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Thread: Too much cycling

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  1. #1
    DIY Member Handymaner's Avatar
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    Default Too much cycling

    I have recently finished installing a Lochinvar Night. Here's a link to the sizing thread:

    http://www.terrylove.com/forums/show...load-calc-help

    I went with the 85000BTU size. It's cycling too much (according to the smart control 127 hours/640 cycles for space heating in the 2 weeks it's been running). Was at about 13/hour for space heating, now it's up to 19! I changed one setting (which did help). In the default settings, it would shut down if it exceeded the setpoint (around 115 with these mild temps) by 10 degrees. A call for heat would come, the boiler would ramp up rapidly as the cool return water flowed. About the time it would get close to 100% the heated return water would start rising the system temp rapidly, the boiler would overshoot the setpoint temp and before it could get the fire down low enough it would exceed the setpoint by 10 degrees and shut down. Then within a minute or two it would fire up again. I changed the setting so it could overshoot by 20 degrees instead of 10 (the max allowed). This seemed to solve the above problem. The only other setting I changed was to set the max heating temp to 155, which I got from Dana's answer in my heat load calc thread. I figure to set it conservatively, and if I get cold I can always raise it. The outdoor reset ramp is still at factory defaults for now.

    As I worked around the boiler I discovered that the thermostat is the biggest problem. I have a Taco zone controller, which indicates when a zone calls for heat. It seems the thermostat only calls for heat for a few minutes, then again a few minutes later. It's a Honeywell RTH7600. I checked the settings, and it is set to #3 on the heating cycle rate (which is Hot water system or gas furnace (more than 90% efficiency). The only other choices are: Gas or oil furnace (<90% efficiency), electric furnace, or gas/oil steam or gravity system.

    With the mild temps it's mostly running on one zone (70' of fin/tube baseboard), as well as some for the hot tub that I just got hooked back up. It runs at close to minimum fire (20%) most of the time with a delta T of around 5-8 degrees. I'm using the pump that came with the boiler for the near boiler loop and the same system pump from the old system (Grundfos UPS 15-42 F) on speed 3. My dealer thought that might be an undersize pump for my set up (~180' of tube/fin baseboard in 3 loops and a fan/coil in the garage). The hot tub has it's own circulator. I don't understand the relationship between pumping and delta T, do you slow the pump (giving the water more time in the radiators to lose heat) to increase temp drop? I'm sure the cycling will reduce when it gets colder and more heat is required.

    The thermostat is on an interior wall about 10 feet from the nearest baseboard radiator. It's the same one that was on the old boiler, although I did order an identical unit for the basement loop during this install. I plan on giving Honeywell a call on Monday. There is nothing in the manual about cycling.

    Any ideas?
    Last edited by Handymaner; 09-30-2012 at 01:28 AM.

  2. #2

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    I'll second the question in respect to the thermostat part since I have a Burnham Alpine and the same thermostats you're using. Are these the best models for the application? Are there better thermostats out there to be used in modcon applications? I also have a taco zone controller.

  3. #3
    In the trades Dana's Avatar
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    The faster you pump, the lower the delta-T will be, and conversely, the slower you pump, the bigger the delta. A 5-8F delta is a bit tight- back off on the pumping rate until you get at LEAST a 10F delta-T out of it unless the ODR is running some ridiculously low temp unsuitable for fin-tube like 90F or something. (Under 120F is a bit sketchy with fin-tube, since it's output is less than predictable, definitely non-linear wtih temp.) If it's plumbed primary/secondary, both pump flows may need tweaking. (High flow on the boiler loop and low flow on the radiation loop can end up with a low delta-T on the boiler even with low return temps from radiation.)

    Bumping the hysteresis to 20F up from 10 was the right thing to do- you have very little mass in the system. But it's the low temp end of the curve, not the high temp that is most problematic from a cycling point of view, since that's when the output of the fin-tube can fall behind the min-mod output of the boiler. At higher temps the fin-tube can put out a lot more. A typical spec looks like this.

