Energy wasted on high water content

Larsofhaus

Hello, all.  New guy loging in but have visited here over the years.

     I have often wondered if there is some kind of formula for figuring out the wasted energy or lost savings when installing a new boiler on an old hot water system with large pipes or converting a steam system with large pipes to hot water.  I can figure out the volume of water in the old and new systems but twice the water volume needed does not really mean you will use twice the energy to heat the house.

 

     Example: Old steam system that no longer feeds the entire building, 2 1/2" & 1 1/2" pipe.  Section to be heated has heat loss of 50,000.  Standing radiation 294EDR column radiators at .188 per EDR = 55.27 gals.  Old steam pipe comes to 48.27 gals. New boiler, Burnham ES23 (boiler input 70,000) content- 2 gals for total of 105.54 gals (879 lbs).

   I want to repipe with 1" copper and Mono-flo tees, pipe content would be 6.77 gals for a total system content of 64.04 gals (533.5 lbs).

     Outdoor design temp is 0*  Figuring a Delta T of 20* on the supply and return, it would take (theoretically) 21 minutes to heat the 105.5 gals of water and only 13 minutes to heat the 64 gals of water. 

     Now, the ratio of the water content, weight and heating time span all remain constant at 1.61.  When I do the Estimated Fuel Usage evaluation, do I need to multiply the "Estimate" by 1.61??

   Am I just being too ANAL here?  I am getting tired of telling customers that it is like boiling 2 quarts of water to make an 8 oz. cup of tea. I know it is costing them more money to run the system with the old large pipes, I just can't prove it with something legitimate, only my assumptions shown above.



Thanks, nice place Dan has here.

Jamie Hall

Ah... well, now...

Fiirst place, steam systems and hot water systems are totally different as regards heat transfer mechanisms.



Steam systems transfer heat entirely (well, 99+%) by latent heat.  That is, the boiler adds latent heat to the water to create steam.  The steam -- a gas -- travels under very very low pressure differential to anywhere it can condense, where it condenses and releases that latent heat to whatever surface (hopefully a useful radiator) it condenses onto -- which then transfers the heat to the room.  The rate is around 6,000 BTU per gallon of water, approximately.  The condensate then returns to the boiler.  The volume of the radiator and the volume of the piping -- other than the need for the pipes to be big enough to have very low head loss (and, in one pipe systems, allow passage of both steam and condensate without conflict), and the radiation having enough surface area to heat the space required -- has absolutely nothing to do with it.



Hot water systems, on the other hand, transfer heat primarily by conduction -- there is no phase change, and thus no latent heat.  In these systems, it is the rate of flow through the radiation time the temperature drop in the radiation which determines the BTU release rate: you can operate at a low flow rate and high delta T, or a high flow rate and a low delta T with exactly the same results.  Again, however, the static volume of the system has absolutely nothing to do with the case; it is strictly a function of flow rate and delta T, just as in steam it was the flow rate (pounds of steam per hour) and the latent heat release.  Pipe size and radiation size do have an effect, but not on heat release.  The effect is related to how much pump horsepower is required to circulate the water at the rate required; smaller pipes, more velocity, more horsepower (the relationship is actually the inverse fourth power of the pipe diameter -- a pipe half the size will take 16 times the horsepower to move the heat, all else being equal).



In hot water systems, however, volume does have one effect which is sometimes significant: it takes longer to heat up or cool off a system with large volumes.  This is why the old gravity hot water systems had such even heating -- they had huge volumes.



Now I will grant you that it does take longer to heat a larger volume of water from one temperature to another.  That isn't relevant unless for some reason response time is a factor -- in which case a smaller boiler volume will respond faster, of course.  But once you are up to operating temperature, again the volume is irrelevant -- it is the rate.



As much as I regret directly contradicting folks, pipe size in a steam system has nothing -- exactly nothing -- to do with the cost of running the system, or the amount of fuel used, provided only that it is large enough.  Pipe size in a hot water system does have an effect: all else equal (that is, delta T particularly) a smaller pipe will cost more to run than a larger one.



I hope this helps clear up the confusion...

Larsofhaus

Think you may have misunderstood

Jamie, thanks for the response but I think you may have misunderstood what I am trying to do.



    I am converting the old steam system to a hot water system. So, if I use the larger pipes, I will be heating up more water than is necessary.



