Cycle times vrs Heat Loss Calc

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Old 01-16-14, 05:29 AM
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Cycle times vrs Heat Loss Calc

With all the info/advice regarding older systems many times having "oversized" boilers installed....myself considering boiler replacement possibilities...I downloaded the "heat loss calc program" and frankly find it daunting to say the least Sooo after reading a recent post "NEW boiler noise and not heating properly, installer won't come back" (WOW...sure hope he's able to find some compitant people to fix that) it made me wonder if there was a way/theory about a current systems cycle times ONCE house is up to temp of course, would be a possible way to tell IF that system is oversized vrs the very involved heat loss calc. I understand efficeincy of old vrs new would of course enter into this. I just wondered if there was a preferred cycle time a boiler SHOULD fire for each heating cycle as it relates to that system being properly sized AND figuring current insulation properties and climate arent changing? As always many thanks...I love reading it all
 
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Old 01-16-14, 05:41 AM
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Cycle times

According to what I have read ,in a perfectly sized heating system the burner would fire for 24 hrs. on the coldest day of the year and put in just enough heat to keep you comfortable. Therefor a burner that can modulate to match the heat lose and not shut off would be ideal.
 
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Old 01-16-14, 02:45 PM
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If you think the computer program is difficult you should have done heatloss calculations in the days before computers. Using a computer program reduces the job to just measuring.
 
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Old 01-16-14, 03:14 PM
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There are several of us here that are fairly well versed in using that program. If you have questions, just ask.

What 'saves' has said about running constantly is the one end of the range, when it's at 'design temperature' outdoors and the heat loss from the home is at it's maximum.

Your home's heat loss is calculated for the 'design temperature' in your area. A reasonably well constructed home will USUALLY exhibit heat loss between 25 and 35 BTUH per Square Foot for something between 0 and 10 degrees F outdoor temp (which might be an 'average' design temp for most of the country, we don't know what state you are even in!)

At all other times when the temperature is warmer than 'design', the heat loss will be less.

So, ANY boiler (except for some of the 'modulating' boilers that can adjust the heat output given outdoor temperature feedback) is going to be oversized for MOST of the heating season.

So back to saves' statement:

When the outdoor temp is at 'design', ideally your boiler would be able to output it's maximum heat and that heat output would match the loss from the building. During this time the exactly right sized boiler would run constantly.

When the temp is warmer outdoors, of course it will cycle.

A 'short cycle' could probably be considered to be anything less than say 5-7 minutes of burner 'on' time.

I would think that MOST boilers, even correctly sized ones will short cycle SOME of the heating season, like when it's 50F outdoors as example. Your home may only need 10K BTUH but your boiler is sized for the worst case heat loss... say 60K BTUH... yeah, it's gonna short cycle and there's not much to be done about that.

If it short cycles during really cold weather, well, your boiler is WAY too big.

ANOTHER thing that can cause short cycles is if only one zone of many calls for heat. The 'load' simply isn't there and the boiler will heat fast and shut off quickly.
 
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Old 01-16-14, 06:15 PM
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So I thought I would go down and time an hours worth of running cycles on my system tonight...first let me state I live in Lower Michigan western lakeshore area about half way up the state...in a timber framed 36 year old 2 story (with a finished basement 1000 SF) basically an open plan with high ceilings...total SF 3000...insulated to 1978 standards roughly R11 walls and R25 roof (fiberglass & some foam) certainly not enough for this climate but expensive and not an easy task to add to because of design (no attic) Windows are original Pella double panes and plenty of them. Heres the plate on my boiler...also 36 years old and running like a top as far as I can tell (:NO NO NO: I'm ready)
Name:  IMG_8516.jpg
Views: 393
Size:  24.5 KB Its a Burnham located in basement room 180 in 120 out piped to 6 zones:
1) Garage currently off but normally in real cold weather I keep the stat @ 41
2) Upstairs (all) currently set @ 72
3) Main Floor (half house living area) currently set @ 72
4) Main Floor (other half) currently set @ 72
5) Front entry, hall and laundry currently set @ 65
6) Basement (all) currently set @ 65 normally I leave this lower @ 58 because we are never down there BUT after a recent break during below zero weather while away I have revised this thinking (another story/thread bad design with zone water running out in area over foundation to keep convector against wall above in living room DOH! big mistake I didn't do it BUT trust me I FIXED it this time never to happen again

I run them all on Honeywell programmable stats and still believe that a set-back saves so I usem at night and when away EXCEPT during very cold weather where I'll letm hold...I find I prefer a cooler house in most situations.

