Fuel waste if boiler size exceeds capacity of radiators?

frogpond

My current gas boiler is massively oversized – 175KBTU/hr. Using the EDR calculations kindly provided by someone on this forum, the maximum output of my radiators is about 75K BTU/hr. Putting aside for a moment that the heatload calculation for the house is even lower, what happens to the extra 100K that the boiler tries to put out but that the radiators simply cannot radiate? Is that all wasted fuel?

STEVEusaPA

The waste is in the short cycling, and even comfort. The boiler quickly runs up to limit and shuts off.
If the heat loss is less than what the radiation can put out, then you have the opportunity to put in a properly sized boiler, and use a lower supply temperature.
And if it's a modulating/condensing boiler, even more savings.

frogpond

Okay, the short-cycling issue makes sense to me. It's not a modulating boiler, so when it's on it's using enough fuel to fire at 175K. But I think my question is a little different: even while the boiler is on, if the radiators are only capable of pushing out less than half of that heat, what happens to the rest of the energy being consumed by the boiler?

To use an analogy: I guess I'm wondering if it is like a pump trying to pump a greater volume of water through a narrow pipe, and the pipe can only handle half of the volume. The pump is still giving it its all, using enough energy to pump 2 GPM even though the pipe can only carry 1 GPM. The result is that the pump is wasting any amount of energy beyond what it would take to pump only 1 GPM.

Isn't it the same sort of issue if the boiler is using enough gas to produce 175K but the radiators just don't have a throughput of more than 75K? And if so, is there a formula to calculate how much extra fuel is being wasted?

Jamie Hall

Not really. The boiler short cycling -- and I presume you are looking at steam, not that it really matters -- is actually "modulating" the boiler output to match what the radiation can put out.

Depending on a whole host of factors, it might be possible to determine the reduction in efficiency -- and hence the amount of extra fuel -- but it isn't simple by any means. Not, if the short cycling is managed well, is it all that much of a hit -- a few percentage points.

There have many -- and occasionally somewhat vigorous -- debates on how best to manage a seriously oversized boiler, and in many ways, oddly, it comes down to the combination of the boiler and the radiation. In some ways, the ideal would be to manage the burner on/off on so short a cycle that the temperature of the water (for hot water heat) never varied much or, in the case of steam, that the residual heat in the boiler metal kept the boiler steaming (it will), in either case giving the burner just enough of an on time to keep it that way.

Most burners don't like to do that. So... if the boiler isn't insanely oversize, one can run a relatively short cycle without real problems. In the case of water, allow the circulating temperature to droop a few degrees -- perhaps as much as 10. In the case of steam, allow the pressure to drop some (in a vapour system, for instance, from 6 ounces per square inch down to 2). The objective being to never actually let the boiler cool off -- because that's where you lose efficiency. On the other hand, if one has really massive radiation (big cast iron radiators!) and a really seriously oversized boiler -- such that the "on" time is less than the "off" time of the cycle, perhaps -- it may make more sense to let the residual heat in the radiation be the guide, and force the boiler to stay off, once it starts to cycle, until the radiation -- not the boiler -- has cooled somewhat (that won't work if you need all the radiation going at full capacity, but that is a vanishingly rare situation). A delay time on letting the boiler turn back on of 10 or even 15 minutes -- if the thermostat is still calling -- may work well.

I hope this helps at least a little bit...

frogpond

That is helpful. The takeaway, I suppose, is that there may be some extra fuel wasted, but it is not as bad as I initially thought and probably not worth calculating. Thank you for your thoughtful reply.

JUGHNE

Perhaps if you thought of this as city traffic.
You have a traffic signal at each intersection, the lights are timed for 20 MPH.
But at the first green you speed away at 40 MPH.
Too fast to catch the next green....you have to brake and stop.
Take off again at 40 MPH....same thing for the next light. All the way across town. (Or thru most of the heating season)

But if you were a boiler going 20 MPH and caught all the greens,
(that is just the right size for the job) you would never have to brake or stop. (But you get to your destination....keeping the house warm enough)
Just steady constant speed, avoiding the braking and restarts.
And avoiding the inefficiency of start and stops.

In the real world the only way the boiler could run constantly is with burner modulation. But if on/off burner and sized close to the heat loss the run time would be increased. If sized to the coldest day of the year then one would expect constant firing on that day......a rare happening as there are only a finite number of boiler sizes and many variables for that design day temperature.
So it is the right size for a small percentage of the season.

My truck gas bills reflect this... 460 cubic inch V-8...7.7 liter.
Town mileage is 5-7 MPG.....down the highway it does the best at almost 10 MPG. Fortunate to only drive 3000 miles per year.

