Domestic Hot Water Priority

 
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Lightning
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Post by Lightning » Sat. Feb. 04, 2023 5:13 pm

At some point in the near future I may incorporate DHW priority with my setup. My goal would be to shut off the circulator (I only use one circulator) while the hot water is running so that the boiler is only doing one or the other. Meaning it's either heating DHW or it's heating the house and outbuilding, with priority on the DHW.

Could anyone give some details on how to do that?

Would I just have the circ shut off if the boiler went below a specific temperature?


 
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Post by hotblast1357 » Sat. Feb. 04, 2023 9:07 pm

How are you making DHW? Plate exchanger?

And I have to ask, why? Axeman can’t keep up?

 
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Post by Retro_Origin » Sat. Feb. 04, 2023 9:29 pm

A standard setup with a triple aquastat runs the circulator thru the c1 and c2 terminals so that the domestic takes priority over the heating system by not powering the circulator unless low limit is met

 
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Post by Lightning » Sat. Feb. 04, 2023 10:43 pm

hotblast1357 wrote:
Sat. Feb. 04, 2023 9:07 pm
How are you making DHW? Plate exchanger?

And I have to ask, why? Axeman can’t keep up?
I'm using the internal DHW coil in the boiler. Yes the Axe keeps up just fine, but if I slow the fan down I'll want to split up the duty.

 
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Post by hotblast1357 » Sun. Feb. 05, 2023 8:08 am

Lightning wrote:
Sat. Feb. 04, 2023 10:43 pm
I'm using the internal DHW coil in the boiler. Yes the Axe keeps up just fine, but if I slow the fan down I'll want to split up the duty.
easiest way would be to mount a strap on aqastat onto your DHW line, either cold in or hot out, and have it cut power to your main circulator. I couldn't imagine giving DHW priority, in my house keeping it warm is more important than a hot shower at that moment.

 
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Post by franco b » Sun. Feb. 05, 2023 9:03 am

Reverse aquastat. Breaks on temperature fall. Honeywell B.

 
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Post by Lightning » Sun. Feb. 05, 2023 9:47 am

hotblast1357 wrote:
Sun. Feb. 05, 2023 8:08 am
easiest way would be to mount a strap on aqastat onto your DHW line, either cold in or hot out, and have it cut power to your main circulator.
I ran that thru my thoughts. It would work to initiate a circ pause, but what happens when water stops flowing? It would sit in the pipe at it's current temp until ambient temperature influenced it, which would take way too much time.
hotblast1357 wrote:
Sun. Feb. 05, 2023 8:08 am
I couldn't imagine giving DHW priority, in my house keeping it warm is more important than a hot shower at that moment.
DHW use is relatively short bursts with the exception of a kid taking a 30 minute shower, so it might not be hugely noticable in the house heating, or maybe it would, I don't know for sure.
franco b wrote:
Sun. Feb. 05, 2023 9:03 am
Reverse aquastat. Breaks on temperature fall. Honeywell B.
I'm thinking that using another Inkbird to measure boiler temp would be easy. I'd have it cut the circ if it went below a certain temp, like 150 for example so that the boiler could catch up.


 
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Post by hotblast1357 » Sun. Feb. 05, 2023 9:59 am

I guess ill ask it here as it was overlooked in the other thread... this is all because you want to slow the combustion fan so that it runs longer... but currently you have the hysteria set to like 2 degrees or something crazy small so that the fan cycles a lot, why not just set it to 10-20 hysteria like "normal"? you fan will run longer and everything will be just fine.

 
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Post by Lightning » Sun. Feb. 05, 2023 10:02 am

If I were to assume on demand water heating at a flow rate of 2 gallons per minute, is the following calculation correct?

8.34 pounds per gallon x 2 gallons per minute x 75 degrees (delta 50 to 125 degrees) x 60 minutes = 75,060 BTU per hour

I need 75,060 BTU per hour minimum for 2 gallons of 125 degree DHW per minute?

Did I do that right?

 
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Post by Lightning » Sun. Feb. 05, 2023 10:11 am

hotblast1357 wrote:
Sun. Feb. 05, 2023 9:59 am
I guess ill ask it here as it was overlooked in the other thread... this is all because you want to slow the combustion fan so that it runs longer, but currently you have the hysteria set to like 2 degrees or something crazy small so that the fan cycles a lot, why not just set it to 10-20 hysteria like "normal"? you fan will run longer and everything will be just fine.
I intended to answer that but I get scatterbrained sometimes lol. It's not that I want the fan to run longer, but of course it will... . I'm very curious if better matching the boiler to the heat load will improve fuel usage. I have a lot of data for pounds of coal used per HDD. It averages 1.5 to 1.6 pounds per HDD most of the time during normal wintertime weather. If I see a dramatic drop or increase due to fan speed, I'll know it was a contributing factor. Some of the things I do are for learning and just general curiosity. :ugeek:

 
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Post by Lightning » Sun. Feb. 05, 2023 10:14 am

So if I need 75,000 BTU per hour minimum for DHW, and I have the boiler doing one or the other (home heating being the other). I could theoretically cut the fan speed to the point that the boiler produces 75,000 BTU per hour instead of 130,000.

 
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Post by Lightning » Sun. Feb. 05, 2023 10:29 am

On my coldest day I'll use around 100 pounds of coal. That is less than half of the Axe's current capacity. A lot of wiggle room there to slow it down.

But if I want to go under 75,000 BTU per hour output I'll need to spread out the DHW heating. That would be the next bridge to cross after this one.

 
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Post by hotblast1357 » Sun. Feb. 05, 2023 10:44 am

i would say you dont need anywhere near 75,000 BTU/H for heating needs, and for the DHW usage, correct you need around 75,000BTUs for one hour, the other 23 hours of the day you need nothing.

at 100 pounds of coal per day, thats only 37,500 BTU/H. 100X9,000 BTUs= 900,000 BTUs / 24 HRs is 37,500 btus per hour.

 
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Post by Lightning » Sun. Feb. 05, 2023 11:53 am

hotblast1357 wrote:
Sun. Feb. 05, 2023 10:44 am
correct you need around 75,000BTUs for one hour,
Yeah, that's what the boiler would need theoretically so it wouldn't fall behind. An easy work around would be to run the boiler a little hotter to store some heat as a buffer for DHW heating.

 
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Post by Rob R. » Mon. Feb. 06, 2023 7:15 am

Lee, is your outbuilding served by the circulator that would be checked off to DHW production?


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