Pickup Factor...

Charlie from wmass

What is unknown about dry steam?

Charlie from wmass

@Dave0176 what is the edr of the bare pipe? The 4" header adds quit a lot of edr on it's own if it is not insulated.

Charlie from wmass

I also realised we figure radiators edr at 70 degrees are temperature. If you run cooler the edr increases. I do not have the charts currently but I believe 65 degree air is a 5% increase in effective edr. I will confirm this later or you guys can beat me to it.

PMJ

That big header size helps a lot to make that system run better. Whatever rate that boiler produces steam at the bigger the downstream piping is the lower the pressure at the boiler will be that is required to deliver it the rest of the way. If the pipes are big enough the pressure can go so low as to be tough to even measure. With sufficient downstream radiation which doesn't need to come close to full to heat the place you can satisfy the tstat long before anything fills and cause pressure to build. It would take way more than 25 minutes to build any pressure in my 2 pipe system even from hot start and I never run more than 10 except on cold starts.

Boiler manufacturers have skimped everything down in size to get the pounds out for that all important lower up front cost. They have left you guys with a much bigger project now to get dry steam out of them at low pressure. I don't know the numbers of how much the installation costs go up as you move up pipe sizes. Probably gets hard to convince homeowners I suppose. But I suggest that over the life of the boiler justifying a big pipe size - matching in total area the old mains you are going into at least (which is probably even bigger than what the mfg says) is easily worth it. Sounds like Dave has a really happy customer.

Charlie from wmass

I always consider a successful installation if my boiler is cycling buy thermostat not pressure. I've also had to have long conversations with customers when they sat and watched their boiler run and did not see the needle on the 30 pound pressure gauge move during the heating cycle. So is anybody willing to run their system it's 70 degrees and see if that affects cycling on pressure?

ChrisJ

Charlie from wmass said:

I always consider a successful installation if my boiler is cycling buy thermostat not pressure. I've also had to have long conversations with customers when they sat and watched their boiler run and did not see the needle on the 30 pound pressure gauge move during the heating cycle. So is anybody willing to run their system it's 70 degrees and see if that affects cycling on pressure?

Huh?

ChrisJ

Hatterasguy said:

Charlie from wmass said:

I've also had to have long conversations with customers when they sat and watched their boiler run and did not see the needle on the 30 pound pressure gauge move during the heating cycle.

The cost of a 3 lb. gauge is a fraction of the cost of the "long conversation".

3 pound gauge never really moved even when I was using an EG-45. The only time that started climbing was running 1 CPH when it was really really cold out, it would get up to near 0.5 PSI.

This was also partially due to the pigtail doing it's thing. Even at fractions of a pound if it starts off reading slightly negative, it's never going to go past 0.

PMJ

Hatterasguy said:

PMJ said:

With sufficient downstream radiation which doesn't need to come close to full to heat the place you can satisfy the tstat long before anything fills and cause pressure to build.

Agreed.

What I get from this is the fact that the pickup factor is almost irrelevant on systems that have EDR that is, say, 50% greater than the heatloss. The boiler could never run on a duty cycle of more than 66% and, at that rate, it cannot completely fill the rads and cannot cycle on pressure (with a standard 1.33 factor).

On a system that has EDR that precisely matches the heatloss, the system must completely fill on the design day and the pickup factor becomes more of an issue.

So, the only time we really need to be concerned with the pickup factor (provided we stay at about 1.33) is when the EDR closely matches the heatloss. Then one might consider a smaller boiler because it must run constantly on the design day.

I realize this is contrary to the obvious logic that dictates a larger boiler when the heatloss precisely matches the EDR. Such is the case with steam.

I think you are correct here. Design days are obviously rare, however, and I think the pressure cycles would be pretty long ones anyway in those conditions.

So taking rads out is generally a bad idea. Managing things is actually easier when there are too many assuming the original piping layout was reasonable.

It would appear then that too much installed radiation, big and well insulated pipes, and an oversize boiler is the system that is the easiest to control and will recover the fastest from bad situations. But oh my, there's that up front cost thing again. I am certain that extra up front on my system in 1926 has paid for itself many times over and I sure appreciate the extra that was put in.

