Too much cycling

[Handymaner]

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?

[mage182]

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.

[Dana]

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.

[Handymaner]

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?

[Dana]

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.

[Handymaner]

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?

[Dana]

Methinks you were mis-remembering Jimmy's dictum. IIRC he was asking people to turn down the heating thermostat to 68F and wear sweaters, turn up the cooling thermostat to 78F and wear shorts & T-shirts. He may or may not have weighed in on setback strategies, but it wasn't until Reagan's second term that programmable thermostats making setback strategies easy become cheap & ubiquitous.

With a non-modulating boiler there is a real savings to overnight setback since:

1: The house loses less heat when it's colder inside (smaller delta between interior & exterior temp, at same R-value)

2: During the recovery ramp the boiler has a nice very long burn, running at pretty much it's steady-state (maximum) efficiency minimizing cycling & standby losses.

But a modulating condensing boiler's maximum efficiency is NOT when running full blast. A mod-con's raw combustion efficiency increases when

A: The return water temp from radiation is minimized

B: The firing rate "sweet spot" is somewhere between 1/5-1/3 of it's maximum rate (at any return temperature.)

Dialing in the outdoor reset helps by optimizing both A & B- the lower the water temp, the lower the return temp (A), and with the lower temp the radiation puts out less, so it's modulating around a lower firing rate (B). Using a set-back strategy screws up both, since in order to recover from setback the firing rate is of-necessity going to be significantly higher than the steady state heat load which compromises B. In order to have reasonable recovery times, to get the heat out of the radiation and into the rooms the temps have to be bumped up, compromising A.

So with a mod-con boiler using an outdoor reset strategy and a constant thermostat temp uses less fuel, unless the setback period is quite long (are you gone for 16 hours/day in one stretch?), but only if you take the time to set up the ODR correctly to roughly track the load, putting a low-temp limit high enough that the boiler doesn't start short-cycling on zone calls.

With 10 minute minimum burns you're doing great, and even 5 minute burns you'd be doing OK as long as the burns/hour are still in low single-digits. Every burn throws away a small amount of heat/fuel on every flue purge and ignition cycle, and if it's doing a dozen burns an hour it's doing pretty lousy- even if the combustion efficiency is high during the short-burst burn it's still throwing away a measurable fraction of the total heat. (Rule of thumb is that a short cycling boiler is getting 15% lower average efficiency than it's raw combustion efficiency. It's a very squishy rule, but it's about right more often than not.)

With fin-tube there's the secondary issue that at low water temps the output becomes very non-linear and sensitive to things like dust-kittens furniture & drapes proximity, etc, and ODR curves don't track it very well. Most fin tube is still pretty predictable with an AWT of ~120F, not so much with an AWT of 100F. (Cast iron baseboard / radiators are much more linear at low temp than fin-tube.) Setting the boiler's output temp floor at 115-120F will keep you from going crazy tweaking the ODR curve, yet still delivers mid-90s efficiency from the boiler. If you go any lower the curve is almost sure not to fit when it's warm outside, and may cause you to bump the overall curve higher than is optimal for efficiency during the real heating season. But you can play around with it a bit.

[Handymaner]

Just a report. We've got into the cold season. The last several weeks have been highs in the teens and lows just below zero. I think I've got the outdoor reset curve down, system temp is at mid 150's at 0* outdoor temp. It just BARELY keeps up, once in a while loosing a degree below t-stat setting and running pretty much continuously (up to 53 min/cycle including the shoulder season short cycles). Our first gas bill was less than half last years usage with a slightly lower avg outdoor temp. This last billing period we had 15 degrees lower than average temps, 20 degrees or more lower than last years temps with slightly less than last years usage. Pretty good savings, I think.

