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Matcartier
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| 09/29/2009 12:28 AM |
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| Recently had a mod-con installed in my home to replace the ancient oil burner. I am planning on an addition to the home which will have radiant heat. The existing house has baseboard. When I asked my plumber if he knew how to plumb the boiler for the radiant heat he said he would install a mixing valve. Was I wrong for kicking him out? Does a state of the art boiler need to have a mixing valve to accommodate the low(er) temps necessary for radiant heat? |
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Dana1
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| 09/29/2009 1:45 PM |
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If different zones need to run different temps, how is any boiler to know which is being served? If multiple zones with different temp requirements are calling for heat at the same time, by what algorithm can the boiler deal with it? Mixing valves can be a useful (even necessary) design element.
If you have sufficient baseboard length to deliver the heat at a low enough temp you might get there without mixing valves by going with lower flow, bigger delta-Ts across the radiant. Not sure if that's the case in your setup.
In order to get the most out of the boiler with a baseboard system using the boiler's outdoor reset function ramped to deliver the temps required for the baseboard is probably the right thing to do, but those peak temps may be ridiculously high for radiant on colder days.
Without more info on the system or the plumber I can't say for sure if you were overreacting, but a mixing valve in a dual temp system doesn't seem anything like a major design faux pas (quite the contrary.)
(But I'm just a hack at this, not a pro... ) |
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Matcartier
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| 09/30/2009 9:53 AM |
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Lower flow/ Delta-T, not really sure, that's why I had the plumber ( who will be recieving a fruit basket from me soon ).
Just seems like a waste, heat the water to cool it down. But I guess that's the story with "hybrid" systems like mine. |
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Dana1
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| 09/30/2009 11:15 AM |
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Posted By Matcartier on 09/30/2009 9:53 AM Lower flow/ Delta-T, not really sure, that's why I had the plumber ( who will be recieving a fruit basket from me soon ).
Just seems like a waste, heat the water to cool it down. But I guess that's the story with "hybrid" systems like mine.
"Pluming & Heating" on the side of the truck doesn't necessarily mean they're great heating designers- I've dealt with my fair share of real hacks. There's a lot more to it than just the plumbing, but at least this guy wasn't planning to run it all at 180F all the time, eh? ;-)
If the boiler loop to the mixing valve is short and low-flow there isn't a huge efficiency hit by having started with hotter water- it's the temp of the RETURN water entering that ultimately determines the combustion efficiency (which is the lion's share of the boiler's overall thermal efficiency.) You want the bulk of the return water to be close to the temp of the return from the radiant when only that zone is calling for heat. If boiler loop is very long and high temp, distribution losses can be as high as the heat delivered to the radiant. (Just ask the f'n' idiot that installed the radiant zone on my addition a dozen or so years back, with 50' of uninsulated pipe full of 180F water to the mixing valve, not to mention the 4x oversized unbuffered cast iron beast he was hooking it into... Want his number? I won't be using it! :-) The thing short-cycled like crazy with only the radiant calling for heat, and kept the then-uninsulated basement nice & toasty too. Things are different now...)
The return water from the other zones will be higher than from the radiant but may still be low enough to reap some condensing efficiency using the boiler's outdoor reset function to roughly track the heat load. Some of the control programming is quite flexible, and it's useful to play around with it to see just how low you can set it and still stay warm in your higher-temp zones. If you can get it to where the return water temps are under 120F most of the time, you'll hit the low 90s for average efficiency, otherwise the best you're going to do is about 87%:
http://www.csemag.com/articles/blog/1250000325/20090201/Boiler-Efficiency-vs-Temp-2.jpg
Whether short loops or long, high-temp or low, insulating the distribution plumbing is always a good idea.
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Blueridge company
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| 09/30/2009 1:32 PM |
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You are running radiators assume temp 145+
you are running in floor assume temp 110 (you have not identified your plan for pipe install on new addition, staple up, Floor panel system, gypecreet, slab on grade)
At present design you will need to set boiler heat temp at 145+ for radiators and Mix down for in floor. Your plumber is (was) correct.
If you want to get max efficiency from your condensing boiler run the whole system at low temp and maybe add radiator capacity.
A condensing boiler is at its best when run cold, setting the initial temp of 110 will do that.
Question is how cover the radiator section. You have not identified the boiler. Many mod con boilers have the temperature control on front. A dial if you will. No more difficult in function than changing the setting on the dryer for your cloths.
If you run the boiler cold and install a 3 way valve to mix the floor you can run your radiators cold through most of the year and turn up the heat in the dead of winter only when the low temp on radiators will not keep up. The mix valve will protect your floors in the event you increase the heat.
This is not automated, but still effective.
Just some ideas,
Good luck,
Dan
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Dan
BlueRidgeCompany.com |
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NRT.Rob
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| 09/30/2009 1:39 PM |
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mod cons have reset curves. there is no need to manually adjust anything. the reset curve, set for the baseboard, "runs them cold" when it's not the dead of winter.
You can't do a better job than a reset curve with manual adjustments. the 3 way valve is there, it should be used, and the reset curve set for the baseboard's requirements which, hopefully, will still be in condensing range most if not all of the winter. |
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-=Northeast Radiant Technology=-
NRTradiant.com |
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Dana1
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Posts:702
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| 09/30/2009 2:43 PM |
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What Rob said! Once you've dialed in the right curve for the outdoor reset it's pretty much "set & forget", no further operator intervention required. The details (and flexibility) of programming the curve will vary by manufacturer & model though.
