minimum recommended low limit for warm-start boiler

Reply

  #1  
Old 02-24-10, 06:55 PM
T
Member
Thread Starter
Join Date: Feb 2010
Location: USA
Posts: 23
Received 0 Votes on 0 Posts
minimum recommended low limit for warm-start boiler

I moved into a house with a 22+ year old Weil Mclain boiler. I don't see any model number written on it but there are service tags from the oil company lying around that say it's model # 468.

I had the oil company disconnect the tankless coil and install an indirect-fired tank. They left my high limit at 180 and reduced the low limit to 110. The only reason they didn't eliminate the low limit entirely was because there is some rust on the jacket indicating that there was at one time a leak between the cast-iron sections. They thought this was the result of the boiler cooling down too much causing the gaskets to leak.

I have been reading about cold-start vs. warm-start and the problem of condensation destroying the boiler. I plan to have it cleaned annually.

Is it reasonable to have my low limit all the way down to 110 or will this dramatically reduce the longevity of my boiler?

Also, I bumped up both the low and high limit differentials from 10 to 20 degrees. Even on the coldest days my boiler doesn't seem to be running all the time (must be oversized) and I figured I'd run longer cycles and save fuel this way. Are there any downsides?
 
  #2  
Old 02-24-10, 07:24 PM
NJT's Avatar
NJT
NJT is offline
Member
Join Date: Oct 2006
Location: USA
Posts: 23,539
Received 4 Votes on 3 Posts
Presuming this is oil fired?

You wouldn't have to worry too much about setting the LOW down there because when the boiler is firing at it's low limit, there is no circulation in the system, thus you don't have the cool return water keeping the cast iron cool and promoting condensation.

You said BOTH the Low AND High differential? You could have a system with two diff dials, but most of the aquastats have only a single diff for the LOW setting. The HIGH diff is fixed... or do you have something like a 4081 aquastat ? Is there a model number on the aquastat? (or if you wanna take pics...)

I think some of the older W-M boilers did have problems with leaks between the sections... as I recall, they used O-rings, or gaskets, instead of 'push nipples', and they did have a tendency to leak if allowed to cool.

If you wish to post pics, you need to set up an account at a photo hosting site and upload the pics there and post a link for us to view ... Image hosting, free photo sharing & video sharing at Photobucket works well.
 
  #3  
Old 02-24-10, 09:32 PM
T
Member
Thread Starter
Join Date: Feb 2010
Location: USA
Posts: 23
Received 0 Votes on 0 Posts
Yes it is oil fired.

My old aquastat failed over the summer and I got a new Honeywell L7224U with separate low and high differentials which can each range from 5 to 20 degrees. They were both set to 10 initially.

Regarding condensation not being an issue:

My understanding is that the circulator will start up as soon as the thermostat calls for heat, regardless of the boiler temperature. At that point, if the boiler temperature drops (or is already) below the high-limit minus the high diff, the burner will fire until it reaches the high limit. At the end and beginning of the heating season when the load is low, won't the returns cool down to ambient and cause condensation when there is a call for heat. Similarly, won't I have the same issue in the summer when the boiler fires up to heat the indirect when I hit the shower in the morning or clean the dishes at night? Perhaps this is why it is recommended to clean your boiler annually.

As I think about it more, even if my low limit stayed at 160 like it did when I had the tankless coil, the boiler would still be shocked by a cold return on a loop that runs very infrequently (i.e. the indirect or a regular heating zone when outdoor temps are mild).
 
  #4  
Old 02-25-10, 04:01 PM
NJT's Avatar
NJT
NJT is offline
Member
Join Date: Oct 2006
Location: USA
Posts: 23,539
Received 4 Votes on 3 Posts
Mostly correct, but let me refine a few points...

When the aquastat has detected that the boiler has cooled below the LOW setpoint (-differential), the aquastat will turn on the BURNER, but NOT the circulator. So there is NO water returning at this time.

When either the heating t'stat, or the indirect a'stat call for heat, the respective circulator will run. ( You may have ZONE VALVES and only one circulator... some systems have separate circulators for each zone)

When the indirect calls, there is little chance that cool water will return. Let's say that you have the indirect set at 120... when it cools to the -differential, say 110, it will call for heat. So the return water from the indirect will never be less than say 110... and once the burner fires, it will get hotter quickly.

Yes, you could have cool water coming back when the heating circuit calls.

Here's the thing though... when the burner fires, and the boiler is at the low limit, it begins heating the water. On the fireside of the boiler, condensation WILL occur. It always does. The object is to MINIMIZE the amount of time that condensation occurs. The burner should fire long enough to DRY OUT the flue passes.