So currently here the weather is
Light Snow Fog/Mist

30F

-1C

Humidity85%
Wind SpeedSE 8 mph
Barometer29.51 in (1000.1 mb)
Dewpoint26F (-3�C)
Visibility1.00 mi
Wind Chill22F (-6�C)

Last Update on 16 Jan 7:55 pm EST

and here's my cycles for an hour with the above conditions:

1) 7:05PM Boiler fires and runs 2.5 min. Boiler temp (if right, orginal gauge but still readable barely) I know I need new ones) 185 to 200 rise

2) 7:12 Boiler fires and runs 7.25 min BT looks like a 175 to 210 rise (Im guessing cause I can hardly read it and what are the chances it's still accurate.)

3) 7:44 Boiler fires and runs 5.5 min BT 185 to 200 rise

4) 8:09 Boiler fires and runs 3.7 min BT 190 to 200 rise

Boiler pressure (if right) looks to around 15 to 18 lbs and steady

Burners look pretty good (mostly blue and very much like some other pictures seen recently in other posts) but probly need a good cleaning since it's been several years since they have been serviced.

Probly oversized, especailly considering set backs and not heating all areas of home the same much of the time...but man...sure has served it's purpose and continues to do so! simple & reliable = aok with me EVEN if it does cost a little more...cant be that much and I'm not unhappy about about what we spend for a heating season typically between 1,2 & sometimes 300 during the coldest months.
 
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Old 01-16-14, 11:59 PM
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Boiler steady state Thermal efficiency = Output/Input = (139,000BTUH/180,000BTUH) = 0.77222222 or 77.2%

For dynamic efficiency see Equation 4. Advantages of Modulation Raypak

http://www.flowintel.com/documents/MeasureFirst.pdf

http://www.harscopk.com/uploads/files/short cycling prevention [pdf] Patterson-Kelley

Performance of Integrated Hydronic Heating Systems. Project Report. December 2007. Thomas A. Butcher.

Minimize Boiler Short Cycling Losses eere.energy.gov
 
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Old 01-17-14, 11:01 AM
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Thanks heatworm....interesting stuff! kind of alarming and I quote "There is an old rule-of-thumb which says that a short cycling boiler achieves fifteen efficiency points less than the lowest efficiency achieved in nonshort-
cycling low fire. An atmospheric flex-tube boiler,
for instance, that achieves 72% efficiency at low fire will
see 57% efficiency in short-cycling mode. The loss of
fuel efficiency is staggering. If you want an energy
efficient boiler plant design, there is often more to be
gained from short-cycle than from choosing an ultra-high
efficiency boiler."

Soooo I'm grossly oversized possibly...need new gauges at the very least and is there any adjustments to be made to help the short cycling short of R&R of the unit...for instance as I tried to watch the actual drop in temp between cycles it appeared that the spread wasn't repeatable on the low to the high AND didn't seem to correlate to the given time of the cycle...again a better/new gauge would help with this..BUT isnt a boiler suppose to have a bigger spread low to high dictating the firing cycle time? what exactly dictates the firing cycle time? I realize the amount of colder zone waters circulatling back in must have the biggest effect and lowers the boiler temp triggering the firing right?
 
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Old 01-17-14, 03:47 PM
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Soooo I'm grossly oversized possibly...
More like probably but so are most boiler/furnace installations.

...is there any adjustments to be made to help the short cycling short of R&R of the unit.
Sometimes minor adjustments to the firing rate can be made.

I tried to watch the actual drop in temp between cycles it appeared that the spread wasn't repeatable on the low to the high AND didn't seem to correlate to the given time of the cycle.
Unless the circulator pump was running all this time the number spread is meaningless. If the pump WAS running it points to a non-repeatable aquastat.

BUT isnt a boiler suppose to have a bigger spread low to high dictating the firing cycle time? what exactly dictates the firing cycle time? I realize the amount of colder zone waters circulatling back in must have the biggest effect and lowers the boiler temp triggering the firing right?
Cycle time is the time to raise the temperature of the water that is in contact with the sensing bulb of the aquastat from the cut-in temperature to the cut-out temperature. The time required is dependent upon the total amount of water in the system, the flow rate of that water, the rate at which heat is released into the room and the rate at which heat is added to the water from the fire. Obviously that technical answer does not answer the question that you are really asking so let me put it another way.

