Calculating Savings from Right-Sized Boiler

jp_2

whats your explaination here?

Jim_80

Calculating Savings from Right-Sized Boiler

Can anyone point me to a methodology for calculating how much operating cost savings can be generated by right-sizing a steam boiler?

I live in a mid-sized (50 unit) condo that has a one-pipe sysem. After reading through LAOSH and "We've Got Steam Heat" I am pretty sure our building's boiler is oversized for the connected load. I've not done any calculations for the load yet, but find Dan's stories about the way steam has been historically sized to be pretty compelling

I'd like to try to convince the board of the condo to replace the boiler with one that is the right size. But how do you calculate how much less fuel will be used by a right-sized boiler?

And, if we were to switch to a higher-efficiency model (say 85% rather than 80%), is there a simple calculation to check the savings from that?

Thanks,
Jim

Garret

The only thing I've found that attempts to quantify this is Henry Manczyk's white paper:

http://www.energy.rochester.edu/efficiency/optimal_boiler_size.pdf

50 units is pretty big... and old cast iron radiators can be surprisingly high in EDR... do the math for the connected load before you do anything else.

If the building has been insulated, windows replaced, etc., it's possible you have too much radiation now.... less likely is that you have too much boiler for what's installed.

Mike T., Swampeast MO

If the system is working simply with routine maintenance, typical consumption and providing at least decent comfort then it's hard to justify replacing the boiler until it's in true need.

No simple calculations in this situation.

Brad White_141

A right-sized boiler

that modulates, even if it does not condense, beats a right-size on-off boiler any day.

A right-size on-off boiler is only correctly sized for one day of the year, the coldest. After that is is all up or downhill depending on your perspective.

As Mike T. said, no easy calculation. But a wrongly sized boiler will cost you a lot more than that!

John Ketterman

It's better to have a right-sized boiler, just as it is better to eat right and exercise. But no one can tell you specifically how much you personally will benefit.

If everything else is OK, spending big money on replacing a perfectly good system will never be worthwhile.

D107

Age of boiler

I'm a homeowner who used to live in a coop with a similar situation. You don't say if this is a very old boiler, that's not been maintained well--with very high fuel bills, that may have leaks from buried returns, that isn't vented properly, that's operating at much above 1 lb of pressure. You're right to do your homework now so that when you do need to replace it you'll be way ahead of the game. Checking the rads and their vents, shutoff valves, ventilation and all the things you'll learn in Dan's books is the way to go. I wish the Wall had been around all those years ago--I would have found a steam guy and paid him for a consult--they probably have some tricks you can do with oversized radiation/boiler, etc. if it turns out to be the case.

good luck,

David

jp_2

not too difficult,

maybe?

first off, 'if' you could start off with an accurate heatloss of the building and hot water usage, you could determine what the building 'needs' at a particular time.

then by clocking the boiler, or measuring its input energy, you can make an 'assumption' as to efficiency.

so if the building requires 80,000btu's , and the boilers inputing 120,000, all areas are at design temp, then those other 40,000 btu's are going elsewhere.

but this requires an accuarate heatloss.

edit: so my point is, if you match the load, you can not get any better efficiency nnumbers, more how inefficient the old system was.

Plumb Bob

> so if the building requires

> 80,000btu's , and the boilers inputing 120,000,

> all areas are at design temp, then those other

> 40,000 btu's are going elsewhere.


This is nonsense.

Christian Egli_2

Oversized sweaters keep you just as warm as tight ones

Looking at no particular central heat schemes

If, for whatever day you wish, it takes one pail full of heat to warm your building to whatever desired temperature, then it becomes the task of the boiler to fill the pail. With everything going right, and a system that can actually fill the pail, and a temperature control scheme that stops the filling when the pail is full, it is more then likely that you won't see much in fuel savings differences whatever reasonably sized boiler you use.

Thus, what's the best way to fill the five gallon pail? By calling the fire brigade and their fire hose? By using your ordinary garden hose? Or by diverting the windshield washer skirt from your car? All three will get the job done... though not with the same effect.

