Hi & Low Limit Setting - Oil Boiler, Forced HW - Cast Iron Radiators - HVAC - Page 2 - DIY Chatroom Home Improvement Forum


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Old 12-09-2009, 09:30 AM   #16
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Hi & Low Limit setting - Oil boiler, forced HW - Cast Iron radiators


I have it set to 170 right now, it actually goes to 180 on the dial
Diff set to 20, low set to 140 I think
We have some cold weather coming up starting tomorrow nite
So far everything is working fine
Radiators are not as hot to the touch...which is good with my son around

I think the old settings (diff & low limit) were there from when the HW loop was in use

*edit* low was set to 130...just turned down to 120

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Old 12-09-2009, 09:39 AM   #17
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Hi & Low Limit setting - Oil boiler, forced HW - Cast Iron radiators


Drop that low side also. It has nothing to do with the heat. The high limit is the only thing to do with heating. The low side is nicknamed Fuel wasted setting. To maintain any temp at all without using the hot water coil is a waste of money. Cold start is the way to go unless you have one of those energy wasting, fuel guzzling, ridiculous idea, why are they still purchased domestic hot water coils in the boiler.
Here is a link on how that control works
http://www.comfort-calc.net/Service/Domestic_Coils.html

Last edited by tk03; 12-09-2009 at 09:46 AM.
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Old 12-09-2009, 09:58 AM   #18
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Hi & Low Limit setting - Oil boiler, forced HW - Cast Iron radiators


Thanks, that explains it better then the site I found
I did just drop the low limit down to 120 - lowest setting
Am I correct in thinking that as long as heat is not called for, the Temp will drop & stay at the low limit setting (+/- diff)

Or when it drops below the low limit it will then fire & heat the boiler back up to the high limit setting? Again - when not calling for heat

I'm thinking & hoping it will drop & stay at the low limit
That should say save some fuel, especially at nite & when the heat is not on
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Old 12-09-2009, 10:26 AM   #19
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Hi & Low Limit setting - Oil boiler, forced HW - Cast Iron radiators


Unless you have a domestic hot water coil in the boiler, the boiler should shut completely off when not calling for heat.
The min temp for conventional oil boilers is 120F (130F for NG). Lower than that you'll get soot and condensate and possibly ruin the boiler. When calling for heat the boiler will fire until it hits your high set point (160) and cool with the zone pumps pumping until it hits your low set point (120) then it will fire again. It will keep cycling until your room temp hits the Tstat setting.
I would set it to run as cold as possible to heat the house adequately on a 6F (in the Boston area) day.
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Old 12-09-2009, 10:30 AM   #20
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Hi & Low Limit setting - Oil boiler, forced HW - Cast Iron radiators


The high limit will be met on a call for heat only. It is not used until the thermostat calls for heat. The high limit has a built in 10 differential.

The low limit is what it is maintaining as you said +/- differential. Set at 120 the boiler should run between 110 and 130. You may see a small creep upward right after burner shutdown.
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Old 12-09-2009, 10:32 AM   #21
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Hi & Low Limit setting - Oil boiler, forced HW - Cast Iron radiators


OK, great
This should definitely save some Oil
No sense keeping it at 160-190 all the time - prior setting
Wish I had found these settings/info years ago
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Old 12-09-2009, 10:36 AM   #22
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Hi & Low Limit setting - Oil boiler, forced HW - Cast Iron radiators


Quote:
Originally Posted by tk03 View Post
The high limit will be met on a call for heat only. It is not used until the thermostat calls for heat. The high limit has a built in 10 differential.

The low limit is what it is maintaining as you said +/- differential. Set at 120 the boiler should run between 110 and 130. You may see a small creep upward right after burner shutdown.
You sure about that? My old boiler completely shut down when not calling for heat. I guess ScubaD could just watch it and see what it does.
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Old 12-09-2009, 10:46 AM   #23
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Hi & Low Limit setting - Oil boiler, forced HW - Cast Iron radiators


I'll see what it does, I know its been going down to 130/140 - what I had it set to the past few days
Usually it would call for heat before it dropped down to that Temp

I just made some weather stripping out of duct tape for the temporary door to the great room
Big difference, hoping that will help keep the sunroom warmer
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Old 12-09-2009, 01:03 PM   #24
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Hi & Low Limit setting - Oil boiler, forced HW - Cast Iron radiators


Depending on the BTU input. And piping set you have. Converting it to t a cold start boiler. Would be a mistake.

