room to be comfy and check the pressure at the boiler when that happens. This will be your operating pressure, and will not be more than 8 oz of pressure on a well balanced system. You are now assuming everything is balanced and functioning as it should and you are fine.

From what you said, sounds like you need better venting at that far radiator, and perhaps a bit slower venting on some closer to the boiler. If that doesn't work, you can install a 't' at an elbow closest to your last rad, and install another larger capacity vent on that 't'. This will vent that last pipe faster than just the rad vent. Look up some "steam radiato venting" videos on YouTube.

If your rad is hot, all is fine. The reason boiler is still firing at 1 psi is that your thermostat is calling for heat, pressure is more than adequate, as vaporstat is too high. Pressure shoud be at as low of a pressure as you can get to, usually between 5 and 8 oz per sq inch while room is comfortable on that farthest radiator. Not all rad sections need to be hot. This happens on a coldest day as heat loss is greatest at a lowest outside temp. which means it takes more btus to keep the room at 70 on colder day. Thus more of the radiator needs to be hot when colder outside, as thermostat will call for heat for a longer period of time as it will take more time to heat the space on a colder day.

So, start the boiler on cold, and wait for steam to get to the last rad. Now check the pressure at the boiler when that rad is all nice and hot and the room comfy. This should be your operating pressure, somewhere around 5-8 oz.

The boiler will now cycle on and off producing steam at the lowest possible pressure for as long as the thermostat is calling for heat, while giving time for steam to give off its latent heat and turn back to water and not waste fuel and build unnecessary pressure. This is normal and quite economical. At low pressure you are not wasting fuel by cooking water and overpressurising the system when the system is already at capacity and just needs time to warm up the space.

In conclusion, you don't want the higher psi but the lower psi and good venting. Your 16 oz pressure per sq.inch vaporstat is sufficient, and in fact, set to higher than needed pressure.

Good luck! This is a fun process and once balancing is done right, you will never ever have to do it again.

Attached is my vaporstat setup. This is operating at 7 oz pressure limit, last rad is hot and all is well on a 10,000 sq.ft. building with 3 zones and about 32 radiators of various sizes.

marcusjh

Within the last year or two, I have realized that raising the temperature only one or two degrees makes a huge difference, rather than 8, or 10 degrees (like my parents used to do). Our thermostat takes a little while to realize temperature - in other words, it will show that it's "to temperature", but the furnace will stay running for a bit longer. I'm no expert, but I wonder if this is common with digital thermostats optimized for steam systems.

I wish my parents had known what I have learned in the past couple of years about steam systems, they would have saved money.

gfrbrookline

MilanD, I know that turning the vstat down is good but can it be turned down too much. I have 2 50' mains each vented with a big mouth and a gorton no 2 mounted on 6" *3/4" risers. The main venting should be more than the 3/4" pipe can handle. My Vstat is set to 12oz out 4 oz. in. My radiators on one riser, the one closest to the boiler are clanging, I thought it was a bad floor valve but after following previous advice on the wall the problem has come back since it got colder outside. I have noticed that when the boiler cycles on after about an hour or two and the pipes are still warm it is silent. If the system has time to cool down I hear the rattling/slight water hammer. Could the low pressure be making too much steam too fast and condenses too quickly in the cold radiators so the condensate is fighting the steam? Would a higher Vstat setting actually help in this situation?

gfrbrookline

I should add the rattling/water hammer is only at the end of the first cycle. If it has another cycle or short cycles a few times it does make the noise again.

Fred

Are you sure what you are hearing is hammer? It sounds more like expansion/contraction noise to me. Can you record the noise and post it here? Your Vstat should be set at 12 ounces (Main scale) and 8 ounces (Differential scale). The boiler makes the steam, the vstat just controls how high the pressure can get before it shuts the boiler down (12 ounces) and tells the boiler when to kick back on (4 ounces (12 ounces minus 8 ounces). Are your mains insulated? If not, that can cause steam to condense before it gets to the radiators but I am thinking expansion/contraction and maybe even a pipe that rubs against a floor /sub floor board as it expands or contracts.

BobC

It sounds like this only happens when the pipes are almost cold. That sounds like you have water lying in a pipe somewhere, check all the horizontal pipes with a level - don't trust your eyes.

Bob

EBEBRATT-Ed

@MilanD said " Someone correct me if I'm wrong, but on a well balanced system, short cycling is fine and quite economical."