Some systems such as converted steam, or gravity flow with HUGE pipes, and cast iron radiators have the capacity to return cool water to the boiler for a LONG time. THESE are the systems that are in danger, and should have boiler protection in the way of bypass piping installed.

If you have fin-tube baseboards, keep in mind that stuff heats up pretty quick. There's not tons of water in it. When the pump starts up, with your boiler already at say 110, it will be only a matter of minutes before the baseboards are above say 125 degrees... and then yer out of the condensing zone (with oil, gas is a bit higher).

OK... a little bit about the differentials... your aquastat operates a little differently than the older variety (i.e. 8124), and it's a bit easier to explain...

LOW LIMIT:

The DIFF is SUBTRACTIVE. If you have the LOW set at 110, and the LOW DIFF at 10, as the boiler COOLS DOWN, the burner will fire at (110 - 10) or 100. The burner will continue to fire until the temperature in the boiler is at the LOW setpoint, or 110. Another example: LOW still at 110, DIFF at 20. The burner will fire at (110 - 20) or 90, and fire to 110.

When the boiler COOLS DOWN to the LOW - DIFF, the CIRCULATOR will be DISABLED, meaning even if there is a heat or indirect call at that time, the circulator will NOT run until the boiler heats to the LOW setpoint, at which time it will be re-enabled and allowed to run on a heat or indirect call.

So, by leaving your low setting at 110 and increasing the LOW DIFF to 20, you are allowing the boiler to cool even more before it fires. Personally, I don't like that. I think you will be much safer condensation wise if you increase the LOW to 130, and set the DIFF at 20. When the burner fires at 110 (130-20), it will run for a long enough time to heat the boiler to above condensing, and dry it out, then, because of the 20 diff, it will also stay OFF longer.

HIGH LIMIT CONTROL:

Leave the HIGH limit set at 180. Set the DIFF at 20.

I'm willing to bet that MOST of your heat calls the thermostat will satisfy before the boiler hits high limit. Have you spent any time watching the boiler to notice what temps it satisfies the thermostat?

If your system DOES regularly hit the high limit, the 20 DIFF will tend to prevent it from 'short cycling'. The burner will stay off longer, while the circulator continues to run and circulate the hot water. Then, if the heat call still continues, and the boiler cools to 160, the burner will fire again... as many times as needed until the thermostat satisfies.

As an example, my personal system almost never hits high limit. The highest I've seen it this winter when the temp was around 10 outdoors was 165. That's because by the time the water gets that hot, the thermostat is happy and shuts down the system. Granted, I've got good insulation, and more baseboard than needed. Leaky older homes with just enough baseboard may hit high limit regularly.

If you have the newest 7224, you should find backup for this info on pages 5 and 6, and figure 7.
 
  #5  
Old 02-25-10, 05:14 PM
O
Member
Join Date: Dec 2007
Location: A Galaxy From Afar
Posts: 421
Received 0 Votes on 0 Posts
For a boiler, I always thought that less then 32* F was too low. Other then that...

Al.
 
  #6  
Old 02-25-10, 06:22 PM
NJT's Avatar
NJT
NJT is offline
Member
Join Date: Oct 2006
Location: USA
Posts: 23,539
Received 4 Votes on 3 Posts
Whatcha mean kemosabe? Cold start? yeah, in a perfect world, with a boiler with push nipples instead of crappy neoprene o-rings... at least I think that's what you mean?
 
  #7  
Old 03-01-10, 03:16 PM
T
Member
Thread Starter
Join Date: Feb 2010
Location: USA
Posts: 23
Received 0 Votes on 0 Posts
Thanks for the advice!

Your suggestion about keeping the low limit high enough to dry out the flue passes makes sense. Obviously, this is irrelevant if the indirect calls for heat often enough to prevent the low limit from ever being hit. I guess I'll find out this summer as it will be the first with the indirect.

As far as my boiler temp in the winter, it seems to hit the high limit before the zone stops calling for heat. Perhaps this is because the amount of baseboard is on the low side relative to my boiler's output capacity of 173k BTUh. My house is 30 years old and only 2180 sqft (not including the 3rd zone in the finished basement which I don't use very often).
 
  #8  
Old 03-01-10, 05:51 PM
NJT's Avatar
NJT
NJT is offline
Member
Join Date: Oct 2006
Location: USA
Posts: 23,539
Received 4 Votes on 3 Posts
You're gonna be surprised how long that indirect hold the heat. If you aren't using hot water, the boiler could easily have time to cool between indirect calls... but yeah, see how that goes... and adjust if necessary.
 
 

Thread Tools
Search this Thread
 
Ask a Question
Question Title:
Description:
Your question will be posted in: