Why no set-back temperature controls on boilers?

Mark_129

We have an oil-fired boiler. It has a setpoint of roughly 180F, and is always holding the water between roughly 170F and 190F. It provides heat for the house and hot water. It does a good job.

Is there a control system that would allow us to reduce the setpoint of **the boiler** during periods of non-use (such as at night, or when we're away during the day)? For example, in the morning with showers and dishwashing the setpoint would be 180F, but during the rest of the day it could be at 120F (or whatever the lowest practical temperature is) since there would be no need for hot water during that time.

The benefit: reduced standby losses. The savings: maybe $1 per day, depending on how low the setpoint can go during periods of non-use. Not tremendous. But worth it since I could replace the controls in an hour. Or, I could rig up something myself if no product exists on the market.

What are the limitations on this? Is it rust? I realize that the cooler the heat exchanger on startup the more likely there is to be some condensation during start-up, and then corrosion due to the sulfurs and other nasty trace stuff in the heating oil. But how much rust as a function of lower temperature? Are there any studies? (I'm an engineer, so I'm looking for quantitative reasons why yes or no).

Thanks for any input - and any control products (or boiler product) recommendations that provide this functionality are appreciated as we will be switching to gas within the next 2-4 years.

NOTE: I am **not** talking about the setpoint of the house! We already have the very common setback thermometer on the house.

Steamhead (in transit)

What you probably have now

is a boiler with a "tankless coil" that makes your hot faucet water. These are cheap to buy and install but also quite inefficient, as you've seen. Maintaining temperature all the time causes air to draft thru the boiler flues when the burner is off, cooling the boiler down which makes the burner start up again. This is called "standby loss". Tank-type gas-fired water heaters have this same problem.

This is a control issue which is common to tankless coils. It has nothing to do with whether you're burning oil or gas.

The easiest way out of this is what's called an indirect tank. It is a tank with a heat exchanger in it. Water from the boiler is pumped thru the heat exchanger, and water to be heated for domestic use circulates on the other side. The two never mix.

Since there is no burner in an indirect tank, it does not have a flue in it for air to draft thru when the burner is off. This means the indirect will hold its heat much longer. The boiler never fires during the summer unless the tank cools down, which might be once a day or less if no one uses any hot water in that time period.

An indirect can be added to your current boiler, which would do away with the boiler having to stay hot all the time. Indirects are not cheap, but some have lifetime warranties. If you get the indirect now, when you replace this boiler you won't have to buy the indirect at that time. So you can spread out the cost of the upgrade this way.

"Steamhead"

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Mark_129

RE: Indirect

Thanks for the info about the indirect. However, this would *increase* my heat loss, since you still need the boiler to keep the indirect heated. And the boiler is still running at 180F all the day when I want it to setback at about 120F when it is not needed. The indirect adds surface area, and therefore adds heat loss. The additional heat loss is small with a well-insulated foam tank. I'll probably do this someday if/when I go with solar hot water.

I'm pretty sure there are boilers that do this setback thing, but I haven't been able to find any controls that I could use to retrofit my existing unit. It's a Burnham boiler, 0.75 gal/hr of oil rating (~100kBtu/hr). I'm looking for something off-the-shelf before I invent it.

An indirect adds capacity. A tankless would be another way to do it. Either is big bucks compared to simply turning back my boiler setting when it's not used.

Steamhead (in transit)

Not with an indirect

If the indirect does not call for heat, the boiler does not fire up at all. Not the burner, not the circulator, nothing.

Most indirects are pretty well insulated- better than tank-type water heaters. The Triangle Tube indirects we use a lot never feel warm to the touch even when fully heated.

The problem with turning down your present boiler is that you'd have to turn it back up if you wanted the coil to work. Most people want the process to work automatically. That's why indirects exist- they're set up to call on the boiler so you don't have to.

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JK_3

to make it more eff.

to make the set up thet all steamed up is refering to work would also mean changing the control unit that now maintains the temp in the boiler or at least adjusting it. now your settings are probly min.= 160 max = 180 or there about or mabye you have a max = 180 with a 10 or 20 diff. that maintains the temp. if you had an indrect you would want a cold start control that would not let the boiler fire untill a call for heat or hot water. if you are going that far you could incorperate boiler reset controls so that in warmer weather you would run at lower temps (as low as 140 without condensing issues) and set domestic priority to run higher only an call from tank.

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Unknown

But , with an indirect

you have standby loss of another appliance . And The 1/2 degree an hour temp loss on indirects is off by alot , in my opinion .

I'll have to refresh my memory , but I believe a BNL report stated there was very little , if any efficiency gain by adding an indirect to a system . It wasn't clear if a different control stategy was used though .

