most fuel efficient aquastat settings for oil boiler w/ indirect hot water tank

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Old 03-04-11, 11:39 AM
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most fuel efficient aquastat settings for oil boiler w/ indirect hot water tank

Hi folks,

I am a new homeowner with an oil fired boiler (H.B Smith series 8) and am trying to squeeze the greatest efficiency from my heating system. I have browsed through the threads in the boilers section and have learned enough to know it is a complex issue, but have not managed to discover an answer to my several questions.

One vein is the optimal hot water for my 3 zone baseboard heating system - I have read that the hotter the water the more efficiently the baseboard heaters radiate it (i.e. 190F), but also that the boiler is more efficient at low temps (i.e. 130F), which seems mutually exclusive. (Where does the "loss" go with 130F water? Back into the boiler and up the flue somehow?) I also understand that the boiler is more efficient in its operating termp range, rather than when cold.

I am not sure how to make all this converge into the most energy efficient settings for the aquastat. Comfort is not really a big issue - we are hardy folk and keep the tt at 45F in 2 of the 3 zones and 60 in the bedrooms (unless we have company, of course). Temp swings also do not bother us much, especially downwards ones. One suggestions I read is to choke the system so that the circulator is running about 60% of the time with low water temps and the boiler is coming on and off at low duty cycle, providing slow btu's into the emitters and running in its operating temp range, but I am not sure that is the right move in my circumstance.

I also understand that having an overpowered boiler can be less efficient and I am wondering if we can change our nozzle to a lower flow one to reduce fuel consumption and maintain slow heating. I think it was somewhat oversized when spec'd and given how we "heat" our house and additional insulation we have added, it is probably well oversized.

One thought was to set the lo as low as possible with a large differential, which would let the burner run longer when it comes on. My concern is with the boiler getting too cold and condensing - or will the longer run burn off the condensations with a large diff? I assume that the DHW will not be adversely affected since it has its own zone and the hi will probably be at least 160. We have a toddler and keep our DHW at 110F.

I also do not have a bypass for boiler protection or adjustment right now, which seems like both a good idea for the boiler and a useful way of choking the system to provide slow heat. I plan to put in a manual one soon and perhaps an automatic mixing valve in the long run.


Any suggestions or feedback would be greatly appreciated. If this has been covered, a pointer would be great. I tried a dozen times to search, but the anti-spam captcha does not seem to work - I tried visual and audio repeatedly to no avail.


Details:

H.B. Smith series 8-4 boiler with Weil McLain indirect fired hot water tank on its own zone, honeywell R8184 aquastat. 15 [email protected]
 
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Old 03-04-11, 01:18 PM
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I am not an expert here but that honeywell control you gave is the oil burner control. Do you have the actual aquastat #?

Regardless if you not using the domestic coil there is a way to disconnect the low.(If not you can replace the aquastat to a high limit only. This way It will only work off the high and be a cold start boiler.

Others will tell you though, usually a warm start boiler likes to be a warm start boiler. Not sure if leaks will occur or not, but I would think not.

Is your boiler newer?

If you do away with the low then the boiler will only kick on for a call for heat or HW. Right now your aquastat is probably for a domestic coil if the link I posted is indeed your boiler. 180F is the standard for water temp.

Smith Cast Iron Boilers > Residential Products > Series 8

The oil guys will be on soon to give there input.

Mike NJ
 
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Old 03-04-11, 04:52 PM
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aquastat model

Sorry about that. I could not find any number on the aquastat, so I assumed the 2 connected boxes were part of a single device - my bad and I know what happens when you make a donkey of me.

Here is a pic of the unit since I am unable to find any number on it: http://i844.photobucket.com/albums/a...g?t=1299286202
(Maybe there is a secret spot with the model number?)



I think it is fairly old, without being ancient, but it is hard to tell by eye. I will try to find a mfg date on it somewhere. Here is the tag
http://i844.photobucket.com/albums/a...g?t=1299286202

The link you posted does match mine, but apparently there is an option to put in a coil for on demand tankless hot water. Mine does not have this option. As I mentioned, the indirect water heater has its own zone and its own circulator (which I failed to mention). I think this means it will turn on the boiler until it gets the heat it needs based on its thermostat, meaning it will fire up and keep going up to the hi limit as long as the DWH is calling for heat.

If I should keep it at a lo of 130 to keep it warm starting, I will. I am more concerned that it has not had a bypass since it was installed, since I can tell that pretty cold water is being introduced into it when one of the cool zones calls for heat, especially first thing in the morning.

Cheers and thanks for the help!
 

Last edited by wolvie; 03-04-11 at 04:58 PM. Reason: clarity
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Old 03-04-11, 05:05 PM
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With baseboard the need of a bypass is about nil due to low water volume and multiple zones. If the boiler is over sized it is even less important.
To convert to a cold start boiler is more efficient but if it is older it may leak at the coil gasket. There normally will not be leaks between the sections if the boiler has nipples instead of gaskets as they expand and contract together.
 
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Old 03-04-11, 07:37 PM
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If I should keep it at a lo of 130 to keep it warm starting, I will.
If you want to disable the low, turn off power to the boiler. There is a blue wire by the diff. Put a small screw driver in the little slot above the blue wire to release the tension and pull the wire out and cap it. Now you are working off the high only. Should save some oil.

As rbeck said the coil gasket may leak. Thats the plate behind the aquastat. That can be replaced if need be.

In a few cases, certain cast iron boilers may cool off and begin to leak between their cast iron sections during the summer( but you have a indirect and may not be an issue since the boiler will come on in the summer for a HW call.) If not this may be a reason to keep the LO wired and active on these boilers.



Mike NJ

Mike NJ
 
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Old 03-04-11, 09:51 PM
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I would be glad to skip the installation of a bypass if my baseboard system is incapable of thermally shocking the boiler. Is that true even if multiple zones come on at once first thing in the morning? I will probably try to stagger the overnite setbacks to prevent that.

I am also happy with the idea of making it a cold start boiler, but only if that is a safe option. How can I be sure of that? How bad (expensive) a problem would the leaking be, if it happened? No point saving $50 in fuel then paying $300 to repair the leaking gasket and sections.

For the moment, it seems that turning the low setting down as low as possible (110F) is a good starting point. I can consider disabling the low completely by removing the blue wire for the summer if that is safe.

No one has mentioned whether condensing flue gases will be a problem, however. Am I just suffering from condensation paranoia? I understand it can be acidic and eat through your flue pipes, chimney and boiler.

Also, no one has mentioned if the hi limit is more efficient at 190 or say 150, given the competing efficiencies of the boiler and the baseboard heaters.

Thanks for the responses!

Cheers,

David
 
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