Tuning of the outside reset

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Leon82

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I put some values in the odr settings in the whn 55 with fin tube baseboard.

0 degree 160
70 degree 97.

The thermostat is one of the Honeywell programmable and after reading about it online and watching it spaz out and cycle the boiler for about 1 minute I set it to a 72 degree hold.

It was 42 degrees last night and the temp climbed to 70 before I went to bed.

I originally put a jumper so both zones would activate because the bed zone is too small. This made bedroom get to 71 even though that tstat is currently off. So I removed the jumper and set the bed tstat to 68 so that small zone will stop and the large zone would always run before going to bed. I also dropped the 0 degree to 157 to try and get it to 68. I'll find out when I get home from work

This morning the boiler was still running at 25 percent output the bedroms were at 68 and the house was at 70 degrees. The thermostat was still set to 72 hold. According to the sh counter it never shutoff

The house zone it set to 15 degree delta t with a viridian pump and the bedroom is set to 12 to try and keep both rooms the same temp.(I'll probably have to add some baseboard to the other bedroom but that's another project).

The boiler is only seeing about 4 delta t at pump speed 1 running this low temp. I may swap it for a vr1816 which is more variable with its speed

So when it gets cold again I'll probably have to play with it some more.

I know at a certain point the fin tube doesn't work with the cooler water like a ci rad would. Do you think the odr high temp is too low?
 

John Molyneux

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No reason not to experiment with it and plenty of reasons why you should.

My strategy was to keep the t-stat at a constant 68F and bump the top of the ODR curve down by 2 degrees every so often until I woke up one morning and the house was at 66F. That probably would have been fine because the temperature bounced right back with some solar gain and human activity. But I like waking up to a warm house so I bumped the ODR back up by 2 degrees.

I have some baseboard on a very small basement zone and it's not great from a Delta-T perspective but it does actually keep up with the T-stat with very low supply water (100F, for example).
 

Leon82

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Well the house hit 72 by the time I came home from work so I lowered the odr low temp by 10 degrees.

This morning the house area was at 68 and 29 degrees out side and the thermostat still on the 72 deg hold. The boiler was running at 32 percent.
 

Leon82

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The house has been hovering between 68 and 70 so I don't think I can do any more until it gets cold again. It was 53 degrees today and only 37 last night.

I had some extra fittings collecting dust so I made my own neutralizer.

IMG_20160203_183601.jpg
 

Leon82

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Once and a while I get these crazy ideas and I just got one.

Last night and this morning it was about 50 to 53 degrees. So the odr curve is calling for low temp water. This morning the boiler was at set temp met, and about 85 degree system sensor temp.

This is will probably not be very effective for fin tube.

So currently I'm using sh1 temperature profile which will let the boiler run continuous at 45 degrees and colder.

So I'm thinking of making the sh2 cuve flat like from 100 to 120 degree water so it intersects at the 45 degree mark. And send a wire from the end switch in the switching relay to sh2 and sh1

The boiler control manual states it will override and use the higher set point when 2 sh calls happen at once and are set for different temps. So on warm days the flat curve will override the steep cold weather curve at 50 degrees. When it gets colder the steep curve will override the flat curve.

This would not run continuous when its warmer out but I think its better than running the really low temp water thru the fin tube.
 

Leon82

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So this represents the ODR curve with x being the outside temp and y being the water temp.

So the sh2 curve is priority at over 40 degrees because its setpoint is higher than the sh1 setpoint when calculated for the ODR.
Mastercam was screwy but I meant to have it at 40 degrees. at under 40 degrees it should switch to the sh1 curve because its ODR calculation will then be higher.

I think this will do what I want because the house symbol is over the sh2 on the boiler screen currently as its 48 degrees outside. snow tomorrow so I will see if it actually switches over

Perhaps the next generation control will offer a graph type input.

I also put some insulation in the hole in the wall for the thermostat. I think a draft may have been behind the weird behavior. ( I witnessed it call for heat for about 30 seconds) .


curve.png
 

Leon82

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This morning It was snowing and I noticed it did not switch to the sh1 curve. The ODR said 37 degrees so it must be a little off. so I shifted sh2 curve up by 5 degrees for outdoor temp and lowered the water temp 5 degrees.