I still think a coil boiler ( if it can meet the hot water requirements ) , insulated to the max , with minimal standyby loss ( maybe with vent and burner dampers ) and with intelligent temp controls is the way to go for oil in the future .

But in the meantime ............ Field controls makes an oil vent damper now that can keep more heat from escaping the boiler . You can also do like Steamhead and JK suggest and add an indirect and make the boiler cold start . Another option since you already have a coil is you can add a storage tank . And use the same control strategy you'd do with an indirect . Or , for a bit more simpler approach , you can add the Beckett Heat Manager .

Steamhead (in transit)

All good ideas

but if we go with the indirect, when the time comes for a new boiler we don't have to get a replacement boiler with a coil. The indirect can interface with pretty much any boiler.

Also the standby loss of an indirect is less than a tank-type heater or boiler. A vent damper on the boiler/tank type heater reduces the standby loss, but I'm not sure if that could equal an indirect that does not have a flue to begin with.

I remember reading that BNL report and I don't think it addressed control strategy either. I've never heard of an indirect used with a boiler that maintains 180 degrees though, but I'm sure there's one out there somewhere :-0

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Darrell_4

Adding the indirect, and CHANGING THE AQUASTAT CONTROL, will allow the boiler to drop to ambient temperature when there is no call for heat. The relatively small standby loss of the indirect will more than make up for keeping a hot/warm boiler and internal coil at standby. Get your watch out and turn down your house thermostats...time how often the boiler fires for no other reason than to keep a hot boiler standing by just in cast you need hot water. Do the math...gph of the burner times burner run time times cycle rate per hour/week/month/year equals what your current setup is costing in real terms. After you put in your indirect, with it's own zone control, get your watch, turn down the house thermostats and time how often the boiler fires for no other reason than to make hot water...take a book with you to the boiler room and probably a drink and a snack...it will be awhile. Do the math...gph of the burner times burner run time times cycle rate per hour/day/week/month/year equals what the new setup costs in real terms to standby. It would be interesting for you to report back to us. Better yet, take a calculator and a Cabelas catalog with you to the boiler room. Calculate how long it will take to pay for the upgrade with the savings, and then figure out what cool toy you are going to buy.

Unknown

Hey Darrell

I actually did just that . Timed the boiler while it idled at 180 while I worked on the house . It averaged 5 min runtimes about every 4 hours ( only tracked for 12 hours , sorry ) with no call for heat . That's about 30 min a day , firing about .60 an hour ............. Let's call it .3 gallons a day wasted to maintain temp .

What you forget to factor in is the energy needed to raise the temp in the indirect , as well as the energy needed to raise temp in the boiler . And the times the boiler fires due the indirect's standby loss . Oh it does happen . I know firsthand when I wake up on a summer morning and the boiler is running to raise the temp on my Phase 3 ......... when noone used the hot water . Or when the same thing happens in the afternoon and night ( I'd guess 2 or 3 times a day , for standby loss ) . I might agree with you that you can get some saving with an indirect and cold start control ( it most definitley will give you more hot water ) . But the cool toy might just have to be a little cheaper than you think

Sissy

I agree with steamhead, but in terms of just cycling the boiler fewer times throughout the day to maintain temp automatically. The least expensive way, at least initially would be a beckett heat manager or similar control. I can't be sure but I thought I read a guarantee of 10-15% savings. Just a disclaimer I personally would never recommend a tankless boiler to someone concerned with efficiency.

Darrell_4

Nope, didn't forget, just didn't mention it...

The next thing to time is to run about 2 gallons of hot water...and see if the boiler fires or the indirect fires and for how long... it will also make a difference depending on how big the boiler is...mass, you know.

I have done many such upgrades...an indirect water heater zoned properly and changing the operating limit control so the boiler will completely idle with no call for heat. All of them that have done the math have paid for the upgrade in less than a year. You can also add a bit of ODR this way...or the Beckettt Heat manager.

One mistake people often make is to set the indirect directly on the cold concrete floor...there is little to no insulation on the bottom so you are heating the floor quite well. Set the thing on a piece of 2" insulfoam. If standby loss is driving you nuts, insulate the tank. My indirect, a WH-7 with a 1" fiberglass water heater blanket added to it, is coupled with a 25 year old Burnham 203 and during the off heating season it fires the boiler for less than ten minutes in 24 hours by the numbers.

This is what is so fun about this trade...there are so many ways to get to the end result! And so many, shall we be nice and call the existing systems starting points!