KC_Jones

And I am over here listening to people with all the excess radiation thinking I wish I had that. I have 3 rooms without heat. I think in total I am good for radiation, but it isn't in the correct locations. None of my bathrooms have radiators. One of them (half bath first floor) can get really cold. I have had to put a portable heater in there a couple times. My bedroom has a 40 EDR for 180 sq ft. It can get warm in there sometimes. My problem with what I have isn't the boiler keeping up it's the radiation keeping up at design. I have seen my thermostat not satisfy for days on end. The EcoSteam only fires the boiler per it's calculated heat loss, but the rads were hot all the way the whole time. They couldn't do any more.

Fred

I do think you guys are on to something as it relates to near boiler piping/size and amount of radiation relative to heat loss. I don't have a drop header but my original header is 45 inches above the water line (to the bottom of the header). It is a 4.5" header. One of the take-offs from that header (the longest main) is also 4" for about 15 to 20 ft. The rest of the mains are 2.5". The boiler is over-sized, about 720 sq. ft of radiation and the boiler is rated at 866 sq. ft. I have a 3 PSI gauge on it and a 0-16 ounce Vaporstat. Lots of venting (now 5 Big mouths) All mains are insulated but, as best as I can tell, the risers to the second floor, in the walls is not insulated . My design day is about 0 (actually 1 degree) but we have had several days over the past 3 years where we have had 5 to 7 days of -15 to -20. Anytime we are at or above design day, my boiler will typically run for 20 to 25 minutes per hour and pressure never rises above 2 ounces (some cycles don't even register on the gauge. Below design day, I have seen pressure get to 5 to 6 ounces (after running for about 35 minutes) and I saw the boiler cut out on pressure (12 ounces) three times last winter, when we were down at that -15 to -20 range. Just adding to that conversation.
EDIT: The boiler will short cycle on set backs, which I don't do.

ChrisJ

@Hatterasguy
Something else you didn't consider.

A radiator is specified as a certain EDR in a 72F ambient I believe, correct? With 215F steam.

What effect does an ice cold wall behind the radiator have? I'm betting it's huge.

My walls are empty plaster walls with clap board on the outside, no sheathing, no insulation. I bet they suck heat out of a 212-215F radiator big time. Like a failed checkvalve on a well pump pulling against a water heater without a vacuum breaker.

How about yours?

ChrisJ

Hatterasguy said:

Fred said:

The boiler is over-sized, about 720 sq. ft of radiation and the boiler is rated at 866 sq. ft.

I get a pickup and piping factor of 60%. No cycling on pressure.

That's four of us.

Ice cold walls I tell's ya.

KC_Jones

Chris not sure what is going on in your house, but in the middle of winter that wall is far from cold. Honestly the colder it gets the warmer the wall is, in my house. That being said, my kids room is insulated and the rad is in an enclosure. That rad stays toasty MUCH longer than any other one in the house, so you may be onto something.

How about this, maybe it's everything that has been mentioned. All the factors combine to make the situations work. We would literally need a lab to effectively test all of this. It is a fascinating discussion though.

ChrisJ

I don't believe I have a misunderstanding of anything.

A cold wall sucks heat out of a hot radiator via infrared, literally. You're saying the wall gives up heat to the outside of the building. Ok, and in doing so it sucks heat out of the radiator. Not sure why you're confused there, Hat.

The only reason that wall is "far from cold", although I claim B.S. on that one because my walls do get physically cold everywhere but behind the radiator, is because of the radiators heating them.

On top of that, my balloon framing creates drafts in the wall which likely cancels out a lot of the R value the 4" air gap may have.

PMJ

ChrisJ said:

@Hatterasguy
Something else you didn't consider.

A radiator is specified as a certain EDR in a 72F ambient I believe, correct? With 215F steam.

What effect does an ice cold wall behind the radiator have? I'm betting it's huge.

My walls are empty plaster walls with clap board on the outside, no sheathing, no insulation. I bet they suck heat out of a 212-215F radiator big time. Like a failed checkvalve on a well pump pulling against a water heater without a vacuum breaker.

How about yours?

