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Old 01-15-2012, 09:42 AM   #1
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DHW From Coil, when does boiler fire?


I originally posted this in the plumbing forum, but realized it is better suited here.

This has always had me a little confused, so I would like to hear from the experts. Even though I get DHW from the coil, the boiler is NOT keeping the DHW hot, UNTIL someone calls for it (i.e. turns on a HW faucet). Is that correct? My (admittedly limited) understanding is it doesn't matter what the aquatstat is set to, it will NOT cause the boiler to run until there is a call for heat. If I never used any DHW during the summer, my boiler would stay silent, yes? (assuming it's 80 degrees all summer, so I don't need any baseboard heat in the house).

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Old 01-15-2012, 09:59 AM   #2
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DHW From Coil, when does boiler fire?


Quote:
Originally Posted by dougp23 View Post
I originally posted this in the plumbing forum, but realized it is better suited here.

This has always had me a little confused, so I would like to hear from the experts. Even though I get DHW from the coil, the boiler is NOT keeping the DHW hot, UNTIL someone calls for it (i.e. turns on a HW faucet). Is that correct? My (admittedly limited) understanding is it doesn't matter what the aquatstat is set to, it will NOT cause the boiler to run until there is a call for heat. If I never used any DHW during the summer, my boiler would stay silent, yes? (assuming it's 80 degrees all summer, so I don't need any baseboard heat in the house).
If it's like my setup, an aquastat keep keeps the boiler between a certain temp. My understanding is that with a typical DHW coil and boiler you have a high/low/diff. aquastat.

the hi temp is when the burner stops while heat is called for
the lo temp is when the burner starts when the heat is NOT being called for
the diff temp is the difference from low that the controller waits to lock out the circulators.

So in my case in the summer, or whenever no heat is called for, the aquastat allows the boiler to drop to 180*... at that point the burner turns back on to heat things up...

In my setup there is no DIRECT link between the DHW use and the burner firing... there is an INDIRECT link in the form of the boiler water temp dropping (as DHW is used) and the aquastat turning on the burner.

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Old 01-15-2012, 11:13 AM   #3
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DHW From Coil, when does boiler fire?


This was my understanding too, until I was told, that NO, these are ON DEMAND boilers. The only time they run is when there is a demand. And it seems to be true.

This morning I went into the basement, and my boiler said 138 degrees. Now that is way below the low value, and I get DHW off the coil. Once I started the shower, the boiler kicked on, and ran until the water was about 195. It has not run since.

Again, I hope some experts will weigh in!
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Old 01-15-2012, 11:27 AM   #4
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DHW From Coil, when does boiler fire?


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Once I started the shower, the boiler kicked on, and ran...
Is there anything (like a flow switch) on your DHW water pipe?
something that is telling the aquastat to have the burner fire?
(and maybe the circulators not to)

Quote:
until the water was about 195...
That w/should be the hi limit kicking in
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Old 01-15-2012, 11:27 AM   #5
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DHW From Coil, when does boiler fire?


Post brand and model number of the boiler.
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Old 01-15-2012, 11:34 AM   #6
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DHW From Coil, when does boiler fire?


What model boiler do you have?

All the DHW coils I've seen have been installed as bubbler said. The boiler always stays hot, not necessarily 180, but hot.

What are the settings on your aquastat?
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Old 01-15-2012, 11:36 AM   #7
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DHW From Coil, when does boiler fire?


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What model boiler do you have?

All the DHW coils I've seen have been installed as bubbler said. The boiler always stays hot, not necessarily 180, but hot.

What are the settings on your aquastat?
There are tankless gas boilers/water heaters, that don't maintain any temp unless there is a heat call, or a call for domestic hot water.
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Old 01-15-2012, 12:17 PM   #8
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DHW From Coil, when does boiler fire?


Thanks everyone!

It's a Weil-McLain WTGO-3 with a Beckett burner.

The power comes in, hit's the safety switch. One line goes to the aquastat, one line goes to the circulator, and one line goes to the burner. A Taco controller gets power before the safety switch for 3 zones.

So the DHW does not communicate with the Taco, it must be tied in somehow to the aquastat.

I do have a mixer valve on the DHW line, it's a Honeywell AM1 series.

On this system, the aquastat is very electronic, with an LED display and all, so I don't know what the hi and lo ranges are.

On my other home's system, I can initiate a call for heat. If the boiler is cold, it will fire. If I turn the tsar down, the boiler shuts right off, even though it hasn't reached operating values. So that seems to be on-demand.
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Old 01-15-2012, 12:26 PM   #9
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DHW From Coil, when does boiler fire?


Ok. It has a tankless coil in it. And it does indeed run at times to maintain the boilers water temp, even when you aren't using hot water. The boiler itself holds 14.9 gallons of water. And the tankless coil is surrounded by this water.
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Old 01-15-2012, 01:50 PM   #10
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DHW From Coil, when does boiler fire?


Quote:
Originally Posted by dougp23 View Post
I originally posted this in the plumbing forum, but realized it is better suited here.

This has always had me a little confused, so I would like to hear from the experts. Even though I get DHW from the coil, the boiler is NOT keeping the DHW hot, UNTIL someone calls for it (i.e. turns on a HW faucet). Is that correct? My (admittedly limited) understanding is it doesn't matter what the aquatstat is set to, it will NOT cause the boiler to run until there is a call for heat. If I never used any DHW during the summer, my boiler would stay silent, yes? (assuming it's 80 degrees all summer, so I don't need any baseboard heat in the house).
A boiler that heats domestic hot water using a "water back" or tankless coil cannot remain silent during the summer until someone turns on a hot faucet. It would then be too slow to provide domestic hot water.

If the boiler temperature is not kept at at least 120 degrees then something is malfunctioning or out of adjustment.

The aquastat (immersion thermostat) can and should operate independently of hot water faucet usage and room thermostat calls for heat except for selecting its low and high range for winter versus summer/..
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Last edited by AllanJ; 01-15-2012 at 01:58 PM.
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Old 01-15-2012, 01:55 PM   #11
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DHW From Coil, when does boiler fire?


Thanks Allan.

I think I had a little confusion with my Mom's system, where her DHW is off a propane HW heater. Her boiler can remain silent all summer since none of her zones will call for heat. I am told this is bad for her boiler, since all that water just sits in her boiler never moving, causing corrosion and all sorts of bad stuff.

I think this AM when I started the shower, the aquastat was prob at it's low range, so it fired off. It had nothing to do with me turning on the faucet!

Thanks again everyone, I am clear now on how that all operates!
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Old 01-15-2012, 03:42 PM   #12
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DHW From Coil, when does boiler fire?


Quote:
Originally Posted by dougp23 View Post
Thanks Allan.

I think I had a little confusion with my Mom's system, where her DHW is off a propane HW heater. Her boiler can remain silent all summer since none of her zones will call for heat. I am told this is bad for her boiler, since all that water just sits in her boiler never moving, causing corrosion and all sorts of bad stuff.

I think this AM when I started the shower, the aquastat was prob at it's low range, so it fired off. It had nothing to do with me turning on the faucet!

Thanks again everyone, I am clear now on how that all operates!
Not bad for the boiler at all. No corrosion can happen, unless there is air in the boiler.

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