Jamie Hall

Not a bad analogy, @JUGHNE -- although it can be stretched a bit further. Suppose your car had only two possible engine settings -- on, full song, and off. If you were clever, you could switch it on just long enough, at intervals, to maintain the required average speed. You could lengthen the intervals of off and on, and have more speed variation around the average, or you could switch it on and off more often, and have less. And never touch the brake, all the way across town!

You'd have to try various combinations to see what gave the best economy (for reference, if you can find it, back in the '50s there was a Mobil crosscountry economy run -- which used similar techniques!).

And further yet -- a surprising number of World War I airplanes did have engines which were either on or off -- no throttle -- and that is exactly how they were flown. Go to Old Rhinebeck Airdrome in New York state to see some.

I sympathize with you on the truck -- one of mine has the 454 Chevy V8, and the mileage isn't a whole lot better than your 460...

frogpond

Haha a very thorough analogy. Thanks again guys.

STEVEusaPA

@ChrisJ disagree again?...lol

Gilmorrie

Here is an axiom - almost universally true, despite what boiler salesmen will say. If your boiler runs reliably, you can't economically justify replacing it to achieve fuel savings. Even if the replacement boiler miraculously puts out heat without burning any fuel, the payback period will be too long. Do the math.

ChrisJ

STEVEusaPA said:

@ChrisJ disagree again?...lol

Just making sure you're paying attention.

JUGHNE

Jamie, are you familiar with "hit and miss" engines, the flywheel carried them thru a few revolutions until the governor sensed a drop in speed and then would fire a "pop"?

Then on the subject of ROI.....I made my 16 year old grandson calculate (paper & pencil BTY) my MPG as we filled up. He said I should update and would save so much money on gas. So we discussed this....a new van might get twice the mileage for an investment of maybe $30,000..... minimal....then 7 % sales tax.
So I spend about $1000 a year on gas.....I might save $500 a year.
The first few years the new van plate tax would approach that amount.
Plus the $2100 sales tax the first registering.

In 2007, I paid $2250 for this 1984 1 ton retired ambulance van. Every thing is heavy duty, always stored inside, 69,000 miles.
Best thing is that it has the 10" roof extension.....stand up .
Today the grandson found out what sales tax and plates cost, and annual tax on a newer car....but he gets about 30 MPG.

I am thinking that a new boiler has about the same financial situation. Especially to go from the old 80% CI to 95% mod con.

ChrisJ, so now we know who is throwing up the "disagree" tag.

Jamie Hall

Yes indeed -- hit and miss engines! The basic principle we're talking about!

And you are so right on the economics -- which is why I still have, and run, that '94 K2500 I mentioned!

ChrisJ

@JUGHNE I haven't averaged less than 40mpg on a tank of gas since 2012 and I drive 20000 miles a year.

30 would upset me a lot.

Also never owned a hybrid yet

EBEBRATT-Ed

@JUGHNE

I agree the numbers don't lie

EdTheHeaterMan

The cost of ownership is used to justify the difference between the higher efficiency and the standard system. This is assuming the boiler is being replaced anyway. So now you are only justifying the difference between the two systems.

Back to the original poster. Is your current 50-year-old system economical to operate? How many Gallons per year?

Now if the oil burner is in the ash pit of the 50 YO coal conversion type boiler, you can raise the flame to the originally designed location. When the "Dead Man" had a picture of it on the drawing table, the coal fire was near the middle door. If you put the burner there and plug up the hole at the bottom of the boiler, (where the ashes fell to the bottom) I have seen dramatic savings because the 80% flame is closer to the water and makes steam faster.

Has it been determined if this is a steam or hot water boiler? Gas or Oil? A 50-year old boiler could be a modern style package boiler or it could be a leftover pre-WW II coal-repurposed with oil burner.

Pictures of the boiler will help us to see if that project is possible.
I believe the only reason to replace a steam boiler is if it leaks. 50 years is nothing for some boilers. How many of us have pulled out 75 or 100 YO boilers?

Just saying.

EdTheHeaterMan

@Jamie Hall there are 2 issues discussed here. "very confusing" @bleeder is talking about Steam and the age v replacing, so a Mod-Con is not an option.

The original poster, @frogpond is talking about oversized boiler and has not indicated water or steam and/or oil or gas.

So how do we address this confusion? Maybe we, as experienced members, should ask off-topic questioners (especially with a low number of posts) to begin a new discussion.

STEVEusaPA

Where's the 'head spinning emoji?
I sent a note to @Erin Holohan Haskell to recommend @bleeder posts get moved to his own thread.

