L7224U Aquastat settings - oil fired cast iron boiler

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Old 12-06-07, 10:13 PM
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L7224U Aquastat settings - oil fired cast iron boiler

Any recommendations on high / low limit settings and differentials high / low on my Honeywell L7224U aquastat to minimize fuel oil use this winter?

http://customer.honeywell.com/techli...0s/69-1957.pdf

I have a Slant Fin Series 78 cast iron oil fired boiler (ca: 1977) (with a domestic hw coil) that supplements a wood fired stove. I have three circulator zones. The wood stove probably provides at least half of the heating requirements during a cold 25 degree day. The boiler really is needed mainly to bring up the house temp. from about 60 degrees setback overnight to 68-70.

My domestic hot water coil is not currently used to supply hot water and is ball valved closed on the supply and output side. I now obtain domestic hot water from an electric hot water heater tank that is also valved into the system and is also controlled by ball valves on both supply and output (same as domestic coil). My water supply is from my well and the pump tank also provides for expansion.

I'm guessing that much of my burner run time especially on milder days is just to maintain a minimum standby temp. in the boiler as I don't notice the circulators running much. The burner is underfired at 1gph from its 1.55gph rating. Stack temp when it runs is fairly high at 550 or so. From what I read in some other posts I need maintain minimum water temp to prevent boiler condensation.

Any recommendations on what aquastat settings would work best under these parameters, as far as looking to minimize fuel use? Sorry for the wordiness of the post.

Thanks.
 
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Old 12-07-07, 04:31 PM
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Tobee, first off, I'm concerned about the fact that you have the tankless coil sealed shut with valves. I've recently read in manufacturers literature that the domestic coil must NOT be sealed, and they also state that it can cause property damage, and possibly death... I think their concern is that pressure can build in that sealed tankless coil and it could blow... personally I would take their advice.

Is the coil still connected to the domestic plumbing ? or has it been disconnected, such that you can simply open the valves to allow the coil to be open to atmosphere ? If you can manipulate the valves in a way as to 'pre-warm' the water into the electric, why not do that ? You could still run cold start, because even warming that 57* well water a few degrees will save you a bit of electric.

That is a pretty high stack temp... has the boiler been cleaned lately ? Soot accumulation can cause inefficiency and high stack temps. But maybe that's normal for you boiler...

You can disable the Low Limit settings in the 7224U if you wish to operate the boiler in a cold-start application. Check the manufacturers literature for instructions to do so.

If you do disable the Low Limit, be aware that operating the boiler in cold start _may_ cause the boiler to start leaking. If this happens, turn the Low Limit function back on.

High Limit should be around 180* or so. Running it at much less than that could cause your return water to be cold enough to cause flue gas condensation in the boiler.

You will save a bunch if you do convert to cold start, but consider the cautions ...
 
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Old 12-08-07, 05:17 PM
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Thanks for the reply and concern. My domestic HW coil is valved so that it is in parallel with the electric hot water heater. The suplly and outlet valves are set to partially open to the domestic supply more or less to keep pressure in the coil equalized to the system.

Unfortunately (as stated) I can't set the valves so that it is in series with the electric water heater. They are both plumbed in parallel with the supply. From what I've read the electric water heater does not like supply water that is hotter (160-170) than the 140 degrees that what the electric thermostats are set at.

I've noticed that my boiler leaks a little in the past when it was shut down (not cold start mode) that I thought was due to expansion of the joints, not condensation.

So you might recommend a 180 high limit? What about LL and differential settings. Thanks again.
 
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Old 12-08-07, 06:45 PM
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I don't understand why the water heater would have any complaints with hot water going in ... do you remember what you read that said that ? I'd like to look at it myself. I should think if the water was hotter than the setting on the water heater all it would do is save you electricity by not having to run the heating elements ...

I say that if they are piped in parallel, just open em both up, ESPECIALLY if you have to keep the boiler hot to keep it from leaking. I mean, it's gonna be hot anyway, might as well use SOME of the heat you are making instead of throwing it all away up the chimney!

Yes, the high limit is fine at 180*.

Set the low limit to minimum or 120*, diff at minimum.

If you have the electric set to 140*, you do understand that there is danger of scalding, right ? Is there a mixing valve on the outlet of the water heater ?
 
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Old 12-12-07, 11:30 AM
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Thanks for the replies.

Somewhere I thought I had read where pre heated hot water should not be used on the supply side of the electric HWH. Can't find that info. now. However my boiler was never installed with a mixing valve on the hot water output which means that when the boiler is being fired that the water temp is higher at that location >160 degrees than the max thermostat setting on the electric water heater so it still doesn't seem like a good idea to run it in series. The EHWH also does not have a mixing valve.

We have a ranch type house that we bought as resale (didn't build it) that does have half axxed plumbing arrangements. Part is on slab which makes replumbing it more than just a piece of cake. There are long plumbing runs of more than 50 ft to both the main floor utility, kitchen and baths which serves to moderate the hw temp by probaby 25 degrees.

I'll try the UL of 180 with 15 Diff and LL of 120 and with minimal diff as the boiler is mostly in place to provide supplemental heat on demand and maintain a minimal standby temp. I actually haven't noticed any signs of the boiler leaking when it has been shutdown for some time.
 
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Old 12-12-07, 12:28 PM
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I have read somewhere that glass lined tanks do not want hotter than 140 water entering the tank. The 140 might be off a little I can't remember exactly. It may have been a little higher due to pissible damaging the glass lining.
Is the system water going into both boilers when the wood boiler is running?
 
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Old 12-12-07, 05:29 PM
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Water Temps

I have no idea why higher temperature water entering the water heater tank would not be fine. Energy Kinetics uses an electric water heater for their domestic storage tank which gets supplied with some very hot water, often over 180.
 
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