Outdoor Reset Controls- Worth it?

**Greatwhitewing** · 04-25-2011, 08:48 AM

Sounds like a dumb question but one thing I have learned about this business is there are opinions on every possible side of a subject. I though reset was a no-brainer.

Plan on replacing 20+ year old tankless oil boiler by Burnham. Going with indirect or storage HW. Not really a thread on merits of tankless system.

I am getting quotes for systems using Peerless, Buderus, Visemann, Weil-McClain from 4 different companies (some are quoting two different brands). TWO are oil companies that do boiler replacements as a side business and two "so-called" energy companies who don't sell oil.

Oil guy 1- Reset controls suck, not worth the cost
Oil guy 2- reset are great
Energy company 1- Reset controls suck, not worth it
Energy company 2- reset controls are a good value.

What am I to believe?

1800 foot home in MA, pretty well insulated. All are going with smaller capacity boilers then present. Oil companies not doing heat loss calcs, one energy company doing it as a matter of course and the other will do it when they get the order.

Thanks
homeowner

**jadnashua** · 04-25-2011, 09:06 AM

Unless you can do your own heat loss, forget those that would just replace what's there.

An outdoor reset controller does two things: lowers energy use, and makes the house more comfortable. A boiler that is 'right' sized and one where you can adjust the supply temp to the needs, means that it can run longer. Longer runs mean fewer cycles, fewer cycles means the whole thing lasts longer and provides more even heat, and, wastes less fuel at the end of the burn. Ideally, the boiler would run full-time, but not sure if any of the oil burners are modulating like what's available with gas, but a smart controller would shut the burner down so that the house just reaches temp as the water temp reaches the low point where it's useful for radiation.

**Tom Sawyer** · 04-25-2011, 12:20 PM

Well now here's a little tip. Anyone telling you that reset controls suck is a guy that should be selling fries at McDonalds. Not only do reset controls save a substantial amount of money they are also reqired on all new equipment sold after 2012. Buderus will give you the most bang for your buck when you figure in the overall superior quality of the unit. Go here and you can get an accurate size. http://www.nora-oilheat.org/site20/index.mv?screen=home about half way down the page is thier ffa calculation program. It's free, it's dead on accurate and it allows you to make comparisons as well as projections.

**Dana** · 04-26-2011, 08:30 AM

There are a couple of a downsides to outdoor reset control in oil-fired systems with low-mass radiation such as fin tube:

1: As the output temp drops the boiler will short-cycle since the boiler's output BTUs outstrips the radiation emittance at those temps, resulting in diminished boiler efficiency and higher maintenance costs

2: When the temp of the water returning from the boiler is lower than ~140F there's a risk of acidic destructive condensation on the flues and in the boiler's heat exchangers, resulting in shorter (sometimes MUCH shorter) boiler or flue-liner lifespan, so there's a lower limit of ~155F on the boiler output temps. Then when you consider that most systems are over-designed for the actual heat loads, and the radiation could actually support the load during the coldest 1% of heating season hours with water temps 140-150F, there's no real advantage to the outdoor reset approach in such systems.

A better approach for many/most high-mass oil system is to use "smart" heat purging controls on the boilers that "learn" the system and anticipate the end of thermostat calls for heat. This type of control turns off the burner prior to the end of the T-stat call and allows the stored heat in the boiler's thermal mass to finish it off, parking the boiler near it's low-limit rather than halting abruptly at full temp, abandoning a lot of heat in the boiler. On a new call for heat the boiler is further purged of heat down to the low-limit before firing up the burner. This ends up maximizing the lengths of burns utilizing the thermal mass of the boiler at a high hysteresis, reducing the number of burns (minimizing efficiency-robbing short-cycles), and by parking the boiler at a lower temp between calls standby losses are also minimized. The Beckett Heat Manager or Intellicon 3250 HW+ are both retrofit controls for heat-purge operation. Some newer boilers have similar controls integrated into them (eg. Energy Kinetics System 2000).

The resulting difference in as-used efficiency with boilers oversized for the space-heating load using a heat-purging approach to boiler control is significant. See system in this Brookhaven Nat'l labs comparative test.