I don't agree. Short cycling wastes fuel and beats the equipment to death.

Oversizing boilers is not good. Especially on steam, it leads to technicians and others willing to raise the operating pressure in order to prevent short cycling.

gfrbrookline

Not expansion since it clatters for about 60 seconds. All pipes and radiators are pitched correctly. The unit on the top floor has Honneywell TRV's without vacuum breakers, could that be the problem?

gfrbrookline

all mains are insulated.

Fred

gfrbrookline said:

Not expansion since it clatters for about 60 seconds. All pipes and radiators are pitched correctly. The unit on the top floor has Honneywell TRV's without vacuum breakers, could that be the problem?

We really need to hear the noise. During expansion and/or contraction the noise can be heard for 60 seconds or more as a pipe heats or cools and creeps rubbing against a floor board.

JUGHNE

Do you have any check valves hiding somewhere in your return lines. They can give a chatter like ringing a bell slowly.

gfrbrookline

No check vales.

cant figure out how to add sound to the post but the best I can describe the sound is a slight water hammer followed by a boiling sound at the end of the heating cycle. Boiler shuts down for about 90 sec then kicks back on. The following cycle is silent but short about 3 minute with no clanging

gfrbrookline

i should have said a slight recurring water hammer may once every 3 seconds for 30-90 seconds.

MilanD

@gfrbrookline - are you the original poster of the question? I'm rereading this thread from November.

Your hammer, if in fact is loud hammering sound and not pipe expansion has to do with either sag somewhere, as Bob said, or depending if in the rad, to it not being pitched enough or maybe valve not open all the way or hanging up the condensate return. Cold water will both colapse the steam as the steam hits it, and the rushing steam will pick it up and smash it against the fitting. You may need to raise your radiators while attached to the riser to lift the risers up. Someone just posted today that doing .5 to 1 inch of lifting of rads solved all hammering.

Sometimes it also may take some condensate that's arrested somewhere in the end of a radiator slightly counterpitched to get back to the wet return, and can mix with steam towards the end of the cycle and colapse the steam. I have one of my 3 mains that gets hot as the system heats all the way to the main vent, then as the condensate from the setback and rads returns (pushed by steam) it cools the previously hot return, and then as the hot condensate follows, the return heats up again. So, it could be some condensate hanging back somewhere until it finaly gets mixed with steam that's collapsing the steam. As bob said, check pitch and sag in rads and pipes.

As to vstat beung turned down too much, this will only shorten the cycle and should not cause any of the hammering problems you described. At least, not to my knowledge. A well vented system should be able to operate on about 8 to 10 oz, sometimes less. You know it's good when all your radiators are just enough hot and the pressure starts to clumb from the original start-up to op run pressure. There should be 3 pressure steps: startup at 0 to say 2-3 oz. Then, 3 to 5 or 6 oz as the rads are getting hot, then 6+ as the rads are filling and getting full. On the well sized and well vented system, you then play with what that last pressure should be. This will vary on the rad venting speed.

Hope this makes sense.

PMJ

> @EBEBRATT-Ed said:
> @MilanD said " Someone correct me if I'm wrong, but on a well balanced system, short cycling is fine and quite economical."
>
> I don't agree. Short cycling wastes fuel and beats the equipment to death.
>
> Oversizing boilers is not good. Especially on steam, it leads to technicians and others willing to raise the operating pressure in order to prevent short cycling.

I think we need a definition of what a short cycle is. I know for sure that evenly spaced "shorter" cycles don't waste anything.

MilanD

> @EBEBRATT-Ed said:
> @MilanD said " Someone correct me if I'm wrong, but on a well balanced system, short cycling is fine and quite economical."
>
> I don't agree. Short cycling wastes fuel and beats the equipment to death.
>
> Oversizing boilers is not good. Especially on steam, it leads to technicians and others willing to raise the operating pressure in order to prevent short cycling.

I get this for oversized boilers. Boilers sized well should not be doing this, unless after some deep set-back as the tstat is waiting for the temp to catch up and the op pressure is achieved, or if venting is inadequate. I'm not sure how, on a gas fired boiler, shortcycling is damaging or hard on the equipment. There is no way around this on a correctly sized system, after a large set back and pressure build to where rads are all hot. As to fuel consumption, I'd rather short cycle than build unnecessary pressure on the system completely filled with steam and giving out tje heat. To me, that extra fuel use to build unnecessary pressure would be more wasteful.