Terry Trommeshauser

setback control

consider the Tekmar 261 boiler control.
add a timer to the un_occ terminals and you get your setback.
you can also set domestic as priority firing the boiler up to full temp if the dom tank calls. (you can also NOT set priority, but the DOM water will not be hot if the timer has set back the boiler)

you can even use a T256 (has no dom priority or "excersize functions/option though)

joel_19

Viessmann

my controll has a setback feature for the indirect and i do set it back i have a non V indirect and the heatloss sure seems like more than 1/2 per hr as you can feel the heat coming through the top. I plan on adding more isnulation to the tank

heatguy

timer

Mark, Steamhead and Darrel got it right but if you insist just get a 115 volt timer with pegs and have a blast turning your boiler on and off and wasting energy you can also do it with 24 volt thermostat same idea.i would not go to a engineering web site and then argue with the people that are giving me free advice

Unknown

Thing is, is a combustion appliance maintaining 170 average temp will never, ever be able to be insulated to such a degree as to lose less heat than a closed tank set at 130 average temp.... assuming you take anywhere near as much care in insulating the tank as the boiler. That is, in standby loss, indirects should always be able to win. If that is not the case, something is wrong.

For cast iron, you still have the heat charge of heating up a cold start boiler that is lost without a purge control, on the indirect. That's significant for sure, and addressed in lower mass exchanger designs like the system 2000. But for most cast iron, that will reduce your DHW efficiency quite a bit in the summer when that whole charge of heat is wasted once the DHW demand ends.

However you also have to address the total destruction of the boiler's ability to use reset in the winter if you install a coil. the heating load dwarfs any domestic load by an order of magnitude at least: and you are reducing your heating efficiency making it run at high temp for all those demands as well as doing your DHW.

For that reason alone, the indirect makes sense. Your DHW production may be less efficient, but you can do a lot more with reset controls and lower temp operation and such on the heating end, if you aren't stuck maintaining the boiler at 170 average for DHW.

And with low mass or purge controls, the only downside of that arrangement are lessened as well.

GBriggs

Tankless Coil boiler reset controller

Hi Mark, Please look at the Honeywell AQ2000 controller. There is a standard application for this. By using the OCC and UNOCC periods you can have reset when you are not home and have a tankless boiler when you are. You can review this at www.customer.honeywell.com. Thanks Gary

Unknown

No need to change the aquastat

We did this job 2 days ago . They had an aquabooster in there . We changed it out to a 35 gal. indirect . We also used a Taco 3 zone relay with priority . We were able to reuse the dual aquastat , and wiried it so it's cold start AND has the low limit function working . Funny thing is the guy I worked with this day installed that boiler back in 1990 . It was in the days of reusing everything for a service replacement . Nowadays we start from scratch and give them all new from floor to ceiling .

Time to heat up the indirect was 18 mins 30 secs ( 70 degree temp rise ) . That was with the boiler starting at 180 . It'll be longer when it runs truly cold start .

Like I said , even with cold start controls I doubt you'll see a huge saving just by adding an indirect to an existing system . I don't have hard numbers to back that up , just MHO .

Unknown

I agree

that you need to integrate some sort of reset control to save the full potential of fuel . But for those homes where it's impractical to have a storage tank , I'd like to see a smart control that'll ramp down the boiler temp when hot water is not needed ( maybe with an add on to integrate it with a reset ) . Add that to a boiler that is sized for the home and can meet the hot water needs of the family with a coil .......... that has at least a few inches of insulation , and has a vent damper ? It won't be the best system out there , but it'll be an improvment on the really good ones

Mark_129

Anyone Used This? - Found What I'm Looking For

Anyone ever used the following? Looks like a fairly sophisticated unit. http://www.rdcontrolsystems.com/hotwater-1430-series.php

Viessmann also appear to have this capability, but I'm not going to get a 2nd mortgage to install them right now.

My current unit is a Honeywell Aquastat L8124A,C. Perusing Honeywell's website shows no advanced controllers that do what I want to do.

Any other options out there?

Recap: I have a simple, oil-fired boiler. It Provides heat and hot water. Now that it's summer, I want to "setback" the *boiler* temperature during periods of no hot water use, which are very well defined and known by me, the homeowner. My review of existing control shows no simple way to do this. I would rather not rig up my own system since I have better uses for my time.

Changing to an indirect is not an option...this year.

Mark_129

Better Yet...Honeywell

http://www.forwardthinking.honeywell.com/products/water/water_products.html

Available from Grainger and other sources...finally...appears to be what I'm looking for. Anyone installed one of these? Any feedback? FYI - other mfrs such as Lochinvar appear to offer similar capabilities.