Convert from 2 zones to 1 to reduce short cycling?

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Old 11-30-12, 07:39 PM
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Convert from 2 zones to 1 to reduce short cycling?

Hi all Hopefully a simple question with a simple answer.

I have an oil fired boiler with single circulator and 2 zone valves (upstairs : downstairs). As is typical, the boiler is oversized (94K BTU for a self calculated 50K heat loss). Had intended to add indirect HW and radiant zone but no $$ as yet to do those projects.

I want to manually open the 2nd floor zone valve so that when there is a heat call from either thermostat, I will have a greater volume of water which should mean that the boiler will run longer.

Of course this is super simple to accomplish but is there some reason I should not do it?

Thanks for any advice. Bill
 
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Old 11-30-12, 08:54 PM
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Of course this is super simple to accomplish but is there some reason I should not do it?


Yes , the temp coming back to the boiler may be too cool. Delta tee....

But over sized maybe not.

But the standard I believe for delta T values is 67 ft of element per zone.

How do you stand in that respect?

Plus example: I have the same situation as you. But I live in a cape. If I did one large zone, upstairs would be ice cold. It loses more heat then down. T stat would warm downstairs before upper floor made temp.......

The children sleep up there. I dont like when my children whine....LOL
 
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Old 12-01-12, 07:57 AM
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Lawrosa I have 43 ft of baseboard on 1st floor and 48 ft on the 2nd so with 91 ft Im almost 50% over the 67 ft per zone. With a 100% oversized boiler perhaps it would be worth an experiment. Id keep an eye on how long it takes for the cold start boiler to get above 140 degrees.

BUT you may have inadvertently given me an alternate answer to the problem. In checking upstairs baseboards, I saw that much of the baseboard in 2 of the 3 bed rooms were covered in dust and cobwebs not so hot for heat transfer out of the pipes into the room. I guess that would then send still warmed water back to the boiler and with a decreased delta T, take less firing time to reach Aquastat max set point.

Ill wait until my wife goes out Christmas shopping and take care of this myself. I like to eat so why make waves.

Thanks, Bill
 
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Old 12-01-12, 08:47 AM
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If your over radiated for the home, clean and vacuum all BB. You can lower the temp on the boiler to save some fuel.

I lowered my boiler high limit to the 150f range. I only need 160f water temps to heat my home. After the boiler kicks off at 150f it soaks some and temp raises to around 160f.

Just have to be careful not to lower too much due to condensation issues. I belive you want at least 130f min coming back to the boiler. Your best keeping two zones IMO.

Although I wonder if a boiler bypass is installed then you can do as proposed. It will run lower temps through the system side while allowing the boiler heat properly...?

Others will chime in, since I am not a pro in this area. Just stating what I did on my system. and my personal knowledge of this theory.
 
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Old 12-01-12, 09:47 AM
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I have 43 ft of baseboard on 1st floor and 48 ft on the 2nd so with 91 ft Im almost 50% over the 67 ft per zone.
Wait... you said they were two zones, didn't you? So the 43 and the 48 are in PARALLEL at the boiler, yes? If so, no problem.

How short are your short cycles?
 
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Old 12-01-12, 11:06 AM
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Short cycling

You can also close down or close the dampers on the base boards in the warmer rooms to ballance your system. I have one zone in my 1000 sq ft 2 story house, and it works fine.
Sid
 
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Old 12-03-12, 04:08 PM
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I havent taken any action yet. Work in a big orange box store so weekends are not project days.

Sidny: Thanks, Ill be sure to give those upper bedrooms more attention.

Trooper: How short are your short cycles? The burner is generally running about 2 minutes then will be off for7-8 minutes while the circulator continues to run. Sometimes burner is only on for 1 minutes. I believe I should be looking for 7-10 minute burner times but perhaps that isnt plausible due to the oversized boiler.

Lawrosa: Although I wonder if a boiler bypass is installed then you can do as proposed.
There is no bypass although Im expecting to re-plumb next summer when adding another zone. Im concerned about dropping the max temp since there was noticeable smoke pipe condensation 2 years ago. We already adjusted the baffles to bring the stack temp up.

My immediate plan is to perform the overdue baseboard cleaning and monitor the cycle times still using 2 zones to see if anything changes. Then, when the weather actually turns cold, Ill manually open one zone valve and see how the cycles change.
 