    I didn't look it up, but you're saying mid-mod is something like 20% of the 85K full-fire, which would be ~17KBTU/hr. With 70' of fin-tube and a continuous call for heat it should be able to run at min-mod pretty much forever with an AWT of ~125F (shoot for something between 130-out/120F back and 135F out/115F back when tweaking flow rates), looking at the heat emittance per foot at those temps. Set the minimum output temp on the curve to something like 125-135F, set the pump to it's lowest speed, stick a dumb T-stat on there that won't over-anticipate & cut out early, then see how it behaves on loop delta-T, numbers of cycles/hr, and time the burn lengths. If you're getting 10+ minute burns out of it at roughly the programmed minimum temp, drop it 5F at a time until its under 10 minutes, then back up a couple of degrees.

    Maybe I don't know how to do math, but 640 cycles in 127 hours seems like 5 burns/hr to me, unless that really means 127hours of total burner time in 640 cycles, or about 0.2 hours per burn, which would be ~12 minutes, which would be GREAT! I'm not sure how you're coming up with 13 or 19 burns an hour in any kind of math on what info you've delivered here. At 13-19 cycles/hr it would have seen 640 burns in a under three days, not a coupla weeks.

    Bumping the hysteresis to 20F up from 10 was the right thing to do- you have very little mass in the system. But it's the low temp of the curve, not the high temp that is most problematic from a cycling point of view.

  4. #4
    DIY Member Handymaner's Avatar
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    No, you are right on the math, I guess I was being confused by the decimal. It is .2 (I saw .19 and thought 19) hours per burn. So 12 minutes is OK? I thought that would be way too much. If it's actually "GREAT!" then I guess I have nothing to worry about.

    The setpoint was running at about 113-115 when it was in the 50's outside, which has been most of the time since the boiler was up and running. I've also seen 125 when it was a bit colder. I am open to any suggestions on how to set the outdoor reset ramp, so far I've left it at factory defaults. If my 12 minute burns are ok, maybe I should leave it alone for now.

    Sounds like I should slow the pump some. I've seen delta T's of 4-5 degrees on the low side, rarely over 10 degrees-never anywhere near 20 (except on DHW I will see 16-18). My question is this: So far it has been usually only 1 zone, occasionally 2 that are open. There are 4 zones on the same pump. If I slow it, then will it be too slow with all 4 zones open? What issues would this cause if the delta T was too high (well over 20)? I could vary the speed according to season (slow it in mild fall and spring temps, speed it back up in winter) if that would help. The boiler does have the capability to control variable speed pumps. I asked my dealer about that, but he discouraged it. I think he said that was normally done with whole building controllers or something.

    The boiler came with a pump for the near boiler loop, and the manual said for my BTU size to run it on speed 2, so I have not changed that. I set the DHW pump the same. It uses a setpoint of 180 for DHW, but I have never seen it get that high. At 100% fire (which it always is on DHW), it will start out in the low 140's and work it's way up to the mid 160's as the tank warms up, with the a fore mentioned mid teen delta T's. Should I slow the DHW pump to increase the delta T?

  5. #5
    In the trades Dana's Avatar
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    If you're getting 12 minute burns out of it with 113-115F water you're doing pretty good. Try the pump at middle speed, see if you can't get a somewhat bigger delta-T out of it. When you have more zone valves open it'll increase the delta-T somewhat, but you should have no problem meeting the heat load at design temp even with lower flow. If the delta-Ts are over 25F when it's cold out, with the boiler cranking near the top temp of your reset curve with all zones calling for heat you can bump the flow back up a notch and call it "done". Cranking high flow/low delta-T is just burning power for no good reason.

    You'll have to read the manual on setting up the outdoor reset curve- it's not rocket science but they're not all the same, and I've not set up a Lochinvar. Heat load is pretty linear with outdoor temp, and fin-tube output is pretty linear with water temp once you're above 130F or so, you know about what the heat load is at design temp, and it's essentially zero load at +65F, so from there you can use middle-school math to plot or calculate the load at any point in-between. Often they only give you 2 outdoor temperature points to play with for defining the curve, along with a min & max output temps that are independent of the curve. So, whatever outdoor temp points they allow you to program, first, calculate your heat load at those temps, then consult a fin-tube spec to figure out what water temp would be needed to deliver the load at those temps with the amount of fin-tube you have in the system, and plug those numbers in for the water temps at those outdoor temps.

    eg: Let's say you've estimated your heat load at an outdoor design temp of -15F to be 60,000 BTU/hr, and at +65F outside it's 0F. The total range is (65F- -15F=) 80F so the heat load is about (60,000/80F= _750BTU/hr per degree below 65F.