MilanD

> @PMJ said:
> > @EBEBRATT-Ed said:
> > @MilanD said " Someone correct me if I'm wrong, but on a well balanced system, short cycling is fine and quite economical."
> >
> > I don't agree. Short cycling wastes fuel and beats the equipment to death.
> >
> > Oversizing boilers is not good. Especially on steam, it leads to technicians and others willing to raise the operating pressure in order to prevent short cycling.
>
> I think we need a definition of what a short cycle is. I know for sure that evenly spaced "shorter" cycles don't waste anything.

Maybe, cycling on pressure? Again, my supposition that a full system waiting for tstat to turn it off is better of cycling on pressure than building pressure needlessly.

Jamie Hall

The question of short cycling at the end or towards the end of a heating cycle always seems to bring out some confusion, if not problems. It really shouldn't.

In an ideal world, the problem could be avoided -- but let's consider the nature of the beast, and why it might be a consideration.

If we start from a cold or at least cool condition, we have a lot of really cool iron in the system -- the boiler itself, the piping, and the radiation. As the boiler fires, first a good deal of heat is used just to warm up the boiler and the water in it. After a while, that water begins to boil; as soon as it does, it starts to move out into the mains -- where it promptly condenses. The better the mains are insulated, the less time it takes for the temperature in the mains to rise to the point where little steam is condensing (some always will, as there is always some heat loss even with insulation). At this point, the steam starts to get into the radiation. So long as the radiation isn't all hot -- not all the radiators are hot all the way across -- some of the heat being produced by the boiler is going into heating up the radiators. The rest is going to heating up the space. As the radiators heat across, less and less is required to heat them, and more and more goes into space.

Now. There comes a time when all the radiation is hot. At this point, there is no longer any need for steam to heat the radiation, and the radiation is now condensing as much steam as it can.

What happens next? Since the boiler is still making more steam than the radiation can condense, the only thing that can happen is that the pressure will rise. In our ideal world, at this point the boiler would be throttled back, so that it is producing exactly as much steam as can be condensed. The only realistic way to do this is by sensing the steam pressure, since the condensation rate in the radiation will vary with space air temperature. So... we introduce a pressure control device, such as a vapourstat. In some instances, there may be a two stage or multi stage burner firing rate, but most residential boilers don't have that. Other instances might have two or more boilers staged. Most residences don't have that, either. So -- our pressure control device senses the rising pressure and simply shuts off the burner. After a short period of time, the pressure drops as some of the excess steam is condensed, and the burner is turned back on again. And so on, until another device -- the thermostat -- is satisfied. Note that condensation -- and thus heat output from the radiation -- does not stop when the burner does.

Therefore we can see that any fuel which would have been burned had the pressure control not shut off the boiler would have been almost entirely wasted (it isn't quite entirely wasted, but that's a fine point having to do with the rise in boiling point with pressure).

So... if we can't throttle the burner, the only alternatives are to either waste fuel or cycle the burner on and off.

A legitimate question is what effect this has on the life of the burner. That is not an easy question to answer, as there are so many different burners. It probably does shorten the time between maintenance somewhat -- but it would be a very poorly designed unit where the time dropped to less than a year, even for high wear parts -- of which an ignitor which operated only during startup would be the only real example. Since maintenance should be done once a year anyway, there is no additional cost.

Now a few additional thoughts. First, if the boiler really is properly sized to the radiation, the cycling of which we are speaking will only happen at the end of a cycle long enough that all the radiation is up to temperature, and thus only on a really cold day -- often colder than designed -- or when bringing the space temperature up after a really significant setback (or power loss). Second, adding more venting won't affect it. Adding venting is of value at the beginning -- and if there is cycling on pressure before all the radiation is filled, additional venting is needed. Third, if the boiler is significantly oversized, this cycling will occur sooner -- on shorter cycles.

Last, it might be added that in the bad old days many coal fired boilers had pressure sensitive draught regulators, which reduced the draught -- and hence the firing rate of the coal -- as pressure rose. They were ingenious gadgets. The efficiency and air pollution were horrendous...

MilanD

@Jamie Hall

Excellent explanation.

One question: why would draught regulators not be efficient? I get coal and pollution, but didn't draught regulator in fact smother the coal on pressure rise and prevent it from burning and thus reduce the amount used, or extend the use of each load that was fed to the fire chamber?

MilanD

I have also read the old Chicago 2 pipe system materials that speak of reduction in fuel used (coal) as the system operates on a few ounces only...