There is no "right sizing" of oil-boilers to any but the leakiest and least-insulated 1800' homes in MA. Even the smallest oil-boiler would be 2x oversized for my 2000'+ circa 1923 antique bungalow (complete with antique windows) in Worcester. Using fuel use against heating degree day weather data for your zip code (or the K-value sometimes indicated on oil seller's billing) and your exising's systems efficiency ratings it's fairly easy to sketch out an accurate upper limit to your design condition heat load. (Or use that NORA FSA calculator Tom pointed to- which is a bit buggy and it takes some insight to use well/properly.)

Better micro-processor based outdoor reset controls will use the boiler's mass and adjust the hysteresis to limit short cycling, but if your radiation is capable of delivering design-day heat at or below the return water min-temp for the boiler (often the case), there's no efficiency or comfort advantage to using them. The old school presumption that you need 180F water at design condition has very little basis in measured reality. Sloppy (or no) heat loss calculations on installations-past tend to err heavily to the high side, and 2-3x oversizing of boiler & radiation over reality is sadly the rule rather than exception in MA. At 140F radiators & baseboard put out somewhat more than half their 180F numbers, so if you're 2x oversized or more (likely), that means you'd never need more than 140F water OUT of the boiler, which is already at the non-destructive limit for the return water, and outdoor reset buys you exactly nothing. (Or rather, nothing efficiency-wise over other intelligent approaches, and possibly higher maintenance costs depending on how well it limits the numbers of burn cycles.)

Outdoor reset on the radiation-loop temp only (not boiler temp) using smart mixing valves (eg Tekmar 360) and PID algorithm thermostats can result in stabler indoor temps and higher comfort, that also results in a modest reduction in fuel use, but also increases the amount of pumping electricity used. It can be cash positive or negative depending on the particulars, but it IS more comfortable. In those situations the low-limit on your heating loop temps depends somewhat on your radiation type. Big-old radiators, cast iron baseboard, or radiant floors all work fine down to sub-100F temps, but the output of fin-tube is too non-linear to work well below ~120F.

**Greatwhitewing** · 04-26-2011, 01:45 PM

Dana, wow ton of info in there. I know I don't get it all but first of all would the Buderus 115ws/3 be considered a high or low mass boiler? Is the logamatic controller a "smart" system anticipating shutdown etc?. Brochure is a little thin on hard data. lots of marketing speak. I am venting through my existing chimney. So about 85k of heat.

Wouldn't even a reset only down to 155 be a big savings over time?

I have baseboard hydronic heating. My old system at 0 degrees outside runs at my SWAG estimate of 75% of the time. Thing scares me when it works that hard..lol the old system says about 96k output on the nameplate, no idea how accurate that is.

For MY situation, as you know it now, would reset be wise investment for me? I should have my detailed heat loss analysis in hand by this week.

Would like some really sound reasoning when I tell my contractor if do or don't want reset and/or intelligent controls.

One more question- would you use circulators or zone valves? Three current zones plus one for potable and maybe one spare since the boiler room will now be colder!!!!

John

**Dana** · 04-26-2011, 03:06 PM

At 155F design-day temp requirements outdoor reset buys you nothing, efficiency-wise since you can't operate with return-water temps below 140F (a modest 15F delta-T from 155F) without compromising your chimney or the boiler itself. You may in fact have to set it up to run higher temp output to keep it in the safe zone. Outdoor reset only buys you efficiency when it can lower the average operating temp of the boiler (and to some extent the distribution plumbing to the heating system) from say 180F down to 155F. I'm betting that if you've tightened up the place and insulated since the initial system was installed, the baseboards won't even need 155F water, and a different approach should be taken.

The Buderus 115/3 WS has 8.7 gallons of boiler water, making it moderately high-mass. I've never really looked into it's control methods, but Buderus boilers are better insulated than most, and are one of the more sophisticated designs. Still at 85K of output it would be nearly 3x oversized for my 0F heat load, and would run well under it's rated AFUE if run without buffering & heat-purge.

And I GUARANTEE it's more than 3x oversized for individual zones in a 3-zone 1800' "pretty well insulated" home anywhere in MA (or Whitehorse Yukon, for that matter.) At 8.7 gallons you're looking at ~73lbs of water (call it 80lbs to account for the thermal mass of the iron too). With 85KBTU/hr of burner output behind it only takes ~2.5-3 minutes to go from 140F to 180F at ultra light loads, which would be an efficiency-robbing short-cycle. And there's no cheating the physics with a clever control algorithm- you either need more mass, or more radiator in the system. BTUs in must equal BTUs out, or the temp will rise.