Becarefull of low limit setting being too low. With most of the water being in the rads. If the boiler can't raise the water temp fast enough. You will get condensation in the boiler flue passages. And it will gunk up quickly, and you will loose efficiency, and can clog up the boiler.

Most non condensing boilers want a return water temp of 120 or 130.
With the low limit set to 120, you may not get a return water temp of 120 on many heat calls during the milder temps.

If your boiler has a tempering loop piped in. Its not a problem. If it doesn't, it can be a big problem.

Also. If you have an indirect. Sometimes with the low limit defeated, or set too low. You will cool the indirect down because the boiler will absorb the heat from the indirect. And you will notice this while taking a shower(unless your a canuck, then any water temp above 70 feels hot ).

If you don't get your boiler cleaned every year. You don't want to set it to 120. As that will make the soot pack on the boiler harder.
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Old 12-09-2009, 03:56 PM   #25
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Hi & Low Limit setting - Oil boiler, forced HW - Cast Iron radiators


No HW feed from the boiler that was removed
We do have it cleaned every year, PO (1.5 years) did not & possibly person who owned it before them
We moved in Nov '03 & had it cleaned that Spring - had not been cleaned in a while per our Oil guy
We do use 1/3 of the oil that we used to...not sure how much of a difference that makes on cleaning ?
Less oil used = less dirty ?

Radiator Temp has always dropped to room Temp (cool to the touch) before the heat came on again
It can be 90 minutes or longer between heat cycles, depending upon the weather

This boiler (1988) is on its last legs per our oil guy - who has been great
I wanted the addition built & installed before we bought a new boiler
We need another zone for the addition
We do not have gas to the house

Thanks again for the input
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Old 01-06-2010, 03:59 PM   #26
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Hi & Low Limit setting - Oil boiler, forced HW - Cast Iron radiators


In our industry we think the 130 return temp is set in stone. I don't have a problem with bringing 100 water back all day long as long as the average water temp gets above 135 - 140. To quote the great Gil Carlson from B&G, the father of boiler protection and p/s piping. Flue gas condensation is a result of extremely cold water entering the boiler or cool water at a high flow rate. The flow rate can be changed to compensate for the lower return temp. See the link below for maybe a better explanation.
http://comfortcalc.11.forumer.com/viewtopic.php?t=22
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Old 01-06-2010, 04:21 PM   #27
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Hi & Low Limit setting - Oil boiler, forced HW - Cast Iron radiators


He would need to redo piping, or reduce his GPM to get a 140 average temp if 120 was is average return water temp.

Slowing the circ may not give enough water flow.
Redoing piping can cost more then it would save him.
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Old 01-06-2010, 04:26 PM   #28
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Hi & Low Limit setting - Oil boiler, forced HW - Cast Iron radiators


Just throw in a simple boiler bypass pipe and a valve between the bypass and the boiler.
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Old 01-06-2010, 04:33 PM   #29
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Hi & Low Limit setting - Oil boiler, forced HW - Cast Iron radiators


I'm not sure what the bypass accomplishes ?
Since the water coming back from the radiators is at room temp - 60-70
It's been that way ever since I completed renovations, new windows, insulation
Heat comes on...reaches temp...stays off for 1 hour to 2 hours (or longer) depending upon outside Temp
On days when I am home alone the temp goes to 68/69 in the AM
Then set point goes to 65 & it may be 2-4 hours before the heat drops to 64 & the heat comes on again

Arte you saying once the heat is running & circulating the return water Temp should be ~100?
That will not happen either until the cast iron radiators are saturated
Once saturated the temp thru the entire system is stable until the heat goes off

So....I'm confused
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Old 01-06-2010, 04:36 PM   #30
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Hi & Low Limit setting - Oil boiler, forced HW - Cast Iron radiators


Quote:
Originally Posted by tk03 View Post
Just throw in a simple boiler bypass pipe and a valve between the bypass and the boiler.
Lots of work, for a little savings.

Ok to do with a mixing valve on a boiler install. But changing over an existing system. Long time for pay back.
And most home owners are going to forget to shut the valve whenit gets cold out.

Easier just to leave the aquastat set up 10 F higher.

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