The goal is to fill the pail up to the rim and no more and to do this in the quickest amount of time because we're racing with the outdoor hourly changes in weather (very rapid changes in the East and Midwest but slow paced in places like Seattle and Western Europe). Any water spilled during the filling and any filling beyond the rim constitutes the wasted heat.

Obviously, grossly oversizing the boiler splashes heat all over the place. It gets the place hot quickly, but due to difficult fire hose flow control you might cause overheating of your building - this is the real big source of waste and inefficiencies, it's not in the boiler room, it's in the heated space upstairs.

But there are many cures

You control the water hose by inserting a valve. If it's just an on-off trigger, you have a flow control that works fine on the windshield skirt but not on the fire hose. There you need finer control that allows you to taper down the flow to a trickle so that there may be no overfilling, you need a valve that modulates.

On the garden hose, a nice modulating nozzle gives you all the control you want, no splash, no overfill. Modulation is always a good feature on a boiler.

To go quicker, instead of one garden hose, you can use more than one and turn them off, one by one, as you get near to full. That's boiler sequencing, yet another form of control.

If your pail is transparent and you can draw some level marks on it so that you can better see the level rise, you have a form of control called outdoor reset whereby you anticipate how quickly the pail fills with heat.

See, even if your boiler is a bit oversized but you have adequate control, there may not be any major fuel wasted on overheating.

However, if the boiler is undersized, you may not be able to heat fully on the coldest days of the year (which may or may not be a problem, it's a personal choice to make and a personal risk to assume) The big advantage of being undersized is that you can prevent a lot of the costly indoor overheating, simply because the tiny boiler can't take you there.

N.B. You have one pipe steam, and that brings in one specific problem. One pipe steam boilers can't be undersized for their radiators. It's the boiler size that gives the system it's capacity to vent the air. That's real important, because if you undersize, you won't get heat to the radiators because they're air bound. The boiler won't know, it'll keep on burning, and you'll keep on paying the fuel bill.

*** Steam has the great advantage of being near instantaneous. At 10, 20, 50 MPH, it takes only seconds for the steam to leave the fire in the boiler and get to your radiator. No waste of time. When the fire goes out, the heat stops coming, about that quick, and though steam carries an incredibly huge amount of heat per pound, steam weighs nothing, your whole system in action may contain only the equivalent of a thimble full of water. There is no mass, thus no inertia, and still there is huge capacity - good or bad, hot water systems are exactly the opposite.

About the cost of overheating

It is entirely possible to compute an approximate cost, it requires minimal data acquisition, and a little dose of calculus. The results you get are related to those of whatever state agency it was that used to promote lowering indoor thermostat settings in exchange for fuel savings. If the thermostat is set at 65F and a thermometer shows 75F, that's 10 degrees in excess and it costs you (60% - I don't know an actual number) in excess fuel. And, for instance, a 5 degree excess might only cost 20% more fuel.

Here's what I'd do

Whether your boiler is oversized or not, if you suspect there are heating imbalances in your system and if some apartments seem to overheat, then the first thing to do is review the venting scheme (mains and radiators) and perhaps add a control system and some radiator thermostatic valves.

After doing that, you then may or may not look closer into a new boiler. Because whatever you do, a new boiler won't have optimal performance without optimal system venting. And yes, buy a boiler that comes with the highest believable efficiency rating.

Since the vents are the only moving parts, they're the only ones to wear. They deserve being maintained and you deserve the big dividends that come to you for doing so.

Isn't that lots of blah blah to end up saying the boiler should match the radiation according to the rules of the trade?

For the rest, keep warm. For shrinking the woolen boiler, simply wash in hot water.

jp_2

Oversized sweaters keep you just as warm as tight ones

what?

apparently you do no not live in a cold climate, a jacket too big on a windy day leaves cold winds straight up your back, and that ain't a nice feeling.

edit: so Christian?

your first paragraph seems a bit vague, are you saying short cycling(heating system oversized) has no effect on 'cost to heat' as long as it does not over shoot?

Jim_80

Lots to consider

Thanks for all of the advice so far. Here's some more information on the system.