What's your zip, and how many gallons of oil/year do you use? Assuming you don't supplement regularly with wood stovers or something it's pretty easy to come up with a quick & dirty (yet still fairly accurate) whole house heat load to do a sanity check on the pro's numbers. Unless they did a lot of measuring it's just plain easier for them to err to the high-side on most of the assumptions, and end up with numbers 2x reality or higher (in fact it's common.) That's why the NORA FSA calculator is so useful- it uses the boiler to measure the heat load.

Even the smallest oil boilers out there have 55-60K of output (more than 1.5x my heat load at 0F), so if microzoning you may in fact be better off with an Ergomax E23 (or the TurboMax equivalent) for the indirect, and plumb it as the hydraulic seperator, slaving the boiler only to the tank's aquastat. That would add 26 gallons of mass to the system, and it would be involved with EVERY boiler burn, not just a hot-water heating burn. If you run the buffer tank at 140-145F water, the heating system and distribution system gets 140-145F water, and the boiler output temp can be 160-170F water and still run at it's max efficiency, since it's the RETURN water temp that determines it's raw combustion effiency. With heat-purge control and cold-start, the standby losses would be less than if you were running it at 155F unbuffered since all cycles would end with the boiler closer to the tank temp, not it's operating full-fire temp, and cold-starting allows the boiler to cool off to room temp if the time between calls for heat are long enough. Since 140F is just fine for a domestic hot water storage temp, it works just fine as a single-temp system, and gives you far more hot-water output than a typical 40 gallon gas-fired unit would (due to the boiler's bigger burner.)

A good discussion of the whys/wherefors of buffering a multi-zoned system to limit short-cycling losses lives here:

http://harscopk.qa.jplhosting.net/up...e962d0ef7d.pdf

It's written from a large facility perspective, but the math works the same for multi-zoned residential systems as well. Ideally you'd get the mininum burn time up to 10 minutes or so, but even 5-6 minutes is HUGE improvement over a 2.5 minute min-cycle.

What type of baseboard do you have and how much (zone by zone, if you can measure it)? Combined with a better heat-loss estimate that would tell us roughly what your design day water temp needs to be.

Separate circulators vs. zone valves is a system design detail that isn't amenable to "design by web-forum". There are advantages and disadvantages to both approaches, and which makes the most sense depends on flow requirements & duty cycles, etc.

FWIW: In my own home I micro-zoned it to deal with system balance issues, and ended up running a reverse-indirect as the central buffer/hydraulic seperator. I have it set up for 130F max temp (it kicks on and calls for heat at ~122- 123F), and used a low-temp tolerant gas-fired on-demand hot water heater as a modulating low-mass boiler. With all zones calling for heat with someone taking an endless shower the burner never hits more than 50-55K of output, and everybody is happy. I also have a drainwater heat recovery heat exchanger delivering 20-25K to the incoming cold water stream for the hot water coil, but we're still only talking 80K max input, ever. During heating-only calls it's never putting out more than ~42K (when serving the fan-coil on an air-handler zone). With the modulating burner the min-cycle is about 11-12 minutes only relying on the 7-8F of hysteresis on the indirect's aquastat, but that hysteresis could be increased if need be, with some retrofit controls. But with the modulating low mass burner it wasn't necessary.

**Greatwhitewing** · 04-26-2011, 03:26 PM

WOW again, have you ever written a book in an afternoon? lol

Really appreciate the time.

I can measure my rads but there are older "Suntemp" and some newer "slant-fin" on the additions. If I did a detailed floor plan with windows, rads and footage could you design a suggested system? Or is that beyond the scope of an internet consultation? I am in 01569 zip just 30 minutes south of you.

Tell me if this summary is wrong. The Ergomax acts as a thermal battery first to make the boiler run longer and store the excess heat to be used as needed by the zones. Would a tank in tank potable storage tank work similar?

If it's not against the forum rules you can contact me offline if you interested in bidding the work.

