considering an upgrade to my aged oil fired boiler, currently 240000 btu output, supplies 12 zones via 2 Taco ZVC EXPs and also provides my domestic hot water. Am considering a new boiler (same output) Buderus vs two smaller Buderus boilers with a Taco 702 1 controller. The cost difference to me is about one thousand dollars more for the two boiler set up and i am being told to expect about a one third reduction in my yearly fuel consumption. Anyone with experience with the two boiler set up and its savings, comments would be appreciated.
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What are you heating, a shopping mall? That is a LOT of boiler. What is your heat loss?
Nah not a mall, just an inn in upstate NH about 200 yo house, original windows and certainly lacking in insulation. Taking care of that issue as i redo each area of the house. Have 12 zones, one is DHW, 1 basement and 2 crawlspace zones with small Modines. Temps are not infrequently seen to be 37 below. The existing boiler is 240000 BTU for the 4000sq ft of living space and does the job, just a fuel hog.
Ok, sounds better. What types of emmitters, rads or baseboard?
Try getting a copy of slantfins heat loss and baseboard sizer.
Do a heat loss and size the boilers to that . Personally, in your case, I would use 2 boilers. Most of the time only 1 will run, and you will have a backup.
Nothing like cold customers to ruin your day.
Before you touching that heating system, fix your envelope as fully as you can.Then, get a bead on where you are at with insulation, etc. Load calcs, measuring load, whatever.. but do not size a system for your current situation if you can by any means improve the situation. You could potentially cut your boiler output requirements by half or more, and then all you do here is a real nice upgrade on a real oversized and thus inefficient system.Envelope first, always, always, always.-------------------------------------
-=Northeast Radiant Technology=-
Radiant Design, Consultation, Parts Supply
http://www.NRTradiant.com
understood and appreciate your comments. The improvements will be years in the making as we are basically going room by room. What i am basically looking for at this point is whether or not running two small boilers is more fuel efficient than running one larger boiler of comparable BTU.
Heat loss calcs are currently being done on a section of the inn that is currently gutted and going to be heated by radiant floor heat. Some things will not be done, i.e. replacing the historic sash (instead looking at the "invisible storm window" and other options)
Thank you
Then yes, two smaller boilers will be much more efficient.But I would encourage you to dedicate any cash you can, including any money you would use for a heating system switch, into improving the envelope, even if it means paying others to do the work so it gets done faster.Figure out what cutting your heating bill in half or better would do for you. You may find it makes a lot more sense than waiting.-------------------------------------
-=Northeast Radiant Technology=-
Radiant Design, Consultation, Parts Supply
http://www.NRTradiant.com
Hey NRTRob, could you elaborate on the "much more efficient" 2 boiler system?
I've just inherited a post on my churches' building committee...we have a dual boiler system, and as near as I can tell, both boilers and both boiler circulators seem to run all the time in heating season, which would seem to be less cost efficient than a single boiler system.
It's a little hard to tell what's going on with the system, as the building once had radiant floor heat, which was abandoned in place and replaced with baseboard radiator zones; it's a bit hard to tell what's active and what isn't. The system does have a modern Tekmar dual boiler control with outdoor reset, but I have a feeling that it's doing little more than acting as a set-back and zone controller (and summertime pump exerciser), since the minimum boiler temp. for the Weil-McLains is set to 180, and it doesn't seem to rotate the boiler firing. Would I be correct in thinking that unless I have a modern Modulating/condensing boiler that can use the lower settings on the minimum boiler feed temp. control the whole minimum temp./outdoor reset function of the Tekmar is pretty much bypassed?Thanks....
Set the minimum to 150 and the max to 180. That alone should save about 10 percent of your fuel consumption.
Well.....
Although I have many years in construction, and a passing familiarity with most mechanical systems, I'm not an HVAC mechanic, so I have to tread slowly making changes in a system that at least heats the building ;-)
I did wonder at the fact that the "minimum" setting on the Tekmar control goes from (IIRC) about 45 up to 180 degrees, while, at least as near as I can tell from a quick Web search, the typical minimum recommended for basic boilers including the installed Weil-McLains I'm working with is 180 degrees.