We've made some good investments over the past couple years....an excellent new Tekmar control system and a more robust venting scheme. All was installed by Bill Dooley of Foley Mechanical, and they did an excellent job. We're already seeing 20% to 30% savings in our fuel bills, which adds up quickly when the winter gas bill is $15K/month. We're predicting a 2-year payback, which is pretty good.

The issue is that our boiler is 24+ years old, and I want the building to be prepared for replacing the system when the boiler gets too expensive to maintain. If we want, I think we have a good 4 to 5 years left on the current boiler, but I'm trying to figure out if it makes more sense to replace in the next 2 years rather than in 5. Given that we're going to make the investment relatively soon, should we try to start saving money now by right-sizing and improving our efficiency (current boiler runs at 78% or so)? Or, is the savings not worth the interest we would earn on letting our building's reserves sit in the bank for the next 5 years?

Thanks for the help.

Perry_2

Maintenance of old boilers

is usually pretty cheap compared to replacing them.

You have a 24 year old boiler. As long as their is nothing wrong with the cast iron sections - it could easily last another 24 years with only minor maintenance (as long as you take care of it).

I'd bank the money in some form of designated fund for a new boiler - with good documentation on what a good new boiler will cost now (they arn't cheap).

You sould also start a similar roof fund, and maybe a parking lot fund for when it needs repaving.

New windows arn't cheap either.

Perry

Brad White_9

Neither is the cost of natural gas

but as soon as I put money in the kitty, it's gone!

I like your preventive maintenence/planned replacement fund, Perry. Like my landlord days... thanks for the reminder of good policy.

D107

good plan perry

and Jim you might add chimney/inspection/repair to your list. Our 90 year old building had very deteriorated lining (probably incorrectly sized to boot.)

We had an HB Smith that needed new hydronic seals after 16 years of service--probably because of poor maintenance in its earlier years.

It might be worth your while to post some boiler pics in case anything obviously problematic sticks out. (near boiler piping, etc.)


David

Christian Egli_2

Drawstrings work great

I want to say one tight fitting garment does not do the trick like layers of looser fitting ones do, but as far as clothes go, fashion has a bigger role to play than comfort, JP perhaps you wear the latest baggy-down-to-the-knee pants? It's cold where here where I live, but only in the winter luckily.

For boiler sizing, if we do not consider standing flue losses (which may or may not be worse the bigger the boiler is), and if we don't consider comfort issues, there shouldn't be big differences in fuel usage whether you use short burst of intense fire or the steadier glow of a smaller fire. All this, of course, providing you can reach and maintain the desired temperature without overshoot. Control and modulation are really important.

There is one more notion I want to expand on which shows an oversized boiler might actually save fuel.

Let's say the thermostat is set at 70F, at which temperature the fire kicks out, then we let the temperature drop back down to 65F before kick in. Nothing out of the ordinary. A large sized system will necessarily swing up and down between the two limits, you get a saw tooth wave form. Keeping the same thermostat settings but with an optimally sized boiler, the ideal temperature curve you'll get will resemble a straight horizontal line at 70F - perhaps the most comfort, but don't fiddle with the thermostat...

Of these two different graphs, which is the cheaper to achieve?

That's real easy to answer: simple mathematical integration of the temperature curve over a time period will show the saw tooth form to represent less BTUs than the solid flat line. Convince yourself with a coloring pencil; while shading all the area between the curve and the ground base line, measure how much of the pencil you wear. The saw tooth form, specifically because of the missing notches represents a smaller area than the solid line. Nothing magic, nighttime setback works the same way, and integration is exactly what the gas and electric meter does every second of the day.

This oversize advantage situation is believable within limits - usually the temptation to use all of the available power is so great that all savings will be lost to overheating. This would shape a saw tooth curve that pokes above the 70F line and increases the area enclosed under the curve.

Jim, you must be a pleasure to work for, you have figured out all that is important to good maintenance.

Perhaps, while you wait for your boiler fund to grow, you could investigate sizing options by downsizing the burner on the current boiler - and see if you can still heat the whole building without problem.

Perhaps even, if your boiler room is large enough, you could keep the old boiler and install the new one. There might be some redundancy and some sequencing opportunities.