**tk03** · 04-26-2011, 06:54 PM

You may want to consider the new MPO-IQ from Burnham. It is also a three pass oil boiler which goes down a little lower on DOE output. The new IQ control will give you pre-purge which starts the pumps only if the boiler is above 140f. The boiler does not fire until the timer runs out, you drop below 140f or the indirect calls. Also will run two pumps, that would be two heat zones or a heat zone with indirect water heater. It does off outdoor reset plug in control, trouble shooting codes, even reads codes on the oil primary control and displays it on the aquastat display. I have used the optional display on occasion. The best feature is the boiler can accept 100f return water temperature which helps increase system efficiency, comfort levels and reduced fuel consumption.

**Dana** · 04-27-2011, 09:06 AM

Being an electrical engineer (with a hydroic heating hobby), I tend to think of buffering reverse indirects more like capacitors than batteries, but they're similar. It's taking the pulsed on/off output of the boiler and averaging it, and letting the rest of the system use that heat at a variable rate.

In Uxbridge you can use 0F as a 99% design temp, or +4F as a 97.5% design temp- either would work. Even 10% oversizing would allow the system to keep up even if the average-temp for the day was -5F, and the smallest boilers out there are still likely to be around 2x oversized for you heat load. Even is sized EXACTLY for the 97.5% number the only time it might slip a few degrees is in the pre-dawn hours of the coldest days of the decade, and it would catch up quickly as outdoor temps rose (even quicker with some sunshine to help out.)

Old schoolers in this area often put a finger up tot he wind and say "lessee, 1800 feet times 35BTU per foot gives ya 63K...", and that is RELIABLY oversized for even less-well insulated places, and usually more than 2x oversized for better-insulated tigher homes. Your true heat load is probably less than 35K. (But with oil-use numbers we could nail it pretty much dead-on.)

Assuming the SunTemp and SlantFin are all ~ 2-2.5" deep from the outer tin to the wall, you're looking at something like 570-600BTU per running foot at 180F, or 300-325BTU/foot at 140F. See: http://www.ntsupply.com/files/products/S600-1.pdf and http://www.slantfin.com/documents/675.pdf First tally up the running lengths of each zone seperately, then total it all up. Then divide 35000BTU/X-feet of baseboard to come up with a max BTU/foot number, from which you can infer the water temp necessary to get that heat into the rooms using the Slantfin & Suntemp specs. If that number is 140F or even 145F, there's no efficiency advantage to outdoor reset (even with a low-temp tolerant boiler like the MPO-IQ), only a slight but perceptible comfort premium that comes from being able to run down to ~120F out to the fin-tube at low heat loads. Low temp tolerant boilers are still running ~140F into the heat exchangers, by variably mixing in boiler output with the return stream when the return temp is lower than 140F.

The output of the MPO-IQ84 is still likely to be more than 3x oversized for any single zone, so it too will short cycle on zone calls unless it's buffered. But it would allow you to turn down the temp on a buffering reverse-indirect to about 120-125F if there was enough enough baseboard to deliver design-day heat at that temp. Smart boilers do a lot, but they still can't beat the fundamental physics, and a short-cycling boiler at 3x oversizing will be losing double-digit percentages of actual operating efficiency below it's AFUE rating.

**Dana** · 04-27-2011, 01:37 PM

You may find this thread from a guy up in Andover MA with a situation useful to review. Similar heat load, situation, & system design problems.

Like him, if you're anywhere near N-Star's gas lines in Uxbridge it may be worth paying for a gas service drop. Modulating gas burners go much much lower than oil burners making it easier to right-size them (at least for the whole-house load) giving both more and better options. The delivered price per therm on N-Star's grid is well under $1.50/therm. (My last bill it was ~ $1.35/therm, up from a about a buck shortly after the economy crashed, and down from about $1.50 before the Allegheny shale gas started to come online.) At $1.50/therm it is about half the price per BTU of $4 oil. (When did you last see $2 heating oil?) If say, you're using 750 gallons of oil/year @ $4 that's $3000. Running at the same efficiency with gas you'd be burning ~1050 therms, for ~$1600, a $1400/year savings. Even if it costs several grand to run the line to our house, odds are pretty good that it'll be worht more than $10KUSD in (after-tax!) savings over the next decade.

Twenty years ago gas and heating oil were roughly at parity in $/BTU in New England (with oil being slightly cheaper on average) but the short and mid-term prospects for oil are much bleaker than for NG. If there's ever a right time to switch fuels, it's before investing in a boiler.