Seasonal manual tweaks aren't even necessary for bang-bang old-skool boilers retrofitted with outdoor reset controls either, but the setup can be a bit more awkward, and cost/benefit wise manual tweaks are certainly cheaper than retrofitting those controls (that might have made sense if you still had your old boiler.) Old-school non-modulating you could get high-single-digit percentage fuel savings out of outdoor reset controls, but with a mod-con operating near the edge of condensing it's well into double-digit savings. Even on design day the heat load in the middle of the afternoon can be less than 20% of what it was at that winter-coldest 5-7AM stretch, and the radiation temp can run a few 10s of degrees cooler than the design-day max. Even in non-condensing mode that's usually worth (rule of thumb) 3% fuel savings per 10F drop, a combination of high combustion efficiency, lower jacket & distribution losses. But going from 130F to 120F return it's good for more than 6% due to dramatically higher combustion efficiency you get out of condensing the exhaust at the heat-exchanger- it's the steep part of the curve, that doesn't slow down until you're under 100F.
DO look at that curve again:
http://www.csemag.com/articles/blog/1250000325/20090201/Boiler-Efficiency-vs-Temp-2.jpg
And bear in mind, that's only the combustion efficiency factor- the jacket & distribution losses rise/fall more linearly with temp, but at a steeper line than the non-condensing portion of the combustion efficiency curve.
Almost ANY radiation will get significant condensing benefit out of a mod-con from outdoor reset, even in mid-winter. (Maybe not much if the radiation needs 200F on design-day, but certainly some even if it needs 180F peak. Going fixed-temp on a mod-con in a dual temp heating system would be burning up money- if the function is available, use it. |
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Matcartier
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Posts:14
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| 09/30/2009 3:59 PM |
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| Dan, the boiler is a Burnham Freedom 90, the radiant is going on a slab-on-grade addition to a preexisting slab-on-grade, so probably some amalgamation of gype-crete and in-slab piping. Not really sure about the specifics of that yet, I'll probably have NRT do the design for it (if Rob's people do Rhode Island). |
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Dana1
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| 10/01/2009 12:53 PM |
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The minimum modulated input the Freedom 90 is 30MBH, which may be a bit oversized for most houses in RI. (That's about my peak design-day heat load in somewhat cooler Worcester. Have you measured/calculated your heat load?) There are others out there that go down to the ~15MBH range, which could provide longer burns, better load-tracking, fewer cycles. Don't be surprised if the designer says you need some buffering for short-cycle suppression to get the max efficiency of the F-90. It's unlikely that any of your individual zones were over 20MBH for any significant number of hours in a heating season let alone 30-40 where it could run at a very high nearly continuous duty cycle.
The thermal mass of the concrete in the slab makes the radiant essentially self-buffering- it's the baseboard zone(s) that may need more mass to lengthen the burns. Your ancient oil boiler was probably sufficiently high-mass that it didn't need a buffer to get reasonably efficient burn lengths, but mod cons have less than a gallon of water in 'em and baseboard doesn't have much either- order of magnitude: 0.025 gallons/foot for 3/4" pipe, or about a gallon for every 40 feet of baseboard + distribution plumbing. If you have 400' of plumbing + baseboard you're still only talking ~10 gallons/85lbs. With that little mass to work with, burns could be well under 3 minutes long @ 27MBH output (90% of 30MBH, the min input) without a fairly substantial hysteresis programmed in. In that scenario a substantial fraction of the total disappears as fixed losses that occur with each burn cycle (flue purges & ignition, etc.)
Maybe I should refrain from turning this into a "design by web forum committee" session. :-) A good designer privy to all the particulars can make this work for you, and should be able to explain the trade-offs of different approaches. Don't be surprised if your heating bills are cut by 50%+ though, between the enhanced efficiency of the condensing boiler and the relative cost per source-BTU of heating oil vs. the (crashing) price of natural gas. |
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Matcartier
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Posts:14
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| 10/01/2009 3:27 PM |
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Thank you Dana, I appreciate the help. In the end as log as I realize savings over the 1950's era oil burner i'll be happy. ... and it would seem that something would have to be tragically wrong for me to not save money. And when the addition is ironed out, i'll have the radiant designer point me in the right direction.
Mat |
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Blueridge company
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Posts:211
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| 10/02/2009 2:06 PM |
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Agreed you need a out door reset, but you have to set a base line temperature on the boiler, this is where I suggest to hold the whole unit low, ideally 110, then you creep up from there in dead of winter, the outdoor reset will follow the design set temperature.
It works off a bell curve. If your delivered in floor is through a 3 way mix valve it will not exceed 110 even if you turn up the heat to help out the radiators.
Dan |
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Dan
BlueRidgeCompany.com |
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NRT.Rob
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Posts:733
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| 10/02/2009 2:19 PM |
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I think you mixed up your emitter names.
the boiler must run at a baseboard reset curve, minimum 100 degrees up to whatever max you need.
the 3-way valve to the radiant floor (there are no radiators) could be adjusted seasonally if it was not a motorized unit. but not the boiler temperature. the boiler temperature must run at whatever the boiler must run at to keep the baseboard going, and that will be set in the reset curve. |
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-=Northeast Radiant Technology=-
NRTradiant.com |
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