The only thing I think could still use some tweaking is the delta-t's. I've got all the pumps set on their lowest speeds (even though the one that came with the boiler for the primary loop said to use the middle speed for the 85000BTU model). I still can't get delta-t's of more 4-5* per loop (one loop on = 4-5*, two loops = 8-10*, three loops = 14-15*, although 4 or 5 loops don't add much staying around 18-19*) The 5th loop is for the hot tub which also has it's own pump, although it draws through the normal system pump. I've only seen all 5 on at once a couple of times recently, it's usually just 2 or three on as the one for the fan/coil in the garage is set on just 50* and hasn't run much at all (until it recently got below zero), and one is in a sun room with the hot tub which is kept at 58* or so.

Is the Grundfos UPS 15-42 F main system pump over sized for my system? (It's the one that was already present in the old system). Is it worth my while to do something to increase delta-t's? Can/should I simply close the ball valves down to reduce flows to the individual loops? I realize this would add some head pressure to the pump, but it's been proven that doesn't hurt (this pump at least) as when I did my install I found the wires running the pump twisted together instead of going to a switch of some kind. Apparently it was running 24/7/365 even when no zones were open! For at least the year and a half I've lived here, and who knows how long prior to that-with no apparent pump damage. My first electric bill after the install was the lowest ever, go figure! Anyway, I'd love some suggestions on delta-t's.

[jadnashua]

With a delta-T that low, you aren't getting into the condensing mode, and thus affecting overall efficiency. You probably can't lower the primary loop pump speed, as that could result in steam bubbles, but you might be able to throttle down the loops some. Dana is much more involved in this than I, he'll probably respond later.

[Dana]

With 150F output you should be able to get at least 20F return from radiation unless, and if need be you can throttle back that flow with a ball-valves to achieve that. It's better if there are ball valves on each zone loop to tweak them individually, but if you only have one ball valve plumbed in series with the pump force all zones on (bump up the thermostat) and see if you can't get it to deliver a ~25F delta when they're all calling for heat. (The idealized configuration would be an ECM drive pump and ball-valve on each zone loop, then the delta would not change with the number of zones calling for heat. But ECM pumps aren't cheap.) Start there before messing with the boiler loop.

(With some trepidation when adivizing a DIYer...)

... The question remains how low you can take the flow on the boiler loop before it starts to sizzle. There's probably a min-flow spec in the boiler documentation, and too much pump would force a very tight delta-T on the boiler due to excessive flow. (This isn't in the manual, but...) ...if you stick your ear to the boiler and very slowlyl & carefully slowly throttle back flow with the ball valve you should be able to hear the hiss when the micro-boil on the heat exchanger turns into a not-so-micro boil. This will vary with the firing rate of the boiler, so if it starts to sizzle, bump the flow back up a bit and keep monitoring it for several minutes.

The system pressure also affects the sizzle- if you're currently running it at 12-13psi, it's OK to bump up to ~20psi to suppress it, but going much above that you may hit the design limits of some components- relief valves typically open up 30psi, and you want at least 5psi margin on that to handle the dynamic pressure or it'll be spittin' at you. (Note, the expansion tank's air charge needs to be adjusted to suit any time you change the system pressure.)

Stopping flow completely and suddenly could damage the boiler in a flash-boil event, so tread softly when cranking back flow, courting violation of the manufacturer's spec. A little sizzle when tweaking isn't going to hurt much, but pops and bangs are worth avoiding. If it sizzles continuously during operation it's losing quite a bit of heat-transfer effectiveness and stack temps will rise, efficiency will suffer. But if you keep the flow conservatively in a non-sizzling zone at all firing rates and it should be fine at delta-Ts up to the manufacturer's max spec. (probably 40-50F, but look it up.)

Do NOT just "set and forget"- check up on it when as temps are cooler and firing rates higher and readjust if need be. You don't want to be wakened at 4AM when it's -10F out by the sound of a banging & clanging heating system.

Unless the boiler is running way above half-fire you probably want to keep the delta-T on the boiler in the ~20F range, not more, otherwise it could potentially hit the max spec when the boiler is running flat-out at max fire.