MilanD

Btw, thank you @Jamie Hall for that excellent explanation. That was my point exactly, once system is condensing at the rate of heat loss, any fuel used to build pressure is wasted. Better to shut off the burners, or on 2 stage or on multi stage units, throttle back the fire.

EDIT: What this means is that on oversized systems, unless one can down fire or stage down the fire, one will have to run on higher pressures, and that this op pressure will be higher than one is on correctly sized boiler, and that cycling on pressure will be inevitable. This means higher fuel bills.

Jamie Hall

@MilanD -- the problem with draught regulators was that the coal did not burn as completely at the low firing rates, and the CO and particulate emissions soared. But they did save a lot of coal!

But then, back in the bad old days, no one worried too much about efficiency anyway -- if one got to 50%, you were doing well!

MilanD

Thanks @Jamie Hall. Great info! I think that saving on coal was considered efficient back then, not how much of the heat goes up the chimney.

PMJ

> @MilanD said:
> Btw, thank you @Jamie Hall for that excellent explanation. That was my point exactly, once system is condensing at the rate of heat loss, any fuel used to build pressure is wasted. Better to shut off the burners, or on 2 stage or on multi stage units, throttle back the fire.
>
> EDIT: What this means is that on oversized systems, unless one can down fire or stage down the fire, one will have to run on higher pressures, and that this op pressure will be higher than one is on correctly sized boiler, and that cycling on pressure will be inevitable. This means higher fuel bills.

But there is another way. Limit the total burn time of an oversized boiler with evenly spaced burns. I've done it for years - 3 cycles per hour and I only need 10 min burns on design day - that's how big the boiler is. No pressure ever. Combined with vacuum between every burn I will put it up against any approach with regard to efficiency. Steam production continues for several minutes after the burner shuts off each cycle while the vacuum sets in and keeps dropping the boiling point. Since the system is in the deepest part of the vacuum on each firing full boil is reached in just a few seconds on each new burn. And the steam is always in forward motion toward the rads unlike vented systems that run backwards every time the burner shuts off and vents cool rads and mains with room air on each cycle.

I've tweaked this for years now and this season had the best one yet nearly doubling the natural vacuum by eliminating some leaks. Quiet, even and efficient heat.

MilanD

> @PMJ said:
> > @MilanD said:
> > Btw, thank you @Jamie Hall for that excellent explanation. That was my point exactly, once system is condensing at the rate of heat loss, any fuel used to build pressure is wasted. Better to shut off the burners, or on 2 stage or on multi stage units, throttle back the fire.
> >
> > EDIT: What this means is that on oversized systems, unless one can down fire or stage down the fire, one will have to run on higher pressures, and that this op pressure will be higher than one is on correctly sized boiler, and that cycling on pressure will be inevitable. This means higher fuel bills.
>
> But there is another way. Limit the total burn time of an oversized boiler with evenly spaced burns. I've done it for years - 3 cycles per hour and I only need 10 min burns on design day - that's how big the boiler is. No pressure ever. Combined with vacuum between every burn I will put it up against any approach with regard to efficiency. Steam production continues for several minutes after the burner shuts off each cycle while the vacuum sets in and keeps dropping the boiling point. Since the system is in the deepest part of the vacuum on each firing full boil is reached in just a few seconds on each new burn. And the steam is always in forward motion toward the rads unlike vented systems that run backwards every time the burner shuts off and vents cool rads and mains with room air on each cycle.
>
> I've tweaked this for years now and this season had the best one yet nearly doubling the natural vacuum by eliminating some leaks. Quiet, even and efficient heat.

@PMJ

Fantastic! What controls are you using to get 3 per hour burns?

Is this a 1 or 2 pipe? What are you using for vents if 1 pipe and how are you keeping the vacuum? And if 2 pipe, same question. I am curious about the possibility to do a vacuum in 1 pipe... Didn't think this was possible on 1 pipe bc of main vents, but I suppose they too can be vacuum vents.

