# Hi & Low Limit setting - Oil boiler, forced HW - Cast Iron radiators



## Scuba_Dave (Jan 16, 2009)

It's a Honeywell controller, circuit board stamped 8805
The Diff control was set to 10
There is a HW loop, but it is not in use - capped off before we bought the house

I'm also wondering if it's better to use the Oil boiler HW loop VS electric in the winter while the heat is running ?
Then shut it off & use electric the rest of the year ?


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## beenthere (Oct 11, 2008)

Your cast iron rads shouldn't need more then 160° water, if they are sized to the homes heat loss.

If the boiler isn't providing domestic hot water, I would set the low limit to 130, and the high to 160.

Actually. If it were my boiler. I would convert the control to make the boiler a cold start boiler, and set the high limit to 140, and see it that is enough to heat the house.

I use to work with a guy that had his water temp set to 110. His rads were so oversized that he didn't need any hotter water to heat his house.

Using the boiler for domestic in the winter. Just makes you use more oil then need be.


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## Scuba_Dave (Jan 16, 2009)

Thanks
I turned it down to 180 last nite
I'ts about 24 out & heat is fine
I'm going to run it today & check the low Temp

Radiatiors are still plenty hot
So I will probably be turning it down to 160
On warmer (50) days heat would turn on & not come back on for about 90 minutes
And the radiatiors are still slightly warm 90 minutes later


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## Scuba_Dave (Jan 16, 2009)

It was still going up as high as 190 with setting at 180
So I have turned it down to 170
But for now the heat is off as I have a fire going in the fireplace
It's up to 70 already....I like
I need to burn off all the wood this year as I have a large dead Oak that will be coming down in the Spring


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## beenthere (Oct 11, 2008)

So your hoping for a cold winter.


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## Scuba_Dave (Jan 16, 2009)

Hopefully not !!
I've already burned off over 1/3 of what I had
A lot was old Maple - burns very quickly & not a ton of heat
I burn that during the day & save the Oak for at nite when its colder
With burning wood I didn't turn the heat on until the day before Thanksgiving


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## beenthere (Oct 11, 2008)

Wow. A 1/3 in a week and a half.

Better have a lot of oak, ash or locust.


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## Scuba_Dave (Jan 16, 2009)

No, I started burning wood the beginning of Oct, so about 8 weeks
Usually we have the heat on sooner


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## beenthere (Oct 11, 2008)

Ooops. I took it as you didn't use any heat until the day before TG.


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## Scuba_Dave (Jan 16, 2009)

Oh...no...my bad - poor explanation
Fireplace grate heater is used in fall up until it gets really cold
Then we turn the oil heat on
Then in late winter when it warms up I switch back to wood again
I just hate running the oil boiler for a day or two - then we get 3 days of warm weather...back & forth in fall & spring

So I try to heat as much as I can with wood until it's cold enough to just leave the oil boiler going


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## beenthere (Oct 11, 2008)

Yeah. I hate when I hear mine kick on.
One of these days I'm going to get an insert like yours.


Been saying that for 10 years I think.


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## Scuba_Dave (Jan 16, 2009)

I lucked out & bought it off Craigslist brand new for less then 1/2 price
Wife was sort of like...what did you get now ??
But she likes it to keep the main room warmer
The other night she even asked why I didn't have a fire going
I didn't take the hint :laughing:


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## beenthere (Oct 11, 2008)

LOL... 

You will if she ask tonight. Or she'll make life tough on ya.


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## tk03 (Sep 30, 2008)

I would turn the control low side down as low as the lowest number and set the diff to 20. This will give you longer run times and longer off times which is better for the boiler. Work the high limit down as low as you can but don't go below 160º.


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## Scuba_Dave (Jan 16, 2009)

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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## tk03 (Sep 30, 2008)

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


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## Scuba_Dave (Jan 16, 2009)

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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## Clutchcargo (Mar 31, 2007)

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 120°F (130°F 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 6°F (in the Boston area) day.


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## tk03 (Sep 30, 2008)

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 10º differential.

The low limit is what it is maintaining as you said +/- differential. Set at 120º the boiler should run between 110º and 130º. You may see a small creep upward right after burner shutdown.


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## Scuba_Dave (Jan 16, 2009)

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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## Clutchcargo (Mar 31, 2007)

tk03 said:


> 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 10º differential.
> 
> The low limit is what it is maintaining as you said +/- differential. Set at 120º the boiler should run between 110º and 130º. 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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## Scuba_Dave (Jan 16, 2009)

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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## beenthere (Oct 11, 2008)

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 :laughing.