    If they only allow you to program at one fixed crossover point of say 0F, that's 65 heating degrees below the 65F outdoor heating/cooling null, so the heat load at 0F would be (65F x 750BTU/hr per degree=) 48,750 BTU/hr.

    If the total amount of fin tube in the system is say, 150', that would be (48,750/150=) 325 BTU/foot.

    So consulting a fin-tube output rating chart, you find tht to get 325BTU/hr per foot out of it takes a temp of about 140F. So plug that 140F in as the defined water temp at the 0F outdoor temp. If they give you two points (most do), calculate the heat load at that temp, look up the water temp that delivers that amount of heat at your second point, and see how it does.

    If it's keeping up just fine when it's in the 30s & 40s outside when you should be getting pretty much continuous burns (since the heat load is about min-mod or more), drop the temps 5-10F and see if the system still keeps up. When you've lowered it to where you're feeling the chill, bump it up 5F, then down two, etc until it seems to track. If later in the season when it's colder out it's not keeping up, bump it up the low-temp program point a degree or two at a time.

    Set the min-temp to whatever it'll take to still give you 10 minute burns (you may be OK). If there's an absolute max setting (not tied to an outdoor temp), it's safe to set it to 180F even if you have PEX tubing. For copper you can go higher yet. If it's tracking the curve it won't hit those temps until & unless it's quite a bit below -15F out. (If you do the math on the above and look it up, the water temp requirements will be only in the 150s at -15F using the example numbers.)

    Do the real math on your real system & heat emitters (ignoring the fan-coil in the garage- it'll be fine at any water temp) and see if you can't tweak it to where it's still delivering the heat, but with 10-20/burns day or even less. It'll always cycle during the shoulder seasons when the heat load is less than the min-output, but once it's turned cool enough that you have a real heat load all day it should be able to lock on and modulate with some VERY long burns, but still not fall behind.

    If you dial it in finely the ODR will make for very long recovery times if you use temperature setbacks, but if you bump up the curve to deliver faster recovery you throw away some condensing efficiency. Dialed right in you'll use less fuel just leaving it at a fixed temp if you're only setting back overnight or while you're away at work. If you're leaving for the weekend obviously you can & should turn it down, but know that if you bump it back 10-15F or more it'll take hours to come back up to temp once you've optimized the reset curve.

    If the delta-T you get when it's running DHW loop is only 15F that's OK, but bump the speed down- as long as as it's running at max fire with a delta-T well-within the manufacturer's max (50F it common, some are a bit lower), all the heat is going into the water. It doesn't NEED to see 180F output ever, unless the tank's storage temp is set to 160F or something. So if it's delivering all the heat into the tank at a lower pump speed, you're burning a bit less electricity for that function. But a 15F delta is still a reasonable number, if that's the only speed that does it.
    Last edited by Dana; 10-01-2012 at 03:14 PM.

  6. #6
    DIY Member Handymaner's Avatar
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    Thanks, Dana for your guidance. You have given me much to think about and figure up. That's just what I wanted, a way to set the curve up for my system. I can do the math as per your example, and like you said tweak it until I'm cold and then raise it just a bit to optimize it. I just wasn't quite sure where to start.

    I wondered about the set back. I've often wondered where the change over point was when it cost more to re-heat than it would have to leave it. I was a kid during the energy crisis in the 70's, and I still remember Jimmy Carter coming on the TV and telling everyone to set their thermostats and leave them. I recall he said to leave it at 72 for cooling and 68 for heating, and that if everyone would do that we would save ______million energy units. Now they really push the set backs, programmable thermostats, etc. I have been setting back about 3 degrees, and I did notice that it takes a long time to recover. I really don't mind the wait, but if it costs about the same to leave it steady then why be cold during the set back?
    Last edited by Handymaner; 10-01-2012 at 10:59 PM.

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