OT- New condensing boiler water temp....minimums
I searched and googled and no luck, so I'm posting here as it seems there are a few HVAC people here not to mention the just plain all 'round smart ones too.
Installed Jan 07 in the shop, a new Peerless Pinnacle modulating boiler. Stainless mostly, and designed to run
at low temps and in the condensing mode.
The local plumb/heat supply people where I got the unit have no clue. They say "Run it about 160-175"....which totally defeats the condensing phase and latent heat extraction at phase change.
If I'm saying that correctly. ...?
I've been playing around with the limit settings on inlet and outlet temperature. I can't find engineering guidelines anyplace for setting the limits. I've been moving them down, and right now it's at 133 +7 (shuts down the burner at 140) high side and 27deg differential.
"Seems" to be doing fine, haven't tried running the shop up to 65 (usually run at 60-62) to see if it will bring it up to that temp, which would be the highest I'd ever want to run at.
I suspect the limiting factor may be the ability to put enough BTU's into the largest loop (shop loop) at the coldest design temp. and achieve the desired room temperature. But I may be totally wrong......
Any theory or guidelines on this is much appreciated.
I appreciate the following probably doesn't apply to your installation, your boilers' copper?
A standard type oil fired boiler with a stack (chimney) shouldn't operate at less than 70 deg cent, mainly due to condensation forming in the stack and rotting the boiler.
Some of the other factors would be: heat loss in your shop; BTU output of your convectors; G.P.M. of your circulator. All things factored in: fuel, both electric for the pump, gas or oil for the boiler; repair/replacement costs for the circulator. Which is cheaper: higher boiler water output temperature with shorter circulator run time or lower water temperature, longer circulator run, shorter pump life?
Seems like a good question to put directly to Peerless. I'd ask them how the efficiency varies with the water temp set point. Simple question - they ought to have an answer...
I didn't catch whether this was oil or gas/propane fired, but there is NO WAY I'd be screwing too far afield with this without contacting someone "official" meaning a direct rep for the guy who built it.
I see there are a few manuals that you can download from Peerless:
You can really screw up, for example, an oil unit. Short cycling, cold operating temps cause low efficiency, sooting, etc. Some gas units are also vulnerable, and can cause premature failure or clogging of secondary heat exchangers. The horrid, defunct (I hope dearly) Amana forced air units--actually a miniature boiler--come to mind. We pulled out several of those with clogged boiler units, very expensive to repair. My own parents had one.
I'm not sure you are correct about loosing efficiency with higher temps. Depending on the design, that might not be very true. If the exhaust temp at higher water temps is still fairly low, that should tell you how much you are losing.
The point I'm trying to make is--it's much cheaper to lose a few points in efficiency over the years, rather than having to replace the heat exchanger/boiler in a few years because it failed.
Of course running at too high temps is equally bad.
WHAT IS the model of this thing?
You don't say what type of terminal equipment you have. Baseboard,convectors,unit heaters? Lower temps work well with infloor radiant-140F max. Out door reset works well with all terminal units. As the out door temp drops the boiler temp is reset higher. Equipment type, sizing ,and building heat loss will determine how low the loop temp can go. I have been out of the business for over 10 years. Back then condensing boilers had a short life from corrosion. They had a primary heat exchanger usually cast iron and a stainless condensing section. If they are still made that way there would be minimum temp to keep the cast iron surface above the dew point. Manufacturer will have guidelines to maintain warranty.
It is a condensating Modulating boiler, you can set the setpoints as low as your min. requirement to give you enough heat. Since it is condensating there is no harm to set lower discharge temp below 120 degrees, or even 100 degrees, as long as it can satisfy demand. An outdoor reset on it will command that on warmer days and might be the best way to set it up. But if you have a indirect hot water system all so on the system you should set up a priority control (shuts all other zones off and ramps to max discharge temp to satisfy the domestic indirect hot water heater and after it is satisfied will revert back to previous control scheme. "Pinnacle" is a neat system should be able to program it to anything you want. I have not actually worked on one as there is not any dealers in our area. I install the Weil McClain Ultra that is virtually the same unit and operating permeters.
You need an outside temperature reset control. This article explains it and is easier than me trying to explain.
