Boiler Outdoor Air Temperature Reset - Why does it work?

Hoping someone here can enlighten me. I understand the concept and how outdoor air temperature resets work. What I don't understand is how they save energy & reduce cycling. If water temperature is lowered from 180° to 140°, the boiler doesn't have to work as hard to heat the water. But, less heat is delivered to the space so the boiler will run longer or more cycles, using the same amount of overall fuel.

I also don't understand how it reduces short cycling. Again, boiler heats water to 180° and shuts off, circulating water until it drops to 160° and then re-firing.

With the outdoor air temp, it could heat water to 140° and shut off, re-firing at 120°. It's the same 20° drop, so the boiler should cycle the same regardless.

What am I missing?

 

You have identified potential fallacies of ODR. But there are more.

A boiler runs at best efficiency and highest thermal output when the water temp is lower. If your system is over-radiated, which many are, you want to run the boiler temp at the lowest that will still avoid condensation in the flue. There is then no reason to jack up the water temp when the outdoor temp low if the house can be adequately heated at a lower water temp.

 

But overall, ODR is still a benefit and energy saving tool, yes?

 

Not if it winds up running the boiler at a higher temp than required to heat the house and also to avoid condensation in the flue.

ODR makes perfect sense if the Btu/hr rating of the radiation is too low to heat the house at the lowest water temp that avoids condensation. For example, if your radiation will comfortably heat the house at a water temp of, say, 170 deg, on the coldest day, why would you want to run the boiler at 180 deg on the coldest day? That would waste fuel.
Your words are a bit mixed up. The boiler will deliver more net Btu/hr to the water (and to the house) at 140 deg than at 180 deg because the boiler is more efficient. Now, the heat emitted by the radiation, Btu/hr, is higher at 180 deg, but the boiler will run longer to put out the same net Btu.

Talking about how "hard" a boiler "works" is not helpful terminology and can lead to wrong conclusions. Best to talk about fuel burned, Btu, and net heat output, Btu.

 

Your missing the important point.. ODR works on shoulder seasons mostly... And most boilers with ODR are modulating.. 5 stage for example..

If its 0F out then the boiler works like a normal boiler would. No real savings and fires 100% to heat the home..

on shoulder season say Its only 50f out. So the boiler will fire at say 30-40% flame on. It will heat your home at a lower boiler temp because its not that cold and there is not a lot of heat loss in the home. ( only if there is enough heat emitters)

Ideally would be to oversize all radiation in the home so on the coldest day of the year you can heat your home with 110f water for example, and the boiler will always run low fire and condensate.. or any temp under 140f..

But wall space is often the factor with over radiation.

 

Mike, you make sense. But, unfortunately, that's not the way ODR is often sold - to people that do not have modulating or condensing boilers. Including to our O.P., who I think has perhaps no or limited experience with hot water boilers, and has several problems with his system that will not be solved by ODR, but he has been given the sales pitch by a service person, likely working under a commission. The limitations of ODR are almost never explained. It's just sell, sell, sell.

 

Yes there are so many variables and all things about the system need to be known... emmitters, boiler type, etc....

If I built a new house tomorrow I would size the radiation to work near 100F or so on the coldest day of the year. Mod cont all the way....( 110-120 maybe in the real world... HO basboard? Two packs? IDK...)

 

Am I the OP in this thread? :-) I understand concepts and enjoy learning more about my home and its systems, but I am no expert. ODR with a modulating boiler makes way more sense, as it can simply stay on and not cycle so much. I have a boiler from the 90's with 80% efficiency - based on that I'm sure it's a non-condensing boiler, and probably does not modulate. I'll check the specs to verify. Your other points also makes sense - simply put, boilers are more efficient at lower temperatures so if you can over-radiate your home you can run the system at a lower design temperature and save energy. My home is over-radiated in most rooms (up to 60 BTU/SF in some). It's all slant/fin baseboard.

So, ODR may still have some benefits but not as much due to my boiler type. And to Gilmorrie's point, I may not need 180° water on the coldest day anyways. I assume I can adjust the ODR curve, including max/min temps, myself?

 

Example:

My boiler is 40-50 yrs old. I am over radiated... I lower my aquastat on the shoulder season to 140F.

This heats home fine...Lower then 140F then can cause condensation issues..When it gets to 0f I raise the aquastat to about 160f..