As I get more comfortable with my understanding of the installed system, I can easily lower the minimum on the Tekmar to 150...but if I am rmrmbering the box correctly, the Tekmar does not have a maximum control setting....so, where might I set that?
Thanks for the help....
No max temp on that control, it is set by the heating curve.
Assuming these are cast iron boilers, set the min to 150. Trust me. 180 is usually the max temp. Don't always believe everything you read on the net. :)
Stats should turn on the zone pumps and send a signal to the tekmar through an isolating relay at the same time.
tekmar will turn on the system pump.
The boiler contacts on the tekmar should pull in a relay that will turn on the boiler pump and the boiler at the same time as required.
I'm obliged for your expertise...
perhaps you can continue....the dip switch on the Tekmar is set for "permanent heat demand" and I'm pretty sure there are no Tekmar 10K zone controls in the system (all the t-stats are Honeywell or Barber-Coleman), so if I'm reading the Tekmar D 252 data brochure right, that means when the system is powered, it's going to keep the fires burning to give 180 degree water minimum, and higher if the outdoor reset calls for it.I don't think there is a timer for unoccupied setback service...can you suggest a good brand/type of timer to use for this?The installers schematic calls the outdoor temp. sensor a "DA sensor ?
and there are "PAM" ? modules in-line with the boiler firing terminalsI'd love to know why they installed a Tekmar with DHW controls on it when the plant has stand alone gas HW heaters, but that's probably lost in the fog of history....
Follow up.....the whole setup makes me want to go out & buy some data logging meters., or maybe software for my old Toshiba, so I can see what the system is really doing....
Use isolating relays from the zones and apply 24v to terminals 1 and 2 and put the dip switch to external heat demand. The boilers will only run when there is a call for heat.
I don't like setback in churches. Would rather keep it constant.
Check where the outdoor sensor is. Should be on the north side of the building, but at the very least, out of the sun.
NO idea what a PAM is. Some sort of relay for the pump/boiler?
DHW was not optional if you wanted 2 stage.
Put the minimum to 150. Trust me. Somewhere on tekmars site IIRC, it talks about minimum temps.
BTW nobody makes a better boiler control than tekmar.
Edited 2/21/2007 11:07 pm ET by rich1
Thanks very much ;-)The outdoor sensor is well located....along with at least 2 other abandoned ones. I hate it when little stuff like that doesn't get removed when the new system is put in."I don't like setback in churches. Would rather keep it constant"....well, we are a very small church, mostly elderly congregation, so there isn't a great deal of activity outside the Sunday service. Seems like a setback would be good. Why don't you like them?I was re-reading your replies---"The boiler contacts on the tekmar should pull in a relay that will turn on the boiler pump and the boiler at the same time as required."
Every time I go in the furnace room, the boiler pumps are running, whether or not the boiler is firing....and there is no "cool down re-circulation" with this model Tekmar.BTW, there are rotary timer switches in the rooms which, I'm told, act as the t-stats....Seems like an odd way to do comfort control to me....All in all, it seems like this installation needs some tweaking to get it to run as efficiently as possible, and perhaps some significant upgrading...I see from Tekmars' web site that they have some nice networked controls now, and some dual boiler controls w/out DWH included....maybe one of the parishioners will leave a bequest....Thanks for all your help
".. two smaller boilers will be much more efficient."
Rob, that is a very broad stroke that is many times incorrect. Without knowing the type of boiler(s), the type of controls, how the boilers are fired and/or staged, what is the part load efficiencies, stages of capacity, etc., etc.; the only "right" answer is "maybe".
One large condensing/modulating boiler will often (if not always) operate more efficiently at a very low firing rate than a smaller boiler operating at a higher firing rate. Then again, a large cast iron boiler with limited modulation will almost always be less efficient than a smaller similar boiler operating near or at 100% capacity, and will cycle less.
The ability to turn down at 5 to 1 ratios or better with modern boilers, and the ability of those boilers to operate more efficiently in condensing temperatures is making the old multi-boiler system less attractive than ever before. Presently, unless some level of redundancy is required, I prefer to design systems around larger, single mod/con boilers than how I would have designed the same system 15 years ago, that being with multiple, staged boilers.