The short answer is, yes you answered your own question. It is simply a matter of the lowest temp.(boiler) that can supply enough btu to achieve desired set(room) temp. Just be aware of cycle times. Condensing usually really only an issue at initial start-up/slab heat-up (am assuming you're referring to slabs, as you mentioned loops). This is when the slab is pulling ALL the heat off the water that's coming back to boiler and there is no residual heat to temper water back into boiler (usually because loop lengths are extremely long). Once the slab is up to temp it becomes maintenence heat supply. Even so, with long off cycle times (because of the thermal mass thats involved) and the low temps required to keep slabs comfortable enough to walk on (about 80-90 degrees), return temps are still usually below dew point. This is typically dealt with using a mixing valve that keeps return temps up to boiler by feeding supply right back into boiler and supply temps to slab down by re-circ-ing return water back into slab. If I understand you correctly your boiler is supposed to be able to do away with the whole mixing valve set-up by being uble to operate below dew point temps. As it is also a modulating (variable firing rate) you should be able to set high limit at desired floor temp and it will still fire at full capacity when it needs to. Limits act on water temps not firebox/flue temp. The limit is set by the MAXIMUM allowable water temp. This is normally set (in conventional baseboard type system) at about 190-200 just to play it safe and stay away from boiling the water off. This allows the most burn time/most heat to be safely "packed" into water before burner shut-off. You can run that high temp because you (usually)never come into direct contact with heat emitter. Just to clarify, the hi-limit is not a safety limit, that is the job of the T&P valve and low-water cut-off (and secondary low-water cut-off under most codes now-a-days). You could not walk , much less stand on a slab that hot obviously, so the highest safe temp would be that temp at which you could "drive" heat into slab and yet not overheat it (dependant on slab thickness/density, loop depth, slab insulated or not). All this is not to be confused with ultra-high efficiency exhangers that remove so much heat from burnt gases that it condenses before reaching atmosphere (where all carbon based fuels condense btw, water is by product of combustion). If in fact the boiler is designed to function this way, then water temps are irrelavant. The heat exchanger is designed to do this irregardless of water temp. I think this is what you are referring to. The latent heat removal is a control issue, in other words usually dealt with by digital or analog timer keeping water circ. going after burner shut-down (some of these are pretty sophisticated, can take into account outside temp and all sorts of other inputs and/or programming, including slab temps). Flue outlet temp is the rule of the day, the more heat removed before it goes up the chimney, the better. The water could be 300, as long as the flue stack remains at whatever design temp it supposed to, doesn't make any difference. The water will reach a point where it can no longer take any heat (hi-limit setting), or the t-stat becomes satisfied and the burner should shut down. Water can take HUGE amounts of heat, especially under pressure (remember water takes more heat to boil under pressure cause it wants to expand and revert to gas). In other words the return temp of the water doesn't equal heat removed from exchanger, or another way of saying would be water temp doesn't equal flue temp. All slabs run below dew point temps, otherwise you couldn't walk on them, your boiler is designed to excell at this as return temps not an issue as it condenses all the time. This could possibly actually be a PITA on wicked cold days if you can't override condensing mode as it will kick down burner if flue temps go up too much with long/heavy demand cycle times. Low limit is simply the temp at which burner comes on during call for heat when the return water temp drops below said temp. In other words circ. pumps will come on but the burner won't necessarily fire unless water actually "needs" it. Hope this answers your question and if I missed something I'm sure someone else will chime in.
OK, here is some more info and answers to -your- questions.
It's a Gas unit.
It's a PI-199 .....199,000 BTU
All stainless combustion chamber and heat exchanger.
Well, heck the whole boiler is stainless.
All terminal units are/is high-output baseboard.
The heated part of the main shop area is the largest loop, it's 70 x 26 and the baseboard is on all 4 walls, except for door openings.
I have thought about this before, and I think you bring up a valid point about the pumps as
far as electricity usage goes. If they run so much that they use more dollars of electric than they save in dollars of gas, there is no savings and no point to that!
There are 2 grundfoss circulator pumps, new in 1/07.
It's set up with an 'inner' and 'outer' loop as per Peerless
But, I have no concerns what so ever about wearing them out.
Here's an explanation of how the condensing mode increases efficiency:
Based on the efforts of myself and the local distributor
CONDENSING BOILERS, run in condensing mode, extract energy from flue gasses until they are
relatively cool. These boilers gain added efficiency
using the energy locked up in the latent heat of
vaporization. When the burner on a gas boiler runs, it
creates COČ and steam from the moisture in the gas
and air. In a properly controlled and designed
condensing boiler, exhaust gasses will cool
sufficiently to condense the moisture in that steam
back out as water. During this phase change from
vapor (steam) back to water some additional energy is
Condensing boilers must be designed to deal with the
effects of this condensate. The resulting cooler flue
gasses are less buoyant, however, which necessitates a
forced draft. But these cooler flue temperatures also
indicate that more energy has been extracted. The low
return water temperatures of radiant promote the
efficiency of condensing boilers.
talking to Peerless about the Monitoring software
and interface cable they offer for the unit, I really,really doubt they would be able to give detailed engineering info as to 'suggested' operating temp ranges.
The software was a disappointment, ran in DOS and didn't do much.....including the one thing I was hoping...
monitor and record cumulative hours the gas valve is open and...would've been even better.... total cfh of gas used.
These units are either made by Heat Transfer Products http://www.htproducts.com/products/index.html or made in Germany and imported by HTP. I think they private label then for Peerless....? If it really is built by HTP, maybe they have engineers that can speak on the subject. But I don't know if it is.....but good idea! I will try to find out!
The outdoor reset controls are interesting. Although they aren't as flexible as I'd like. Usually only 2 ranges of setback, 3 or 4 would be better. I'm considering a setback control in the future, but I've spent it -all- on the boiler at this point. Tougher because it replaced a Hydro Pulse boiler that expired 'mostly' unexpectedly on a cold January morning earlier this year. Well, it was showing signs, but I had hopes it would go a couple more years! Incidentally, that unit was all steel, and ran condensing at times, but the hi limit was at about 155 deg. It was only 100,000BTU and had all it could do to heat the place. The install date was 1982! Not bad mileage, considering the expected life is around 10-12 years. The hydropulse units do not modulate, and I have read accounts of them being replaced with Peerless or Munchkin units and the result is a significant gas savings.
John Fray, you understand the technology.....
The controller will allow a range of temps for the limits, and it's 'to the degree', with the digital readout. Quite unlike most of the old aquastats with the dial wheels.
As I'm turning it down, I notice the condensate pump running more frequently. I suspect that's a good sign and probably indicates more water vapor is being extracted from the fuel?
BTW, there is a Rinnai Tankless for DHW, installed mid 2006.
WOW Jim, wish I was quicker to reply with the details of the system.
Must have been typing at the same time....
No, not slab but baseboard. Thanks for the lengthy and detailed relpy!