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Old 12-03-12, 05:10 PM
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Trooper: How short are your short cycles? The burner is generally running about 2 minutes then will be off for7-8 minutes while the circulator continues to run. Sometimes burner is only on for 1 minutes. I believe I should be looking for 7-10 minute burner times but perhaps that isnt plausible due to the oversized boiler.
Yeah, I would call that short cycles...

Cleaning out the baseboards will help some. More heat coming off the water in the boards means cooler returns, and longer runs.

On these heat cycles, does the burner then kick back on after the 7-8 min off time while circ is spinning the hot water? and then lather, rinse, repeat until the thermostat satisfies?

Have you observed the boiler temp during a heat cycle?

I'm curious as to what 'differential' you are seeing... presuming that burner comes on, boiler hits it's high limit, circ runs, boiler cools to diff, burner fires, etc...

Would be telling to know temps during this.

The condition you have there is actually an excellent application for the "Heat Manager". It will widen the differential and allow more 'off time' as the boiler cools farther. When the boiler fires up again, since it is cooler, the cycle will be longer.

Alternately, replacing the aquastat on the boiler with a modern electronic variety with a 5-30 adjustable differential on the high limit will also help.

What aquastat is on the boiler now?

BTW, going back to your first post:

this is super simple to accomplish but is there some reason I should not do it?
You might end up overheating the upstairs... between 'too frequent' heat calls and the possibility of 'gravity flow' when there is NO heat call (thermosiphon moving the boiler heat into the upstairs loop) it could happen. But try it and see, can't hurt... most zone valves will reset to auto once a heat call comes in... so it might not stay in manual mode.
 
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Old 12-03-12, 05:24 PM
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I you have valves in the piping you can open both zones with a thermostat and balance the heat you want on each floor.
 
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Old 12-05-12, 08:13 PM
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rbeck:
valves in the piping you can open both zones with a thermostat and balance
Currently no valves to balance the zones but will consider for next summer's re-plumb. Thanks for the idea.

trooper:
On these heat cycles, does the burner then kick back on after the 7-8 min off time while circ is spinning the hot water? and then lather, rinse, repeat until the thermostat satisfies?
Have you observed the boiler temp during a heat cycle?
I'm curious as to what 'differential' you are seeing...
Now that it has gotten colder outside, some cycle times are longer BUT until now I hadn't appreciated how the Outdoor Reset worked. Have a Honeywell L7248 with an ODR.
Hi limit 180. With ODR it seems the dif is fixed at 15 and the boiler set point starts at 145. It steps up 10 degrees per cycle until it reached Hi Limit or the heat demand is satisfied.
Just ran through full cycle recording times and temps. Burner run times 3:33, 2:33, 6:16, 2:50. Off times before the burner kicked back on were: 5:32, 6:38, 5:00 then another 3:27 with circ on but burner off before demand was satisfied. Temps were 130-145, 140-155, 150-165, 160-175.
ODR setting are all still Default except I upped the minimum Boiler Set Point from 140 to 145 since had had condensation concerns in the past. Gross stack temp at 350 - 380 seems about right.


the possibility of 'gravity flow' when there is NO heat call
Agree completely but later this month I'll still give the one zone a trial and see what happens.

For now, I'm going to monitor cycle times as weather gets colder and perhaps tweek the ODR settings. Thanks for all the insight.
 
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Old 12-06-12, 02:23 PM
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the boiler set point starts at 145. It steps up 10 degrees per cycle until it reached Hi Limit or the heat demand is satisfied.
This 'stepping up' is the "Boost" function. The a'stat will only step it up if the heat call isn't satisfied within the time setting of the Boost. If there is a heat call and it satisfies first call, it won't step up.

You will most often see the boost kick in during recovery after night time setback.

Do you have the Honeywell ODR that plugs into the a'stat? What's the model on that? I'd like to get familiar with the settings you have available.
 
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Old 12-06-12, 03:45 PM
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I would lose the boost feature, its being counter productive for you at the moment.
See if you can open the differential some, it's been so long since I did a zones CI boiler system I forgot about some stuff.
We always put in an operating limit with a large dif, CI boilers don't mind running hotter. I would rather see a boiler run to 200 F then to short cycle.
At least your storing the energy in the CI sections (to some extent anyways)

You might also think about a buffer tank to add some mass to the system to help your cycling.
It's not out of the question to go grab a smaller boiler and use your existing burner if it will fire down low enough.
Sometimes spending a bit more now saves a ton in the long run right.
I think losing the zone is going to hurt the comfort of your home in the long run.
Besides, once you start getting down to design temps both zones will be on anways
 
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