The trend over the last several decades has been to make boilers (and the water content of the entire heating system) smaller and smaller. This allows for a quicker responding system from the time the thermostat is turned up until the time the desired temperature is achieved. The heat source (boiler in this case) has traditionally been oversized to allow for a "load pickup" (raising the temperature in the house significantly) in a reasonable time. Unfortunately this same thought means that under more normal circumstances the heat source is vastly oversized to simply replace the heat lost under steady temperature setting of the thermostat. As homes have become more energy efficient through insulation and air sealing this problem has been made even worse. Way back when, a residential boiler might have held twenty or thirty (or more) gallons of water and the rest of the system with large pipes and tall cast iron radiators might hold another twenty, thirty or more gallons of water. It took a long time for this quantity of water to heat up or cool down so the temperature in the house was more even and the burn times for the boiler were longer yet less frequent. These longer burn times would allow for a higher combustion efficiency but the designs of the boiler itself would be such as to not really take advantage of the higher combustion efficiency.

But people decided they wanted the heating systems to respond more quickly to load changes. Cast iron radiators were shunned for several reasons and boilers were downsized for space saving and for overall efficiency improvements. The result was a system today might have only a couple of gallons of water in the boiler and maybe eight or ten gallons in the rest of the system yet the burner is still producing a large amount of heat, albeit somewhat less than the older systems. That heat must be absorbed by a much smaller quantity of water and then released much more quickly to heat the house. Unfortunately, what now happens is the temperature in the boiler rises quickly which shuts down the burner even though the system is still calling for more heat in the house. The returning water cools the aquastat, the burner again fires and the cycle repeats.

To try to alleviate this short cycling they industry has devised the modulating burner, one that tries to change the firing rate to match the heat loss of the house. The concept is sound but the problem is that it also introduces a much more complex burner control system and that means higher capital costs along with higher maintenance costs. Prior to the modulating burner several strategies of resetting the boiler cut out and cut in temperatures according to the expected load have been tried,mostly based upon outside air temperature. All these strategies work to some degree and some better than others depending on the individual home's heating system and heating requirements.

I hope that answers some of your questions. If you have more, please ask. I'm pretty slow in responding mostly because of problems in typing that require extensive editing on my part, caused by several physical health problems.
 
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Old 01-17-14, 03:54 PM
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Keep in mind that some of those PDF files are written by companies who have something to sell.

I would be somewhat skeptical of the 15% claim they are making.

"Rule of Thumb"... yes, I use them myself, but where's the data to at least validate that ROT? It must be based on SOMETHING.
 
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Old 01-17-14, 04:10 PM
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This information on boiler short cycling losses , on how to provide a home for boiler heat in a buffer tank confirmed that the experts I dealt with when installing a new gas boiler know only the thinking of 30 years ago .Thanks for this and should be required reading for anyone in the heating business.
 
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Old 01-19-14, 09:38 AM
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I hope that answers some of your questions. If you have more, please ask.
It certainly does and thanks very much! Modulating burners sound great and I understand the theory with thier operation/savings during use in higher spring/fall temps with thier ODR's AND I guess during "steady state" running times and such BUT for me personally I use other means of heat (albeit free standing and insert gas appliances we like the ambiance of the fires) during these times and dont really rely on my boiler system until the cold weather really sets in soo with that in mind and the ole "KISS ethics firmly in place with me. I'm sticking with a tried and true low pressure cast iron boiler for my main heating needs...afterall this one has lasted 36 years and possibly many more! My only question is: and short of doing the HLC (I just dont want to do it) Would it be reasonable to think that I could probably drop back to a boiler with a 130 input and 108 DOE/94 NET over my current...and of course the new one would be a 82% AFUE and probly a Burnham S2...I'm thinking that just may be a better sized boiler over the next up 164/135/117 similar to what I now run.
 
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Old 01-20-14, 02:49 AM
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You're on gas, so if you want a simple way to gauge your heatloss, why not monitor your gas meter and also the heating degree days? WeatherUnderground.com can provide your HDD numbers. Then you just need to WAG the efficiency number and voil, you have your heatloss. Factor in the efficiency gains of the newer boiler and you might be even more accurate than trying to guess what is between the studs and rafters and infiltration. Your gas meter is a useful tool.

As for increasing the run times, this might sound hoaky, but a used, but in good shape unwired 40 gallon electric water would really help there. If you do that just remember that you'll need to increase your expansion tank capacity accordingly due to all the extra water in the system.
 
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