This is an important point Chris makes and I agree except that it is all about the air temp around the radiators. The fact is that when it is really cold outside the rate at which heat is leaving the room is much higher. So that means that the rate at which heat is leaving the radiator and warming the room is higher to keep it at temperature. Necessarily the rate at which the boiler must put steam into the radiator is higher too. Even equally "full" radiators can condense at different rates for different outside temps. This is because it will condense steam faster and the boiler must replace it faster even though it is "full". Why? Because the colder it gets if the air in the room is still, the colder the walls will be and also the air toward the floor though it may only be a fraction of a degree. A full radiator warming air from the floor that is even 1/4 of a degree colder and sending it upward will condense more steam than before. A full radiator with a fan on it will condense more steam than a full one without for the same reason. Multiplied by all the rads in the structure this can add up to a sizeable chunk of boiler capacity that needs to be used up before pressure can actually build if the boiler is big enough.

For illustration of the point, if the point is reached where the radiator is simply not big enough to keep up(like you open a window) the temp in the room will begin to drop and the btu flow rate from that same radiator will continue even higher even though it is still just the same full. The condensation rate will continue to go up and so will the steam flow rate from the boiler. The temperature of the air around it is falling and it will condense steam at an even higher rate yet. So the btu output of a radiator is anything but constant even being what appears to be full. The conditions around it are actually quite variable and fractions of a degree will make a significant difference over the entire EDR of the building.

ChrisJ

The reason this thought popped into my head is when I put warm bottles of water next to the evaporator in my refrigerator they get colder far faster than if I put them on a lower shelf. Not touching the evaporator, but sitting an inch away from it.

In theory, the air on the lower shelf is colder, but having them right next to the evaporator cools them quicker thanks IR. Direct radiation is much better at cooling the water, or, warming the evaporator depending on how you want to look at it.

A hot radiator next to a cold wall must have the same effect.

Charlie from wmass

Unless the room temp drops the condensing rate of the radiator is constant. This is also how we can properly size steam traps, we know the rate per hour for a given edr and room air temp.
The variable and condensing rate of steam is whether the whole surface area of the radiator is required to maintain the temperature to satisfy the thermostat.

Charlie from wmass

What is the consensus on cycles per hour on this system? In the Colder Weather the temperature drop on a 1 cycle per hour vs 3 cycles per hour would be significant

ChrisJ

Charlie from wmass said:

Unless the room temp drops the condensing rate of the radiator is constant. This is also how we can properly size steam traps, we know the rate per hour for a given edr and room air temp.

The variable and condensing rate of steam is whether the whole surface area of the radiator is required to maintain the temperature to satisfy the thermostat.

We all do realize a radiator's output is only something like 60% via convection right? The other 40% via infrared radiation.

That means the room temperature does not have to drop to increase the condensing rate. You put a block of ice near the radiator and it's going to use more steam. Not on, just near. Even a few feet away.

This isn't an opinion, it's a fact.

ChrisJ

Charlie from wmass said:

What is the consensus on cycles per hour on this system? In the Colder Weather the temperature drop on a 1 cycle per hour vs 3 cycles per hour would be significant

I found 1 CPH works nice with my system when it's warmer out, 40s, 50s etc. Once it's into the 20s and 30s I switch to 2 CPH.

3 CPH I found only really works well when it's in the single digits out or below zero. Once we got up to the mid teens I go back to 2 CPH.

I'm sure others will have found different things work for them.


At -8F and 3 CPH it's actually really a beautiful thing to watch, the Ecosteam counting down, saying the cycle is over and then 5 minutes later the thermostat calling for heat. I believe it was running 14 minute cycles x 3 per hour at that point.

Fred

Charlie from wmass said:

What is the consensus on cycles per hour on this system? In the Colder Weather the temperature drop on a 1 cycle per hour vs 3 cycles per hour would be significant

The amount of temp drop, assuming the boiler has the capacity, is going to be driven by a well placed thermostat, isn't it? I have mine set at one cycle per hour and I see a 1 degree variance before the boiler kicks back on, and given everything is already hot, it typically stays in that range, maybe a fraction of a degree more drop before I recover back to the set point. At 2 or 3 cycles per hour, there may be a fractional degree difference but, depending on the thermostat, if it is energized based on a 1 degree drop, it would seem to be a minimal variance. I'm sure the more elaborate controls would make a difference.

PMJ

Charlie from wmass said:

Unless the room temp drops the condensing rate of the radiator is constant. This is also how we can properly size steam traps, we know the rate per hour for a given edr and room air temp.