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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## Scuba_Dave (Jan 16, 2009)

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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## tk03 (Sep 30, 2008)

In our industry we think the 130º return temp is set in stone. I don't have a problem with bringing 100º water back all day long as long as the average water temp gets above 135º - 140º. 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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## beenthere (Oct 11, 2008)

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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## tk03 (Sep 30, 2008)

Just throw in a simple boiler bypass pipe and a valve between the bypass and the boiler.


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## Scuba_Dave (Jan 16, 2009)

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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## beenthere (Oct 11, 2008)

tk03 said:


> 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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## beenthere (Oct 11, 2008)

Scuba_Dave said:


> So....I'm confused


Low return water temps for prolonged periods can cause several different problems.
most common. Is the boiler gunks up with damp soot.


Lots of cold fire boilers are set up like yours.
Can be a little harder to clean.


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## Scuba_Dave (Jan 16, 2009)

Ah, OK
We have gone from 3 tanks of oil the 1st year down to 1 to 1.5 tanks of oil since then
I think as a result its been much easier for our Oil guy to clean
I know he's surprised at how little oil we use
At 1st maybe he thought we were filling up from someone else
But we found him the 1st year & have stuck with the same guy


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## tk03 (Sep 30, 2008)

I am confused what valve needs to be shut. When adjusted always stays the same. What expense? two tees some pipe and a valve? Saves about 9 - 12% in large water volume systems. Nothing to sneeze at.
Boiler protection
Increased comfort
Fuel savings
Where are the negatives?
See link
http://www.comfort-calc.net/Bypass_Piping_Explaination.html


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## Scuba_Dave (Jan 16, 2009)

I see what you mean now.....but 

From the site:


> *The savings is minimal*


and my boiler is going on 23 years old
So I don't see a problem


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## beenthere (Oct 11, 2008)

Going from a 130 return temp to a 120 return water temp won't save much on a residential system.

As a DIY change, cost is minimal. For those that have to pay someone, it is costly.

While a boiler bypass will maintain proper flow through the system.
Slowing water flow to .5GPM through the boiler doesn't not give full BTU to the house, and can cause heat problems. With boilers that are sized to the load of the house. In which case the bypass valve must be shut to get the full BTU to the house.

If the boiler is oversized. It won't have a problem.


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## tk03 (Sep 30, 2008)

You could not reduce the flow to .5 gpm through the boiler or the boiler would grossly short cycle. But let's not confuse gpm flow with btu's. The btu into the home is going to stay the same. If the boiler is 80K for instance and the flow is 100 gpm or 5 gpm there is still 80k btu's to the home. The more gpm going through the boiler the lower the delta T. More gpm means less btu;s per gallon. Reduce the flow and that means more btu's per gallon thus higher water temperature. There are three thins to think about in hydronic heating, they are BTU's, gpm and water temperature.
You can change the flow through the boiler to achieve a different delta T but the btu's to the system do not change. The output from the radiation may change as water temp will change.


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## beenthere (Oct 11, 2008)

Actually. Changing the GPM through the boiler can lower or increase the BTU delivery to the house.

Low water flow through the boiler will cause a higher stack temp(the higher the stack temp, the more BTUs that are in the combusted gasses). This is because not as many BTUs are being absorbed by the water.

Too high of a water flow, and the water isn't in the boiler long enough to pick up the heat/BTUs.

The .5GPM I refered to in my other post. Was to have been typed as .5GPM per 10,000 BTUs of boiler rated output. This would be a 40° rise through the boiler(on paper).


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## tk03 (Sep 30, 2008)

Your analogy is correct but minimal change in btu's not going into the home as compared to need for boiler protection. The flow through the boiler can be whatever it is but the btu to the home is the same other than the small amount lost in the vent.


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## beenthere (Oct 11, 2008)

Whats minimal will remain debatable.

Lowering allowable/safe entering water temp by 10° is only a 7% savings(on paper).
So if you lose 4%(just a % picked out of the air) heat transfer efficiency in the boiler. You don't really get much of a savings from the lower entering water temp.


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## tk03 (Sep 30, 2008)

> You don't really get much of a savings from the lower entering water temp


. 
The greatest temperature transfer is at the greatest temperature difference. So the cooler return water will allow a greater temperate transfer from fireside to water side. The boiler bypass in testing saves approx 9 - 11% dependent on water volume. Decrease water temperature and increase fuel savings. Heat less water as all the gallons in the system are not going through the boiler.


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## beenthere (Oct 11, 2008)

So your saying that you get greater savings(9 to 11%) by having a 10°(120) cooler entering water temp with a 10°(160) hotter leaving water temp.
Then with a 10°(130) warmer entering temp with a 10°(150) cooler leaving water temp. 

If your getting that savings. The savings might be from a more even heat through out the house. From having the lower water temp at the beginning of the heat call. And the customer is turning the thermostat down a ° or 2. But not from a boiler efficiency improvement alone.

What is your combustion efficiency improvement on these units that are saving 9 to 11%?


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