    Your answer seemed to address the problem from the radiator - down, in terms of getting the heat out of the water, and my question is about the cost of getting the heat into the water.although you did start to hit upon my problem, it looked like it was by accident.  Your words below:



"In hot water systems, however, volume does have one effect which is

sometimes significant: it takes longer to heat up or cool off a system

with large volumes. "



      "Now I will grant you that it does take longer to heat a larger volume

of water from one temperature to another.  That isn't relevant unless

for some reason response time is a factor -- in which case a smaller

boiler volume will respond faster, of course.  But once you are up to

operating temperature, again the volume is irrelevant -- it is the rate."



     That is my point.  Taking longer to heat up the water to get enough heat out of the radiator.   HOW much longer?  And HOW much energy (gas)   are you wasting taking that much longer?  How in the world could it not be relevant to have a boiler burning energy longer than it needs too?



    Anyone else have any suggestions?



Thanks again, Jamie



    

Jamie Hall

Sorry if I misunderstood

Passing over the initial supposition that you are converting a steam system to a hot water system, which is something I can't possibly recommend and which I, personally, would turn down in 20 seconds flat if someone offered the job to me...



In an hydronic system you are maintaining a reasonably constant output temperature from the boiler -- controlled by outdoor reset, or a fixed aquastat, or whatever.  That reasonably constant temperature water is circulated through the system by a pump or pumps, in turn controlled by the thermostat(s) and returns to the boiler at a lower temperature -- the delta T.  If there is a very large volume of water in the boiler, that return water will cool the boiler (and the supply water) relatively slowly, and the aquastat or whatever will not turn the burner on immediately.  Conversely, if there is a very small volume, the temperature will drop quickly, and the aquastat will turn the burner on quickly.  Looked at another way, if there is a constant heat load which is less than the burner capacity, and both systems have the same differential in the aquastat, the burner will run less often, but longer, in the large volume system than it will in the small volume system.  But either way, it's the same amount of BTUs on average.



Now let's change the game.  Suppose you are going from a cold start from some reason.  You will, indeed, use up more fuel heating a large volume of water than you will a small volume.  On the other hand, at the end of the cycle, the large volume will take a similarly longer time to cool down than the small volume will -- and, if you are measuring your fuel usage from cold start to cold stop, you will see no difference, assuming one critical thing: your control system is set up to avoid overshoot, so that you are honestly putting the same amount of heat into the space.  If that is true, the total burner run times should be the same.



I hope this helps some...

The Steam Whisperer (Formerly Boilerpro)

It doesn't make any difference.....

as J said, if you are maintaining constant termperatures in a space, the input btu's needed are the same.  In fact, a larger volume system may use less btu's since the heating equipment can run long, efficienct cycles.  Low mass systems tend to cause equipment to short cycle, a real killer for boiler efficiency.

Gordy

Being anal

    As BoilerPro, and Jammie pointed out in thier posts. Plus the larger pipes add buffering capacity to the system. Its all a mute point once a system is up, and running. Cold start MAyyybeee a difference, but once the system is up to temp, and doing its thing in the heating season there is a lot more mass of water, and pipes. This means longer cool down times between cycles which overlap in calls for heat.



 Example: With a larger piped system, and 180* water supply. The water in the pipes may only cool down to 90* before next heat cycle.



In a smaller piped system with 180* water supply the pipes will more then likley be ambient of the building temps before next heating cycle.

No real calculations involved but just a theory on how the systems would differ with more,or less mass.



 Shooting from the hip I think by the time you figure cost to reconfig system. there would be no realized ROI to the consumer over a reasonable period of time.

Mike Kusiak_2

High mass even a benefit in cold start

Even if the system is run as cold start, the high water volume has an added benefit. Because of the high thermal mass of the system, it takes a long time for the system to rise in temperature when firing, and drop when the boiler is off. The result is an averaging effect, which tends to produce relatively constant water temperatures.



The actual water temperature at any given time is dependent on the current heat load, since the boiler will only fire up until the point when the thermostat is satisfied. As the temperature of the high volume of water cannot change quickly, the average system temperature will only rise up until the point when the heat load is satisfied and no further. The net effect is similar to outdoor reset, since the water temperature depends on heat load, which is directly related to outdoor temperature.