Erin Holohan Haskell

Thanks, guys. I've moved @bleeder's question to a new post here: https://forum.heatinghelp.com/discussion/180568/45-year-old-oil-fired-steam-system-worth-it-to-switch-to-gas

ryanwc

Cycling in a mod/con has a variety of costs, though, no? Not enough to justify replacing the boiler, but still important to consider. My understanding is that they throw away heat during the post-cycle purge. Also, that each ignition and start up is more wear and tear on the system. How important are these factors?

Jamie, I have a particular question about this:

>In the case of water, allow the circulating temperature to droop a few degrees -- perhaps as much as 10. In the case of steam, allow the pressure to drop some (in a vapour system, for instance, from 6 ounces per square inch down to 2). The objective being to never actually let the boiler cool off -- because that's where you lose efficiency.

Why would letting my water cool off harm efficiency? As you said in a thread about a marvelously oversized steam boiler, the heat generated is always going into the house, unless there is some gigantic problem, like unburned fuel going up the chimney or a steam leak.

I just got my hydronic boiler reset to an Outdoor Reset mode, and I can hear it cycling very rapidly right now. The initial cycle on a call for heat is somewhat long, to get 90F water up to the 130 setpoint (and then the 10-pt. buffering to 140. But the next call will be only a minute or so, because the water hasn't yet cooled, and it takes little time to bring 132 degree water to 140.

My sense is that my best bet is to set the mandatory post cycle off-time to the highest parameter available, to allow the water to cool significantly. All I can see that doing is improving my ability to get condensing temps (ie, get maximum in-house heat out of the same amount of fuel) while running for a handful of 5-10 minute periods rather than several dozen 50-second periods (reducing wear and tear). And either way, all the heat put into the water will go into the house. What am I misunderstanding?

Either way, I can see that the system would work better if I added emitters to Zone 2. The short cycles are mostly in zone 2. The first floor zone with old fashioned radiators and more space to heat tends to cycle for reasonable times. Or it may be that a storage tank is in our future.

Jamie Hall

Figuring the actual loss of efficiency -- if any! -- in a boiler which is cycling is really messy, and depends very much on exactly how the boiler operates and how it's controlled, as well as how massive the boiler is and how much water it holds.

I very rough terms, the higher the frequency of the cycling the less of a hit there will be on efficiency, provided that the cycling is essentially square wave (either full on or full off). There are tradeoffs with that, though, in terms of parts wear or precision manufacture -- or both. There may be noise problems as well -- one early aircraft jet engine was modulated and controlled by the acoustic resonance of the exhaust and intake pipes to the burner (at, as it happens, about 50 hz) and could be heard for miles.

There really isn't one good answer to "how much", though it's quite reasonable to say that the best result will be with the boiler firing continuously. Even there, though, unless the fuel to air ratio at that firing rate is the optimum ratio, modulating by fuel low may not give better results. In another current thread there is a comment about using oxygen sensors on the flue gas to drive servomechanisms to control that fuel/air ratio within very narrow limits (which is done on current automotive engines, for instance). Other applications (aircraft gas turbines) don't lend themselves to that, so instead they use very complex electronic and electromechanical sensors and feedback mechanisms or the same purpose; these work because the entire engine is manufactured and maintained to absurdly tight tolerances.

To get back to the world of heat... your rapid cycling (I presume that you don't have a modulating burner) suggests that you need a buffer tank in the system, as it certainly isn't helping your efficiency -- nor is it helping the longevity of the burner and its controls.

hot_rod

adding heat emitters will just heat the space faster and not increase the cycle length. It may match the boiler output and the heat emitters capacity, add some fluid volume, but it is not a cure for what ailes your system.

When a boiler is oversized to the load, either downsize the boiler or add insulated storage to park the heat until it is needed.

or put an addition on to the home🏦

ryanwc

I do have a modulating boiler, but it's so oversized (PF 140 = 140k btus), that even modulationg to 25% doesn't bring it low enough for zone 2.

Adding heat emitters certainly can potentially mitigate cycling. The boiler is cycling because the emitters can't get rid of heat as fast as the boiler makes it. If adding emitters allows the system to shed that heat into the house faster, yes it will heat the space faster. It will do that by pulling more heat out of the water before it heads back to the boiler. Cooler return water will delay the process of hitting and then overtopping the setpoint.

The question may be whether I can add sufficient emitter capacity to make a substantial difference, and whether it's worth the cost. And Jamie seems to suggest that cycling isn't inherently much of a problem. I had read differently about condensing boilers - that you lose heat at the purge, and the ignite & extinguish phases both involve some incomplete combustion. But I don't really know. Maybe that's not significant.

Jamie Hall

In boilers with a purge, you do lose efficiency at the purge, and there is some incomplete combustion. With those, there is an interesting -- and difficult to quantify -- balance between cooling the block.... and higher frequency with that fixed loss on each cycle.

Fun, isn't it?