Hopefully I'm not forgetting anything too eggregious on the above discussion (and any pros, PLEASE step in to correct any sins of omission or commission here!), but if it's not sizzling or banging at any firing and has a 10F+ delta-T whenever the output temp is above 130F you should be close to maxing out the efficiency as you dial in the ODR curve.

If you're burning less than half the gas for even higher heat loads than last season it sounds like this was the right move! Even when it's not in condensing mode the fact that it's modulating with load, with very long burn lengths delivers a huge boost to average operating efficiency. An oversized short-cycling 78%AFUE cast iron beast could easily have been running under 60% average efficiency during the shoulder seasons, and an oversized 1960s/1970s vintage 70%-er could have been running in the low 50s, or even under 50% during the shoulder season. A reasonably sized mod-con will be hitting the 90s at low temp, dropping to ~87-88% with the 145F return water you're currently seeing. Efficiency drops pretty slowly from there with rising temp- with 160F return water it'll still be running ~85%.

[Handymaner]

Thanks Dana & Jadnashua,

Let me clarify: I installed ball valves on each side of each loop (supply & return). The pump and zone valves are on the supply side. I'm thinking it would be best to close down the return side? Or does it matter?

You are not suggesting restricting the primary loop, correct? Won't the primary/secondary interface assure adequate flow through the boiler (avoiding flash-boil) regardless of flow through the system? Or am I misunderstanding something? I can see how a low system flow would cause the boiler to overshoot set point due to recirculating the water it just heated without giving it a chance to cool off in the baseboards, but won't the primary loop insure adequate flow across the boiler always? If I'm understanding correctly, closing the system flow down too much would actually reduce delta-T's because of this?

On the ECM pump: does it interface with the boiler? Or does it have it's own temp sensors and controller? Just how expensive are they? Even doing the entire installation myself I had $8700 in it, so a new pump may be worth it in the big picture. Do they happen to fit in the same flanges as my Grundfos? Thanks again.

[Handymaner]

OK, I just did a quick experiment. It's 7* outside this morning. The boiler was at 63% fire, setpoint 148* and delta-T was running 16-17* with 4 zones open. I cut down each return ball valve a little over halfway. The delta-T started dropping! It went from 16-17* to 14-15* and the boiler slowed to 53%. I then fully opened all the valves again, and after a bit the boiler stabilized again at 64% and 16-18* delta-T. The opposite of what I expected I guess?

[jadnashua]

As I understand it, you want to throttle the output of the pump...throttling the inlet might lead to it sucking air or cavitating. With a primary/secondary loop, as long as the primary loop is not running too slow, you should be able to slow down the secondary loops without damage. Dropping it too far, and you won't be able to push enough heat through the loop to keep the house warm, but that's a different issue.

[Dana]

When you slow down the flow to the radiation it's average water temp drops which means it's pulling less heat from the boiler, so it's expected that the firing rate would decrease. But the delta T on the RADIATION will increase with lower flow. You may need to measure this with in IR thermometer and hockey-tape on the outbound and inbound pipes to the zones- you can't measure it at the boiler.

Until and unless the delta-T is on the radiation is increased it won't matter what you do to the boiler loop flow, since there is no possible way it can return water to the boiler any cooler than what the radiation is returning. This is why it needs to be a 2-step process- slow down the radiation flow first, not both at the same time.

If it's running a 16-17F delta-T and 148F output (low 130s return) it would be getting about 87-88% efficiency at about half-fire (which isn't half bad.)

The boiler's modulation control algorithms may be limiting the delta on the boiler, but I'd be surprised if they keep it that tight by-design.

[Handymaner]

OH.....I've only been reading the temps on the boiler screen. Page 2 has input temp, output temp, and difference (which is the delta-T I have been reporting). I have not checked the loops at all. Wouldn't it be the boiler input temp that counts for efficiency? I will use my IR temp reader on the radiation loops and see what I get. What is the hockey-tape for? Does it read the temp better than right on the copper pipe or something? I'll go out and check them right now.