Don't be left out in the cold with no boiler... or jacket.

jp_2

watch what you say.....

JP perhaps you wear the latest baggy-down-to-the-knee pants?

boy, thems fightin words, luckily i take it as a joke or I'd skin you like a oppossum!!!

Ken_40

Fuel savings digression...

If you are making a cup of boiling hot tea, and put three cups of water in the kettle, the energy required to make the three cups boil, is three times what's needed!

Given that it takes 1 BTU to make one pound of water rise one degree F, BUT ~1,000 BTU's to make one pound of water go from 212F water - to 212F steam, a substantially oversized boiler will waste a great deal of energy making three cups of boiling water, when only one is needed.

The basic calculation of wasted energy is simple. Ancillary issues aside, figure how many gallons are present in the existing boiler, how many would be in a new boiler (sized properly) and convert the gallons to pounds of water (~8.3 pounds of water = 1 gallon) and extrapolate until your heart's content (;-o)

John Ketterman

Why is it wasted? Where does that heat go? Does it leave the house?

LarryC

Where does the heat go?

How about up the chimney?

Dan C.

JP

He said it was a steam boiler you size it by the radiation not the heat loss

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jp_2

i'll buy that

but what if the radiation and heatloss are really different?

where do you go?

yes i was thinking more in line with hot water, sorry.

brucewo1b

JP

Then its time to start reradiating(placing radiators to meet the heatloss of each room) or TRVs (to help radiate each room to heatloss), but you still have to meet the load of all the condensing surfaces on cold start.

Charlie from wmass

For the sake of full disclosure We should tell you that steam is around 82% efficent. I have seen people save large amounts by using TRV valves.We had a large home That documented 25% saving on heating costs and this was with a 30 Mills Smith boiler with a tankless. I can only imagine what they could have saved with a new boiler.

Christian Egli_2

Making for a blue moon

I had no doubt you would have fun at my fashion statement... JP... You know more about warm clothes than I do, if I remember right you live far away from me, in a place that sounds real cold. I just hope you don't have to dress in opossum pelt coats, my hide wouldn't do you much good, it's not all that furry on top anymore.

Kindest regards as always and keep warm.

jp_2

Weezbo knows cold

we are kepth warm by lake superior, she, in turn, dumps a couple hundred inches on snow on us.

Christian Egli_2

You know snow

Wow, no wonder you don't like getting a snowflake inside the back of your coat. A couple hundred inches, that's cruel.

jp_2

snow vs tropical beach!!

take it any day!

skiings great, snowshoeings great, dogs love the snow, no BUGS, beer stays cold forever! no refrigeration problems!!!

I've been cross county skiing on a lake in may! but too have been mt biking in the lake in april at 70F, so much for global warming?

subcooler

Modulation

I big steamers modulation has been proven to provide no energy savings over a on off or hi - lo fire burner. Now before you go off the deep end this is why: 1st of all what is the eff. of the boiler with a hi -lo gas train 82% and the mod gas train 82%. 2nd when the hi - lo does it thing and shuts down and the shutters close stack loss is at a minimum. When the mod. goes into turn down, lets just say for the record we are using a 4 to 1 turn down, what is the eff. of the burner now? Are we using excess air to keep that big burner running at a turn down condition. The answer is yes even on the best burner out there. These losses at a low fire continous condition have been proven to burn more fuel than a hi-lo that cycles 2 to 4 cycles an hour. Now this is not a great selling point to advertize with so they don't. On - off and hi -lo's are like old school and not sexy. Where as modulation is like pimped out. And just think of all that electricity not used to keep that big power flame roarin. Why do you think the on-off's and hi-lo's burners lasted so long? Because they are not running 24/7. That is why I chuckle when-ever I here the argument about running a big oversized boiler with modulation at about 25% capacity for like 9 to 10 months out of the year. Like all these little mod-con jobs. And one last thing hate the message not the messenger.

Ken_40

Simply put...

The energy is wasted because it fires a larger flame than required (wasting fuel) and, time! It takes a larger flame - and more time to heat a bigger than needed boiler. The waste is substantial, since it occurs EVERY TIME THE BOILER STARTS UP!