Boiler Water Differential Temperature.. Why?

 
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Post by Lightning » Mon. Mar. 07, 2022 6:54 pm

I'm gonna show my true novice colors here and ask what is the purpose of having a boiler water temperature differential on a coal boiler? As some of you may have seen on my Axe thread, I've decided to experiment with running a very tight differential of only 1 degree. So far I'm seeing very positive results with keeping the fire in a better state of health for DHW demand which of course is very inconsistent relative to home heating requirements. When there is little or no home heating demand the boiler cycles for 2-3 minutes, 2-3 times per hour and over shoots/over temps are kept minimal. Very similar to what a keep fire timer does. This way the fire is kept at the ready for a hard run when it presents itself.

What's your opinions on running a coal boiler this way and what ill side effects could it cause? Thanks for putting up with my cantankerousness... :D


 
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Post by coaledsweat » Tue. Mar. 08, 2022 8:15 am

It is recommended that it be run at 15° minimum for solid fuel boilers. If you are having success then it probably isn't a problem for you. Is your boiler a little oversized perhaps? My only concern is how low the boiler dips on a big heat demand.

 
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Post by franpipeman » Tue. Mar. 08, 2022 8:28 am

In earlier control days it was difficult to keep exactly one degree of temperature , Many times the equipment could only run at 100 per cent or off. That would create a situation known as short cycling, with start up being a hardship on the equipment the equipment would be in a constant state of off on off on off on Solid fuel is even more demanding to be fired in a variable rate.
In most of todays non solid fuel equipment and with the development of variable frequency controllers for firing rates, pump rate they can enable staying on 1 degree by having a very wide variable firing rate of flame in a boiler , or a pump pumping , or a ac compressor and condenser fan valuing its load. Some time it was called dead zone, where no action would occur to enable the device to run longer when demand called for it rather than off on off on.

 
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Post by McGiever » Tue. Mar. 08, 2022 9:27 am

Lee, there can be no such thing as a undesired overshoot in temp if there is capacity for that very thing in storage.
Storage acts as a battery being charged by a random trinkle charger only to be discharged at another more convenient or desired amount and time.

Naysayers would be quick to point out standby heat loss and they’d be right but it’s hard to only have a perfect world for us all to live in.
Sometimes it ends up being only the best we are capable of doing rather than perfect. 😀

 
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Post by Lightning » Tue. Mar. 08, 2022 7:43 pm

coaledsweat wrote:
Tue. Mar. 08, 2022 8:15 am
Is your boiler a little oversized perhaps? My only concern is how low the boiler dips on a big heat demand.
That's debatable, I mean sometimes it is, and sometimes it isn't. During January and February I ran 4200+ pounds of coal thru it. If you average that out to BTUs per hour it works out to about 37,500 BTUs per hour of input for a boiler that is capable of 130,000 BTUs per hour.. Looking at it that way it seems oversized. But when you factor in on demand DHW there could be times that the boiler needs to deliver close to 100,000 BTUs per hour for short periods of time to heat DHW and keep the house warm on a cold day.. So in that respect, then it seems to fit the bill pretty good. Since running the tight differential the boiler dips aren't as far as they used to be. I don't see it fall below 150 degrees very often. In my opinion its because the fire is in a better state of health when a high heat demand comes around.
franpipeman wrote:
Tue. Mar. 08, 2022 8:28 am
with start up being a hardship on the equipment the equipment would be in a constant state of off on off on off
I see.. that makes sense. With other fuels its like gas, propane and fuel oil its full blast or zero. But with this type of coal appliance there is a degree of latency (delay) involved. So I think that's where the difference is (more on that later)..
McGiever wrote:
Tue. Mar. 08, 2022 9:27 am
Lee, there can be no such thing as a undesired overshoot in temp if there is capacity for that very thing in storage.
Storage acts as a battery being charged by a random trinkle charger only to be discharged at another more convenient or desired amount and time.
I totally agree :yes: .. The over shoots don't bother me too much for the reasons you posted.


I'm gonna take a crack at this. I mentioned latency earlier and wish to elaborate on that. So, with these units you have roughly a 30 pound mass in the burn chamber at all times. During a heat call from the thermostat or DHW the boiler temp starts to fall, the combustion fan kicks on at 164.5 then there is a period of time for that 30 pound mass to get hot and start heating the boiler.. Meanwhile the boiler temp continues to fall to the 150s.. Then as the boiler starts to heat, it gets to 165.5 which is the combustion fan off temp. Now that 30 pound mass is extremely hot and continues to radiate heat till it cools down sending the boiler temp into the lower 170s, sometimes higher depending on how severe the previous heat demand was. So there is your 15-20 degree differential in boiler water temperature that coaledsweat mentioned. The differential is caused by latency on both sides of the 1 degree differential that the PID is set at. More differential set on the PID is just added to the boiler water differential caused by latency.

 
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Post by Retro_Origin » Tue. Mar. 08, 2022 8:03 pm

It would seem to me this whole discussion really revolves around the fuel type, since it's coal and that takes a given amount of time to get to efficient combustion performance, that's actually what the discussion is about. Maintaining it as close to efficiency at any given time.

Like in the summer when mine pretty much idles (via the timer) the coal all the way off the grate, that's all inefficiently burned stuff. To me it would seem keeping the whole unit peppy and hot (at the risk of 'losing radiant heat') still is more efficient because you are getting "everything" in the energy source that is there. As I've mentioned before (with my extremely extensive knowledge and experience :roll: :roll: :roll: :roll: ) I think the pot style is the most favorable for various load types since the bed size increases or decreases exponentially rather than linearly like a flat grate. A small hot fire (low load pot style) is better than a large thin/cool fire (flat grate). In other words with a flat grate at 1 minute of stoking my bed might be 2"x8" after a few more it might grow to 3"x8" then after near an hour 5"x8", that's not very much growth square inchwise per time. Unless I'm mistaken the pot style grows the diameter (not just length) of the bed allowing for a faster ramp up time to performance, and better 'small hot fire' capabilites.
This is my limited opinion, I'd be happy to be corrected.

