Economics of Retrofitting a Buffer Tank

stan_12

Tim,

Cutting back, or eliminating night setback sounds like the way to go. As for outdoor reset, I do have an old analog Enertrol reset originally installed on the boiler. I override it with a timer controlled relay in the morning to let the aquastat kick in to its high limit at 4am, and then, after 7am let the Enertrol work during the day until about 4pm, going back to aquastat hi limit (until 11pm) to ensure my wife doesn't complain about heat when she gets home. This kludge is all I can afford to do. When weather gets too warm I have to manually go down and turn up the Enertrol to stay above 140 deg return. I'd like a nice new computerized reset ( like modcon?), but right now I can't pay a contractor, ( and I won't diy anything that wires into gas valve) without an income of my own! However, you've made me feel better knowing I'm only losing an extra 1.5% with the 205 deg. hi limit.

Thanks to you Tim, and all the above pros above that responded so gently to an HO. I wish you guys were in my yellow pages 15 years ago when I needed my boiler replaced!

stan_12

DOE Propaganda Snows Homeowner

I fell for the propaganda of US DOE and state energy agencies 15 years ago, and replaced a 120000BTU boiler with 100000BTU (both atmospheric) which would be more than enough at outdoor design temp of 0 deg for my home (BTU loss 65000BTU using Weil Mclain worksheet at 0 deg. design temp NY metro). With new boiler aquastat at 180 deg. house never heated up in morning after 10 deg. night setback from outdoor temps in single digits to teens. New boiler aquastat never had a problem reaching 180 deg. even at 0 deg. design temp, and cycling. However, boiler never got to fire full time at design temp. Therefore, I assume boiler was still somewhat oversized from the point of view of a pure design temp calculation.

Not being a hvac pro., (just an unemployed computer programmer) but having had to deal with some engineering overhaul work with strategic nuclear submarines many years ago, I put back on my engineering hat, and I believe I've figured out my problem. The I-B-R output of the baseboards was assumed to be 170 deg. average for the boiler sizing calcs. I did a numerical integration of the temp. rise with my 100000BTU boiler installed, and discovered that ONLY at, or below outdoor design temp. would the baseboards ever reach, or exceed 170 deg. With the original 120000BTU boiler, the heat exchanger had enough mass to hold 170 average I-B-R in the baseboards during the aquastat dead zone. My current boiler reaches only 150-160 deg average. That's why the builder's heating contractor "oversized" the boiler. It wasn't oversized, it was RIGHTSIZED! (Why didn't the worksheet have a line item to multiply final boiler net BTU by 1.2?...to punish layperson for using pro. worksheet;-(? ) It took me 13 years to figure this out! To compensate, I have the aquastat cranked up to 205 deg. My home is properly heated now, but I'm paying for a raging stack temp. There is an article (List #304) answering the question of when to use a buffer tank. None of these pro. answers address retrofitting a buffer tank into a standard residential hydronic zoned baseboard system with an undersized boiler for the purpose of lowering aquastat hi limit. My question is: Would installing a buffer tank, (thereby increasing system thermal mass) which would permit me to turn the aquastat back down to 180 deg., pay for itself in 5 years or less? Would it be a better investment to just replace the boiler with a properly sized condensing unit, and forget the buffer tank?

Darrell_7

It sounds like you may be just a tad undersized for the peak demand you ask of your equipment after night setback. Your boiler may be sized just fine for your structural load but takes a hit at peak load when trying to do it all at the same time...in real world terms those are two different numbers. Total capacity to deliver btus at a steady state point of operation vs. the ability to sustain delivery of those btus under the influence of the controls and existing delivery system.

If I understand you your boiler limits off and then before the burner fires again it is "stalled" and cannot recover from the lower temperature. The factory limit is probably a 20 degree fixed limit. The trick is to get the boiler to not to go out on high limit which puts you into a control lag situation from which you cannot recover. To accomplish this you raised the limit very high.

Other "indian tricks" to try:

Get your boiler cleaned. This is almost always overlooked and should be done first. I can get 5-10% out of many boilers just by cleaning them and tuning them properly. If yours is sized to squeek then 5-10% is more than enough to make the difference. A good cleaning and tuning will require some professional expertise with proper instruments and experience. This "modification" should be done every year anyway, (your manufacture recommends it), and, as such, won't cost you anything that you shouldn't be paying anyway.