The benefits of this high mass mode of operation were stated in the early Bell and Gossett handbooks, where a graph of system water temperature vs.outdoor temperature was presented for the usual high mass systems of the time. The graph looks exactly like an outdoor reset curve, but is accomplished without any outdoor sensing. They mentioned that the water temperature naturally varied with outdoor temperature. It was simply the result of the high water content and thermal mass of the system, evening out the on-off cycles of the boiler and resulting in average water temperatures only as high as necessary to meet the current heat loss.

Mark Eatherton

Well said Mike...

Energy can not be created or destroyed, only converted, distributed and dissipated.



The boiler converts it to heat, the pipes distribute it to the loads, and when the load goes away due to thermostat satisfaction, the remaining energy left in the pipes is dissipated to the space being heated, raising the MRT and increasing human comfort...



Other than the loss of head space in the basement, there is no disadvantage to having larger pipes, and as has been stated numerous times in different ways, it is actually advantageous, regardless of the heat source (old conventional versus modern con).



ME

Gordy

Elegent explanation Mike

  Backed up with some researchable facts.



Gordy

Larsofhaus

Thanks, but Anal-ocity still smoldering

     Thank you, every one.

 

     Nice explanations that helped explain away part of the "mental block" I was running into, a section of my mind was basically telling me "it did not matter" since the larger volume of water would be the same as creating a "heat sync". But my anal-ocity is still registering, as far as wasting energy. 



     With the lack of insulation on the pipes, it still seems like there will be some "waste" due to heat loss from the the exposed surface of the pipes. Given the lineal feet of pipe used in the original question, at an average water temperature of 150* the heat loss from the surface of the larger pipes will be about 28,000 BTUH and the heat loss for the smaller copper pipes will be about 7,000 BTUH.



   Is that some thing to quibble over?



   As to the reason for converting the system from steam.  This is a two story commercial building that, at one time, stood alone.  Over the years, the second floor was taken off the steam boiler and had a separate heating system installed. Then, a two story addition was added to the rear of the building with the first floor of the addition expanding the office space and the second floor being used as an apartment. In this two story addition, they installed copper baseboard with the new hot water boiler installed four feet away from the steam boiler,  which was, by now, extremely oversized.



    The Main tenant on the first floor is paying for the heat in the apartment,

and they also have a problem with balancing the system.



   Not forgetting to mention that the original heating system was a Vapor system

with a "Differential Loop" installed. They have obviously been having problems

with this system for years.  Some knuckle head even set the pressuretrol at 5 lbs.



   By the way, instead of using the heat output of the steam radiators to size the new boiler, I did a heat loss on that part of the building. For the baseboard heat in the office addition, because they complained about the way it heated (although I am pretty sure it is just a balancing problem), I did use an allowance of 600 BTUH per foot of element installed.



   My proposal to them was converting the steam radiators to a hot water system,

connecting the office baseboard to the new water boiler with zone control,

circulator, outdoor reset and 7 day programable Tstat, and then, with the apartment

being the only thing connected to the existing hot water boiler, we would run

a new gas line to the boiler from the tenants gas meter.



   Any thoughts on either of these?



   

     

Jamie Hall

Oh dear...

a differential loop?  And the system was running at 5 psi?  That is really a shame -- the presence of a differential loop indicates that the system was a Hoffman Equipped vapour system, and they were wonderful.  They ran, however, at 5 ounces per square inch, not pounds!  I wouldn't be too surprised if they had serious heating issues at 5 pounds -- since at that pressure the loop would be industriously operating to keep the returns at 5 pounds too.  That's part of what it was meant to do, if the pressure in the boiler got too high.  It was (and is, on non-knuckleheaded systems) a valuable, if widely misunderstood, safety device.



Oh well...



You are quite right about the additional heat loss from an uninsulated supply line -- the heat loss from a supply line is directly proportional to the diameter if it isn't insulated, which it should be (even for hot water heat, by the way).  Which is why we recommend that all supply lines be well insulated -- 1" fiberglass is the current thinking.  If you insulate those supply lines, the difference in heat loss will be much smaller -- and I doubt whether your client would be able to recover the investment in fuel savings.



However.



If you are going to change to hot water heat, and I have to admit that it does sound as though you have a decent case to do so, you may want to replace those pipes.  Not because of the heat loss, but because of the probability that they will leak -- if not immediately, pretty soon.  Likewise, you may want to replace the old steam radiation, for the same reason.  This has been a rather frequent call-back issue on steam to hot water conversions -- and it is much better for you, as the contractor, to do all the work of a piece and not have to try to recover the cost of fixing leaks on a call-back!