Should I install outdoor reset module?

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Old 07-14-13, 06:10 AM
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Should I install outdoor reset module?

Hi guys,

Just moved into a new home and I'm taking note of things for service and upgrade. The boiler is an oil fired vitorand with a Reillo burner.

The system provides hydronic baseboard heat and hot water through an indirect hot water tank.

I have a Honeywell L7224/7248 aquastat but no outdoor module.

Do you think it would be beneficial to add the outdoor temp module and if so wired or wireless?

Thanks!

Bryan
 
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Old 07-14-13, 07:07 AM
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It depends on how much you can reduce the water temp without causing damaging condensation in the flue. The theory of outdoor reset is that you run the water temp higher during cold spells and reduce it at other times. A boiler is more efficient at lower temps.

If, like I, you have enough radiation to heat the house at the lower water temp during very cold weather, then I believe you'd be better off just setting the aquastat to that temp all the time, and forget the outdoor reset.
 
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Old 07-14-13, 02:10 PM
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ODR is great if your boiler is sized close to your home's heat loss calculation. Otherwise if your boiler is over-sized as many are, save your money and dial down the setpoint as Gilmorrie says.

What is the cost of adding the outdoor reset module?
What is the BTU rating of your boiler?
 
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Old 07-14-13, 04:21 PM
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ODR is great if your boiler is sized close to your home's heat loss calculation. Otherwise if your boiler is over-sized as many are, save your money and dial down the setpoint
I think that statement is true - if the boiler is undersized, which few are, it can put out more heat with lower water temp, because the boiler is more efficient under that condition.

But I think a more important issue is the amount of radiation in the house - that is often oversized, too. But if the radiation is marginal or undersized, and the boiler is oversized, then you may need to jack up the water temp in very cold weather.

I think the most common situation is for both the boiler and the radiation to be oversized - in which case, outdoor reset should not be needed. Those who sell ODR might not agree.
 
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Old 07-15-13, 04:03 AM
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Thanks Guys,

I'm not entirely sure about the heat loss and its calculation. I do know that the boiler is the smaller of the Vitorand Series, the Vitorand 100. The house is a larger 3200 sq. ft. 2 story with some heated space in the basement, 4 heating zones with the 5th being the domestic hot water. I'd be happy to attempt the calculations if you can point me in the correct direction.

I guess in the interim I should just set the aquastat a bit lower, say 155 until we get into the colder months... Will 155 have an adverse affect on the hot water heating potential?

Thanks,

Bryan
 
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Old 07-15-13, 08:38 AM
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Performing a heat-loss calculation probably wouldn't help answer the questions you've posed. If you want to do it as a matter of curiosity, here is one program: http://www.doityourself.com/forum/bo...alculator.html

The concern with reducing the aquastat setpoint is whether it will lead to damaging condensation in the flue - and that depends upon various factors, such as the efficiency of the boiler, chimney height, flue design, etc. Personally, I wouldn't reduce the setpoint to 155 deg. What is it set at now? Reducing the water temp will save a small amount of fuel, but at what risk?

Will 155 deg water heat the house during cold weather, assuming you want to risk condensation in the flue? That would depend upon the amount of radiation installed in the house - you'd have to measure your heat emitters and find manufacturer's data for them to estimate their Btu/hr rating. Doing a heat-loss calculation won't do that.

Since you're new to this house, I wouldn't tinker with the boiler until you have had a year or so of experience. And, to properly tinker, you should hire somebody that can perform a combustion analysis, which requires special equipment that most DIYers don't have.
 
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Old 07-15-13, 03:47 PM
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I sorta share Gil's position on the ODR. It absolutely CAN save fuel, but under some circumstances it might not actually save much.

With an oil fired CONVENTIONAL (i.e. NON condensing) boiler, the main concern is prolonged periods of water under about 125 or so returning to the boiler. This is what causes the condensation inside the boiler. As long as the boiler comes up to temp during a burner cycle in time enough to 'dry' the condensation before burner shuts off, there is little concern.

You MIGHT be 'safe' with 155, but remember that this may cause HIGHER fuel usage when heating water. The burners could actually fire LONGER to reheat your domestic tank. I'm not saying they WILL, but they COULD. Let's say you've got your hot water tank at 140... there's much less 'driving force' (difference in temp) between 140 and 155 than there is between 140 and 180.

I wouldn't set the BOIL MIN on an ODR at less than 155 with an oil fired system unless I knew for a fact that the return water came up to temp fast enough to dry the boiler out.

There's another condensation issue to think about too...

Condensation in the CHIMNEY. Even if you aren't condensing in the BOILER, you can still condense in the chimney.

This is because as the flue gas heads up the chim, it cools. If it drops below say 125 before it exits, you will get 'rain' in the chimney.

This is a problem with newer more efficient boilers that have lower flue gas exit temps that are connected to large old chimneys.

If the chimney is too large, the barometric damper has to open wider to maintain the draft and this will pull in more cooler room air... diluting and COOLING the flue gases. In addition, the VELOCITY of the flue gases in a too large chimney is SLOW, compounding the problem. The slower the flow, the more heat that is lost as it travels upward.

This is an excellent case to be made for properly sized stainless steel INSULATED flue liners in old masonry chimneys! (I'm in the process of installing one now as a matter of fact. Not a masonry chimney, but 6" 'manufactured'... my new MPO boiler just doesn't have enough HOT gas to fill the chimney, and the condensate pours out the bottom of the pipe...)
 
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