Make sure your baseboard is clean of lint, pet hair, crayons, stray socks, etc and that the carpet is not rolled up under the fin-tube. It will not work properly without full air flow. Make sure that the bottom of the baseboard is open and that the new carpet and pad you put in doesn't narrow the gap too much. This "modification" won't cost you anything, and you'll find all sorts of cool things.

Check furniture placement...seems pretty obvious, but that great big leather sofa blocks alot of heat...and you are on the hunt for a few good degrees.

Do not setback at night. You probably don't save anything in the way of gas...in fact, it may be costing you more because it stalls your boiler in the morning. It is a far easier load for your boiler to bear to maintain a given temperature than to try to recover from a deep setback. And ten degrees is a deep setback. This "modification" won't cost you anything.

If you really want night setback, then stage the morning set-up times so that the boiler only sees a portion of the total morning load at a time. Probably even a 15 or 20 minute staging scheme will work. Zone 1, then zone 2, then zone 3, etc. If one of those zones is a hot water heater then you need to count it as a major demand, (100%), on your boiler...bring it up first, and then you'll have a hot boiler to get started on heating the house. This "modification" won't cost you anything.

If you are using domestic hot water priority in conjunction with setback, turn off the priority and stage the heating set-up. DHW priority is designed to allow a marginally sized boiler to put all of its efforts into making Domestic Hot Water and shuts off the heating zones. Good idea, except when the DHW is satisfied and then the heating zones all come on at once, especially from set-back, it hammers your boiler and it can't keep up. This "modification" won't cost you a dime to try and is fully reversible if I'm wrong.

Replace the factory limit with an adjustable differential and set it at something less than 20 degrees, say 10 degrees, so that the control lag is shortened, effectively making the burner run on limits from 180-170 instead of 180-160. This modification runs the risk of short-cycling and may increase your cycle rate and actually keep the boiler in the "dead-zone" of the limit more than before...or it might work perfectly. The mechanics of your particular system will determine which and the modification will cost you about a hundred bucks for a limit and the time to fiddle with it. It sounds to me like you are handy enough to make this modification without professional help.

A buffer or tempering tank will give you some "fly-wheel" effect, lengthening out the burner on-time before it goes out on high-limit. If your boiler is, in fact, too small then a buffer tank won't help...may even make the situation worse, but, it sounds to me like you have enough boiler, just some control/delivery issues. If you decide you need more volume then it should not be terribly expensive to do this modification...but some re-piping will be required, probably have to involve a contractor.

I'd try the "indian tricks" first. I bet you can get it to settle down.

hot_rod

lower the heat load

as much as possible by adding insulation, weatherstrip, etc. Have a blower door test done to see where the home is leaking. I'd spend my money on the structure before adding a buffer tank or making piping changes.

Zone off unused rooms? Merely closing the damper on the baseboard will limit the output in a room.

hr

tim smith

I think you would be better off limiting your setback at night

say 4 degrees, add a Tekmar outdoor reset control set w/ a minimum of 140 water temp and a max of 205 based on your data, O degree design temp and 62 warm weather shutdown. I feel this would provide you with best savings and comfort level short of changing to modcon. A modcon works great but $$ to install and depending on age of boiler, you may have many years left on that boiler. I would not be extremely concerned about the extra 25 degree stack temp, this may change your efficiency about 1.5% but not the end of the world. Also the reset control will provide you the lower operating temp efficiencies most the time when we are in the above freezing temps. Water temps will mostly be in the 150 to 160 range most the time. Ps, another question, is this multi zone or single zone system. If multi zone, is it piped primary/secondary?? Good luck and good day..... Tim By the way, + all of what Hotrod said too.

stan_12

I like those "Indian Tricks". Except one to change out aquastat for smaller dead zone. The Honeywell aquastat installed was part of a "contractor package" and is basic type with fixed dead zone. My DHW is from a separate water heater which H*me D*p*t subcontractor installed smack up against aquastat?!@ Can't change aquastat until next water heater replace.

As for heat load, I did, indeed, run a blower door test. My biggest air bypass turned out to be the window air conditioners. I had previously thought I did a good job of sealing them, but I was wrong. Now there's plenty of styrofoam and heat shrinked window wrap over that, with outdoor covers well secured. That was a big help. I need to get to put more insulation over the living room ceiling too, but I can't do much about the fact that two of the living room walls are entirely 40 year old glass from floor to ceiling. I put a double glaze on them, to help, but that's no substitute for sealed modern double glazed argon filled windows. Cleaning the baseboards is done every fall, but, your right, I should clean them again.