The variable and condensing rate of steam is whether the whole surface area of the radiator is required to maintain the temperature to satisfy the thermostat.

I think the condensing rate is actually more variable than that. The change once full is not as dramatic as changes in fullness but the reality is that the air temp in the room is not uniform and gets less so as the delta T to the outside increases. Colder walls cause air to slide to the floor faster and pass by radiators faster. Rooms do see more significant air movement which does affect convection at the radiator just as a fan would. How much a body radiates to things around it is related to the temperature difference of the two bodies. If the walls were hotter than the radiator the flow would go the other way. So the rate at which a full radiator condenses steam is not as rigidly constant as you suggest.

ChrisJ

Why is everyone talking about cast iron radiators as if they're convectors? 40% of the output isn't by convection.

I'm really confused right now.


While I agree cold walls do cause cold air to float down by the floor and aid in convection, that's not what I was talking about. I feel that cold wall being right by the radiator could have a very large impact on EDR. But for some reason, everyone keeps talking about air temperature.

I don't get it?

PMJ

This is all true, however a fraction of a degree makes a negligible difference to the EDR. 5F change in interior temperature makes a 4% difference to EDR.

My 1/4 of a degree was something to throw out there. Rooms do have more air movement as walls become colder which affects convection at the radiator. The radiation component of what leaves the radiator is affected by the temperature of the bodies they radiate to. Both components of heat transfer are affected. I agree with Chris that in some and total this is significant and can easily become a 2 digit % of the total EDR.

PMJ

I think what we are talking about is the same as why a puddle can freeze on the ground on a still clear night even though the air right on it is above freezing. The radiation component from the puddle to the black night sky removes enough heat from the puddle for it to freeze - it overcomes whatever convective heat addition there is from the warmer air to the puddle because there is little air movement. Wouldn't happen on a cloudy night. The radiation component of what heat leaves the radiator and goes to the wall is affected by the wall temperature (and any wall or object in its view). If the wall got hot enough it would warm the radiator through radiation.

All this stuff is all actually quite variable. It is a good thing too and explains why excess capacity in a boiler really shouldn't be feared so much.

PMJ

@Hatterasguy, I'm not sure what physics you are referring to. We have established that already full radiators will convect more and radiate more as the temperature of the walls drops - even if the temperature in the room, though truly less uniform, is basically being held. It goes a long way to explaining your observation that significant amounts of "extra" or pickup factor seem to disappear without causing stops on pressure in so many systems. Isn't this a good thing?

ChrisJ

Hatterasguy said:

PMJ said:

I agree with Chris that in some and total this is significant and can easily become a 2 digit % of the total EDR.

The physics begs to disagree. It is insignificant unless the room temp drops severely (no heat for 12 hours).

Please explain how this is true if a cast iron radiator gives up 40% of it's heat in infrared? That's a huge percentage and the temperature of the objects around it must have a large impact.


As you said, I'm way out of my element so help me and others understand. You're good at calculating such things, I'm bad at it. I'm sure you can easily figure out how much heat would be lost so let's find out?

Charlie from wmass

I think two points are being confused here.
1.The radiators are proven to condense a specific amount of steam per minute or hour at a given air temperature.
2. The heat that is in the room escape's at varying rates depending on the outdoor air temperature.
What this means is more steam is condensed over a 24-hour period not that more steam is condensed per minute per edr, unless the room is cooling faster than the radiator can add heat.
If the radiators are restricted in such a manner that either the air cannot get out or steam is fed into them at a rate slower than the condensing capacity of the radiator, like in a vapor system, then even an under-sized boiler could produce pressure without tripping the thermostat on temperature.

DanHolohan

The boiler will cycle on pressure if the venting is inadequate, even if the radiators aren't hot. Remove the vents; the boiler will quickly reach the set high pressure and cycle like mad.

BobC

I assume by removing the vents you mean block them so nothing escapes.

Bob

DanHolohan

Yes, Bob. Okay, Hatt. Thanks, guys. Just needed to clarify.