Lee, am I right in saying you are trying to maintain " a controlled hot fire" instead of a 'given boiler temperature'. I would think given the 'latency' concept you talked about we are more at the mercy of how fast we can ramp up or down the boiler, we're going to get differential regardless since the delayed response time is so great. Finer settings (like what your doing) would mean finer control over the delay. Example, if you drop a payload of munitions at 30,000 feet, you better have your calculations for when it lands a few minutes later...if you fly 10 feet above the ground...screw the calculations, you have no delay (or plane for that matter! Kapow!)

 
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Post by oros35 » Wed. Mar. 09, 2022 9:25 am

Running my outdoor boiler on bit at 1 degree. Works great. I'm sure every system is different. My system at 1 degree diff usually overshoots by about 3-5 degrees after the blower shuts off. Seems to work the best there.


 
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Post by Lightning » Wed. Mar. 09, 2022 10:44 am

Retro_Origin wrote:
Tue. Mar. 08, 2022 8:03 pm
Lee, am I right in saying you are trying to maintain " a controlled hot fire" instead of a 'given boiler temperature'.
Sure.. my intention is to maintain the health of the fire between combustion fan cycles during lower heat demanding times. The only way I can see that possible is to run the combustion fan more often. This can be achieved by running a keep fire timer (which I would like to avoid) or by tightening the boiler differential which of course is what I'm experimenting with.

The advantage of keeping the fire in good health is that it will ramp up quicker and prevent the boiler temperature from going too low. This should help the boiler recover from a high heat demand scenario like a long shower.

Another advantage is to prevent outfires during shoulder month and summertime use for DHW. This remains to be seen but so far looks very promising. On the 70 degree day we had a few days ago the boiler fell into a nice little routine of firing for 2-3 minutes, 2-3 times per hour and the water temp stayed between 164-170 degrees.

The possibility of an outfire would be right after a long DHW call. This is where the boiler would reach full output heating capacity getting that 30 pound mass in the fire pot extremely hot. This will cause the boiler to overshoot into the 190s. Then it will take a couple few hours for the boiler to lose its standby heat and hopefully not lose the fire in the meantime.
oros35 wrote:
Wed. Mar. 09, 2022 9:25 am
Running my outdoor boiler on bit at 1 degree. Works great. I'm sure every system is different. My system at 1 degree diff usually overshoots by about 3-5 degrees after the blower shuts off. Seems to work the best there.
Cool!

 
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Post by coaledsweat » Wed. Mar. 09, 2022 10:53 am

I run my Axeman without a timer. The timer died on me twice and I had enough of it. An Axeman can go 2 or 3 days without a run and still come back. I've done that many times.

 
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Post by Rob R. » Wed. Mar. 09, 2022 10:58 am

coaledsweat wrote:
Wed. Mar. 09, 2022 10:53 am
An Axeman can go 2 or 3 days without a run and still come back.
That is true for the 260. There are plenty of examples of outfitted with the 130.

 
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Post by Lightning » Wed. Mar. 09, 2022 11:10 am

Rob R. wrote:
Wed. Mar. 09, 2022 10:58 am
That is true for the 260. There are plenty of examples of outfires with the 130.
True.. back in September I lit the Axe for the little bit of heating we needed and for DHW. I was having outfires quite frequently.

 
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Post by Retro_Origin » Wed. Mar. 09, 2022 8:30 pm

coaledsweat wrote:
Wed. Mar. 09, 2022 10:53 am
I run my Axeman without a timer. The timer died on me twice and I had enough of it. An Axeman can go 2 or 3 days without a run and still come back. I've done that many times.
We all agree it can go without a timer but why do they advise having one? Do you think it's to prevent the fire from getting too low? They don't seem to recommend things that don't make sense...

 
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Post by coaledsweat » Wed. Mar. 09, 2022 8:55 pm

It's for summertime DHW use.

 
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Post by Retro_Origin » Wed. Mar. 09, 2022 10:03 pm

coaledsweat wrote:
Wed. Mar. 09, 2022 8:55 pm
It's for summertime DHW use.
Not saying 'you're wrong' but that doesn't answer my question. The timer is a 'fire stimulant', nothing more, correct?

If the boiler won't go out for 2 days ( to be conservative) without the timer then it's function is pointless unless it's for maintaining a fire condition.

The boiler doesn't act differently when it gets a heat call or transfer to the domestic coil, so to say the timer is for DHW makes little sense. The boiler will make an effort to hold a temp regardless of how it loses it's contained heat. I think we all know how quickly the domestic can pull the boiler temp down, the timer really isn't going to satisfy that, especially at "1 minute per hour" that AA just told me when I called them on Friday...the boiler is going to have to run/stoke whether it's supplying DHW or Hydronic heat at least once a day so why would I need a timer?

My point isn't to say 'you're wrong' but rather I think they're advising usage of a timer to maintain a fresher fire to prevent overshoots/long recovery times. I could be wrong but seems pretty straightforward to me.

 
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Post by coaledsweat » Thu. Mar. 10, 2022 7:46 am

The fact that it doesn't die in two days doesn't keep the fire in a sweet spot, where you want it to be. This is important during summertime when DHW is the only call for heat.


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