ODR is a waste imo for my unit and I can do it manually.. The savings if I had ODR are nil...

 

Sounds like my question should be: how do I manually manage my aqua stat to find the proper operating temperature?

 

Baseboard Is usually sized at 180f for normal households...

Whats yours sized for?

IMO leave stat on 180F and forget about it... You will save nil trying...

NG is cheap... If you save $40 bucks a year you'll be lucky..

 

I agree with Mike.

There is no "proper" operating temperature, but there are temperatures that are too low or too high. You could play around with the aquastat setting and, if you aren't careful, you could damage the flue and/or boiler if you get into condensing.

Theoretically, you could measure the boiler efficiency at various aquastat settings - but that would require combustion analysis equipment which you don't have. And there are a host of other variables affecting efficiency that would swamp the relatively tiny effect of aquastat setting.

 

For years during the heating season I periodically adjust boiler temp as outside temps changed. Then installed a $150 Tekmar and now have a more comfortable home.

To avoid condensation keep boiler water temp at 140F min. Also use a circulator cutout that deactivates it below 135F.

For those that believe short cycling is bad, there some ODR's have an "AD" or automatic differential setting. When I tried AD on a Tekmar 256 ODR with 180 max temp setting, the system overshot to 190F plus. Took it off AD and set it to 3F delta. Also lowered max temp to 170F to handle overshoot. A separate boiler control shuts it off boiler if it ever reached 190F

With regard to "short cycling" look at any efficiency chart and see that efficiency is always highest at start of cycle and goes down the longer it is run. On boilers and water heaters lowering the deltaT is a good way to increase efficiency. The MTBF of burner components is very high. Factors other than short cycling typically cause failures.

 

Running a combustion analysis test, the efficiency always is observed to be highest at the initial start of the test, and then declines to a stable value. But, I think the results are somewhat illusory.

The combustion analyzers initially read a low stack gas temperature, which normally indicates high efficiency and low stack losses. But, as the boiler fireside and exhaust flue heats up within a few minutes after the start of the test, the stack gas temp rises and the indicated boiler efficiency stabilizes at its normal, lower efficiency. This pattern repeats itself every time the burners cycle. The losses that are "missing" at the start of the test were previously lost, but not observed, during the last system cool-down.

Another factor is that much, usually most, of the stack losses are not tied to the thermal energy of the heated flue gas but, rather, the latent heat of the uncondensed water vapor from the moisture content of the fuel. For a non-condensing, atmospheric boiler, the moisture losses are essentially constant. Reducing the boiler water temperature, through ODR or manually does not reduce the moisture losses.

Keeping boiler temps below just 140 deg F is a little too close to danger for me.

 

Older system controls had a delta T of 5F to 20F. Newer controls and ODR can accurately control to a couple of degrees. My Weil McLain boiler has two wells, one on tankless coil for ODR sensor and other other low, near water return inlet used for low temp circulator cut off.

The circulator low temp control is a PID with 1-2 degree delta. In the morning it typically kicks in with a rush of cold water when the thermostats raise room temperature to day time level.

In 60 years never had a condensation issue even with original control that cutout circulator around 120F.

Boilers commonly run 20% to 25% of the time. Latent/standby looses at 180F during the other 75% of time add up to a lot of wasted fuel. Without getting into deep discussions about efficiency, ODR's cut that loss directly.

 

Thanks all. Very helpful opinions. I am skipping the ODR for now; my home is small and natural gas prices are low. I only spent $400 on heating last year. I will likely include system upgrades such as ODR when I have to replace my boiler (hopefully not for a while).

 

Good plan.

What could you reasonably pay for a fictional gadget to, hypothetically, heat your house with zero fuel? For a reasonable 5-year payback, that would be 5x$400 = $2,000. What was the ODR going to cost?

 

Today's New York State price for Long Island is $2.73 gallon, Oil Heat Institute of America data for average residential is 800 gallons per year. Today that is $2184 per year.

Price of Tekmar 256 ODR is $160 (Amazon raised the price to $214 recently).

ODR savings are typically in the 5% to 10% range, or more. ODR's also offer many control features to optimize boiler performance that are not available on “dumb” aquastats.

Through improvements to home, over the years have saved many $10,000's in heating and AC costs.

My first year oil cost at today's price would have been $5,000. That is not “fiction”.
Improvements at modest cost gives many years of payback and a more comfortable home.