PMJ

@hatteraguy and Charlie, we probably can't get past this point. My whole point is that there is no exact number for the total heat put into the room by a "full" radiator. There is only a reference figure to design systems with that is approximate. The air temp in the room is never uniform nor is the air movement - both of which affect convection. The temperature of walls and windows and objects isn't unform either so the rad's radiant losses aren't constant. As the temperature difference to the outside increases things get less and less uniform. Therefore a "full" radiator can in fact be condensing steam at higher and higher rates per minute while the boiler puts more and more in to match that rate all the while being "full" without building pressure and the nominal or average room temperature holding steady. Obviously a boiler can be big enough to outrun the rads and finally build pressure. But it is also clear that if you continuously decrease the temperature outside very slowly, a radiator in a room will continuously condense more and more steam right up to AND after it becomes "full" as conditions in the room continue to change. The rate will never stop increasing. There is no max point. The rate of increase may be less in the zone after it first becomes full and just before the room temp starts to fall, but it will never actually stop increasing. The issue here really is how much "extra" can be consumed in this zone beyond the standard EDR rating of the rad. I think it is quite significant and the experience of so many of us with our systems shows it. I think it explains the disappearing "extra". I also think it shows in living color why there should be a healthy pickup factor.

ChrisJ

Yes, and a healthy pickup factor is 15-25%.

Fred

On a different note, The variable that is specific to each structure making it difficult to apply a "general rule" on how big a boiler can be to still work fine and not cut-out on pressure is how over radiated the structure actually is. On design day (or thereabouts) my radiators are not hot all the way across. Maybe 2/3's to 3/4's hot. That says to me that I am probably over radiated for the heat loss. Having said that, it is probably reasonable to think I could put a boiler in that is double the output requirement (not that I would, but in theory) and still not cut-out on pressure because the radiators aren't full and the radiator vents haven't even closed. System can't build pressure. Now when it's 15 to 20 degrees below design day, the radiators do get filled and the vents do close. That allows my system to build 5 or 6 ounces of pressure but the tstat is satisfied before the boiler cuts out on pressure. No one would want a boiler larger than that required to fill the rads and keep the pipes hot enough to move the steam as there is no value add, other than being prepared for those occasions when design day is off by something unusual. Clearly, for me and probably anyone with an oversized boiler (and no elaborate control system), recovering from a set back, where the boiler has to run for an extended period, will cause the short cycles.

PMJ

The outside temperature is irrelevant? It is what drives the entire process and determines the temperatures of walls and everything else. These temps are not able to be known exactly - only estimated. Because this whole thing is an estimate a pickup factor applied just to be sure things are covered. Pretty good system actually.

I could agree with your #1 but not #2 or #3. The air temp in a room varies from floor to ceiling more and more as the temp outside goes down. Air movement increases which affects convection at the rad. There is no exact calculation for this. Same for #3. There is no single temp to assign to walls and objects. Again, no accurate calculation possible only guesses. Depends entirely on what kind of structure the rad is in.

I think our disagreement is really about how exactly the output of a radiator can be known - even when full. I say not very and it appears you think quite exactly. I maintain that the temperatures of all the objects being radiated to and how much to each really isn't known. The exact convective conditions on the surface of the rad aren't known either as it is not just about air temperature around it which is obviously quite different from top to bottom. On top of that all these things are constantly changing. It is why all that exists is a standard estimate by type of radiator and size. Too many variables. So people use that standard to estimate and then apply a safety factor.

You have suggested in other threads that exact radiator outputs are quite fixed and knowable. I simply don't agree and respectfully leave it there.

ChrisJ

My neighbor's boiler is big enough it causes Gorton 6's and C's to start hissing before the radiators are even half full and I don't mean 0.5 PSI hissing, I mean 1.5 - 2 PSI hissing.

I vented everything as fast as I could, any faster and the steam shortcircuits and shuts the vents too soon.

No, too big is too big and that's that.

ChrisJ

Hatterasguy said:

ChrisJ said:

I vented everything as fast as I could, any faster and the steam shortcircuits and shuts the vents too soon.

What you need is a third main that goes over to your place.

He'd never know the difference.

He died in June, so, yeah.......

PMJ

Hatterasguy said:

PMJ said:

The outside temperature is irrelevant? It is what drives the entire process and determines the temperatures of walls and everything else. These temps are not able to be known exactly - only estimated. Because this whole thing is an estimate a pickup factor applied just to be sure things are covered. Pretty good system actually.

I suppose I need to clarify.