Hi Tim,If it's not a mod/con boiler, two smaller boilers of equivalent output to one larger boiler will always be more efficient. Not always more cost effective, but always more efficient.If it is a mod/con, sure, you get to run it at a lower fire if you oversize. You also typically double your minimum modulation rate, which is not a terribly efficient way to run the boiler, and I would think it's slashing its expected lifespan as well, unless you're also adding some major buffer capacity.But I'm speculating at this point, admittedly, since the mod/cons are too young to test that lifespan out. Personally I'd rather keep the lowest min mod rate I can, but I can see your arguement too... you may be interested to know though that Tekmar is releasing the 265 staging boiler control next week which will stage two mod/cons and do "paralell staging", where it will attempt to keep both boilers running at low fire and ramp them up together. I'm pretty excited about that, personally!-------------------------------------
-=Northeast Radiant Technology=-
Radiant Design, Consultation, Parts Supply
http://www.NRTradiant.com
"..will always be more efficient"
Maybe it's just the conservative Engineer in me, but "always" is a word I stay away from in general discusions along these lines, though I can think of no contradictory examples.
"you get to run it at a lower fire if you oversize..." I wasn't referring to an improperly (i.e.oversized) sized boiler, but rather one sized to meet the design load. In a house, one boiler is all you need, sized for the worst case design load, accounting for some safety margin. Recently we saw -15 degrees in N. IL, and while that is 5 degrees below the ASHRAE design conditions, most Engineers and/or designers in the area use -15, not -10. CYA in heating is not a bad idea.
"You also typically double your minimum modulation rate." Is more than 5:1 turn down ration ever necessary in a house? No way. The load will vary all over the place and yes there will be loads less than 20% of the boiler capacity. However, to go to the trouble and expense to install two 50% capacity boilers to acheive a 10:1 turn down ratio, or for any other reason that I can conceive is wasteful and not advised.
In an application in which the needed capacity is beyond what is readily available in a single package that will fit through the door or that you can get down the stairs, multiple smaller boiler installations are the only option. In instances where redundancy is necessary or highly recommended (schools, nursing homes, hospitals, hotels, etc.), two 66% to 75% capacity boilers is my preference for the system. A single, 2/3 capacity boiler will provide 100% of the necessary heat, 90% of the time. Controllers like the Tekmar that you mentioned (they are copying what others have successfully done in the commercial market years ago, BTW) to operate two (or more) mod/cons at as low fire as possible maximizes the thermal efficiency and ROI for the more expensive high efficiency boilers. IF operatng these boilers in a manner that increases their efficiency actually does shorten the life of the boiler (I don't beleive this to be true, though like your speculation, it is just an opinion), I think its a wash. Save more money over a shorter period of time or less money over a longer period of time? Do you want to get to the green in two, or three?
Edited 2/26/2007 1:36 pm by Tim
Tim, I would kill for greater turndown in most homes out there today. Most regular size homes are coming in with max loads that are only 50% of the capacity of most of the small range mod/cons available out there... they hit the mid mod on these boilers halfway to design, quite often, and cycle the rest of the time. Granted, that's not the situation we're talking about here.But if I'm working on a big one, with a 150k to 200k load, we'll go with two boilers. Because then we can modulate all the way down to 18-25k load periods, the vast majority of the time, and get redundancy which on a big home is probably a good idea.. they aren't going to heat the house with "dragons" very easily, right? But it generally takes quite a big home to hit those numbers, and would be the kind of project where redundancy would be valued... efficiency is a nice side benefit though.You're probably right though, the 20k range in mid mod values on a house that big probably doesn't represent a very huge temperature band that represent "unneeded cycling" for one boiler vs two... sometimes, I am a bit too much of a purist. You caught me ;)
-------------------------------------
-=Northeast Radiant Technology=-
Radiant Design, Consultation, Parts Supply
http://www.NRTradiant.com
Like one of the other posts said I would seal up as much of the envelope of the dwelling as possible before sizing the new boiler. If you have an old boiler the new one will be more cost effective. If you want to run your domestic hot water off the boiler I would add a water heater so you are not using 240,000 btu during the warm season. Lots of luck.
"If all else fails, read the directions"