Do tell!

nicholas bonham-carter

And there you are, if you have a two pipe system, naturally induced vacuum seems to be a real benefit. On a one pipe system, subatmospheric operation is going to be difficult, therefore generous main venting is needed to make sure that the main supplypipes fill FIRST, and then the run outs to the radiators fill, all at the same time.
The resistance to the escaping air from the mains, measured as backpressure on a low pressure gauge will show how effective the main venting is. Do not be mislead by calculations of the time needed for the steam to arrive at the end of a particular main, and therefore throttle back the venting on that shorter main, because the steam will start to rise in the more vented main first, defeating the whole purpose of balancing the system.
If you have ever had any experience with drip irrigation systems, you will see how important it is for all legs of the system to give out the same amount of water, starting at the same time.--NBC

PMJ

> @nicholas bonham-carter said:
> And there you are, if you have a two pipe system, naturally induced vacuum seems to be a real benefit. On a one pipe system, subatmospheric operation is going to be difficult, therefore generous main venting is needed to make sure that the main supplypipes fill FIRST, and then the run outs to the radiators fill, all at the same time.
> The resistance to the escaping air from the mains, measured as backpressure on a low pressure gauge will show how effective the main venting is. Do not be mislead by calculations of the time needed for the steam to arrive at the end of a particular main, and therefore throttle back the venting on that shorter main, because the steam will start to rise in the more vented main first, defeating the whole purpose of balancing the system.
> If you have ever had any experience with drip irrigation systems, you will see how important it is for all legs of the system to give out the same amount of water, starting at the same time.--NBC

Admittedly this is easier in 2 pipe. However, when I started years ago I read about that I would have mysterious problems with vacuum in 2 pipe too. Condensate and steam would get stuck in places etc. I've had no such problems - only more even results not less and with basically no venting at all.

Also, none of these systems were originally designed to horse air in and out - one or two pipe. It is when I realized this that I focused on keeping the air out altogether. The advantage has turned out to be so large that if I had a one pipe I can tell you that I would simply have to try it. I have 23 radiators. I would probably be wiring the place to run solenoid valve vents on each one to run from my plc. Would be an amazing amount of control.

I can't possibly know it will work in one pipe. Many on this forum are quite sure that it wouldn't. They may be right. I'm just saying I heard this about 2 pipe too. Based on what I've found I know I'd be trying it in one pipe.

PMJ

> @MilanD said:
> > @PMJ said:
> > > @MilanD said:
> > > Btw, thank you @Jamie Hall for that excellent explanation. That was my point exactly, once system is condensing at the rate of heat loss, any fuel used to build pressure is wasted. Better to shut off the burners, or on 2 stage or on multi stage units, throttle back the fire.
> > >
> > > EDIT: What this means is that on oversized systems, unless one can down fire or stage down the fire, one will have to run on higher pressures, and that this op pressure will be higher than one is on correctly sized boiler, and that cycling on pressure will be inevitable. This means higher fuel bills.
> >
> > But there is another way. Limit the total burn time of an oversized boiler with evenly spaced burns. I've done it for years - 3 cycles per hour and I only need 10 min burns on design day - that's how big the boiler is. No pressure ever. Combined with vacuum between every burn I will put it up against any approach with regard to efficiency. Steam production continues for several minutes after the burner shuts off each cycle while the vacuum sets in and keeps dropping the boiling point. Since the system is in the deepest part of the vacuum on each firing full boil is reached in just a few seconds on each new burn. And the steam is always in forward motion toward the rads unlike vented systems that run backwards every time the burner shuts off and vents cool rads and mains with room air on each cycle.
> >
> > I've tweaked this for years now and this season had the best one yet nearly doubling the natural vacuum by eliminating some leaks. Quiet, even and efficient heat.
>
> @PMJ
>
> Fantastic! What controls are you using to get 3 per hour burns?
>
> Is this a 1 or 2 pipe? What are you using for vents if 1 pipe and how are you keeping the vacuum? And if 2 pipe, same question. I am curious about the possibility to do a vacuum in 1 pipe... Didn't think this was possible on 1 pipe bc of main vents, but I suppose they too can be vacuum vents.
>
> Do tell!

2 pipe.

A PLC controls the on/off cycles. I've described it in posts on this forum you can search.

One small electric solenoid vent also run by the PLC so only one opening to the atmosphere in the whole system.

Abracadabra

gfrbrookline said:

Not expansion since it clatters for about 60 seconds. All pipes and radiators are pitched correctly. The unit on the top floor has Honneywell TRV's without vacuum breakers, could that be the problem?

Clattering sounds to me like it's a radiator valve with a loose washer in it.

JUGHNE

I think I have heard that sound with TRV's without vac breakers.
They trap water in the rad and at the end of the cycle they could open and the condensate flowing down could rattle the loose washer. (In my case the water sounded like a toilet flushing....the rattle could have been from a vent float getting hit with water??).