The outside temperature is irrelevant to the calculations we use to determine the rate of convection or radiation. Of course, the outside temperature drives the entire process and the indoor temperature will vary slightly with the outdoor. But, understand that the outdoor is not used in any of the figuring.

PMJ said:

I could agree with your #1 but not #2 or #3. The air temp in a room varies from floor to ceiling more and more as the temp outside goes down. Air movement increases which affects convection at the rad. There is no exact calculation for this. Same for #3. There is no single temp to assign to walls and objects. Again, no accurate calculation possible only guesses. Depends entirely on what kind of structure the rad is in.

The air temp in the room does vary as the outdoor temperature drops. Please put a value on it. If the air temp surrounding the rad at 50F ambient is 70F, what do you believe the air temperature surrounding the rad would be at 0F. With those two values, the difference in convection can easily be calculated. But, we need to agree on the difference first.

PMJ said:

I think our disagreement is really about how exactly the output of a radiator can be known - even when full. I say not very and it appears you think quite exactly. I maintain that the temperatures of all the objects being radiated to and how much to each really isn't known. The exact convective conditions on the surface of the rad aren't known either as it is not just about air temperature around it which is obviously quite different from top to bottom. On top of that all these things are constantly changing. It is why all that exists is a standard estimate by type of radiator and size. Too many variables. So people use that standard to estimate and then apply a safety factor.



You have suggested in other threads that exact radiator outputs are quite fixed and knowable. I simply don't agree and respectfully leave it there.

We have to take a value for the EDR at a specific operating point. If you take the position that the calculated EDR is irrelevant and wildly in error then we cannot go further.

If you take the position that the EDR is valid for a room temperature of 70F, I can calculate the difference in EDR if the room temperature drops to 65F. I can also show the difference in radiation if the wall temperature drops from 65F to 60F. These calculations would show that the differences are truly negligible and will not exceed 5% of the original EDR value.

But, if you are contesting the original EDR value and calling it worthless, then I'm sorry but we're done. If this was the true situation then we're all in the darkness when selecting boiler size and we might as well just install anything because EDR is basically irrelevant.

@hatterasguy, please be respectful now. I don't see anywhere in my comments that I suggested the standard EDR estimates to be worthless. I merely called them estimates which they are. They are the basis for all figuring with these systems. But they are estimates and that is why after all the figuring, pickup factors of double digit percentages are put on top.

I have been able to slowly drag you into admitting that these things are not as exact as you wish them to be. The convection at the radiator is not just about temperature. As disparities in temperatures in the room increase so does movement (drafts) which are impossible to know about. So the exact same radiator will emit different amounts of total heat depending on what structure it is in, how tight it is and all the rest. It will even depend in the same structure on conditions outside on what day. I am not claiming these things to add up to a huge number. But as an engineer with some experience I suspect now it can creep over 10% quite easily.

So this all boils down to how inexact is it since we do agree it is not exact. You seem bent on reducing all these things I brought up by myself and Chris to less than 5% of the whole story and thereby rendering the whole comment line pointless I suppose. I don't know how many pros out there would bet their bank account that they could call what any one radiator was doing to less than 5 percentage points of error under any conditions except totally off but perhaps they would. I'm guessing they would agree that is what the safety factor is for - the uncertainty in all this.

But if you must, have at it with your numbers and minimize for everyone the potential importance of any of the subjects I raised. This exercise has been enlightening for me anyway. It has shown me another way that these systems are actually quite forgiving to a significant amount of extra boiler - confirming what I have observed with my own system for many years.

DanHolohan

This is how they arrived at the radiator ratings. Really no guesswork at all:

https://heatinghelp.com/systems-help-center/how-they-rated-radiators/

Fred

When I said "double the boiler size" I was exaggerating for effect. My point was that I could have what is considered an over sized boiler and still operate quite well until I reach that threshold where I cross the line between the run time of the boiler, the over-capacity of the radiation (relative to heat loss, which probably means more/larger mains/run-outs than required if the radiation were truly matched to heatloss), the set point of the Tstat (Static vs. set-backs), good venting of the mains and radiators. In my case, it seems the outside temps can drop to about -20 before pressure comes into play and even then it is a factor because I have the cut-out on my Vaporstat set at 12 ounces. By all typical standards, I could go up to a cut-out of 1.5 to 2 lbs and still be in an acceptable upper range.