After it rebooted sh1 became priority. so ill have to play with sh2 curve to tweak the intersection point
 

Dana

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At output temps below 125F (average water temps below 120F) lo-rise fin-tube baseboard's output becomes fairly no-linear with temperature. Some outdoor reset curves developed by manufacturers try to compensate for that, others don't, but there's no good way to characterize the output at 100F, since everything from dust-kittens to a window or drapes above the baseboard affects it's output at such low water temperatures. Also, to maintain a fixed delta-T of 12F or 15F at low temps can also be a bit of a problem. It's sometimes better to run with a fixed low pumping rate, say 0.5gpm, unless the delta-Ts get too big.

The min-fire output of the WHN-055 is about 10,000 BTU/hr. The output of fin-tube baseboard at an average water temp (AWT) of 110F is about 150 BTU per foot at 0.5-1 gpm. If you don't have at least 70' or more of baseboard the thing will be cycling the burner on & off rather than modulating once the curve reaches the 115F point with a delta-T of 10F, which takes a toll.

Don't run it any cooler than the baseboards can emit- any incremental efficiency you might gain in raw combustion efficiency get's offset by the efficiency hit from going into a cycling mode. If you happened to have only 50' of baseboard, that's 10K of boiler output/50' of baseboard = 200 BTU/hr per foot, which will balance at about 120F AWT, not 110F, which might work just fine with a boiler output temp of 123F, with a 5 F delta-T, with a continuous burn until the thermostat is satisfied. Below that it may run just fine in terms of keeping the place warm, but it may take 10-20 ignition cycles before the thermostat is satisfied instead of just one throwing away some heat with every flue-purge, and putting unnecessary wear & tear on the boiler.
 

Leon82

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I have began to notice This. At 38 to 40 max odr sensor reading the boiler is able to run constantly and maintain the house temp. Anything higher and it reaches the setpoint with all the baseboard running. It just happened to be at 100 degree system sensor temp. There is about 95 ft of baseboard.

I got it in my head that running fulltime was the best for it. Obviously it will need to cycle at moderate temperatures.

I have the basic Honeywell programmable tstat, which I have set to hold. They are too sensitive. I have stuffed insulation behind them in the wall cavity but from what I have seen don't trust them to not nuisance cycle. I was cleaning up near the boiler and witnessed it call for heat and the Thermostat ended the call for heat in less than a minute.

I am thinking about going back to a mechanical thermostat. I found one of my mercury ones but the trim ring is missing and has been collecting dust for 5 years.

There has not been and super cold weather to fine tune the low temp side of the odr curve but from 25 to 40 degrees the boiler will maintain the house temp without turning off.
Using the sh2 curve did what I wanted and basicly set a minimum water temp of 105. While not changing the sh1 variables.

Now if 105 is not practical I can raise it to 125. The boiler would cycle during 38 degree weather but if it will be more efficient that way I will change it.
 

Dana

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At outdoor temps when the the load of the house is approaching parity with the minimum fire output of the boiler it's better to have the calls for heat cycling than to let force it into to cycling multiple times DURING long calls for heat by running it at a water temp that's too low. With most boilers the difference in combustion efficiency at min-fire between a return water temp of 110F and return water of 95F would be a difference of only ~ 1%. (Say, 97% efficiency instead of 96%.) If dropping the water temp by 10F causes it to run 5-10 or more burn cycles per hour during a continuous call for heat rather than burning continuously, you've eaten up that 1% savings in extra flue purges. At some point (sounds like maybe ~40F, but it might be even cooler) the heat load on the house is lower than the minimum-fire output of the WH-055.

Get rid of that 5 grams of mercury in your junk pile by taking the old thermostat to the proper recycler or hazardous waste disposal site.

Sometimes the differential built into digital thermostats is on a hair-trigger, other times it's about a full degree. Sometimes the cheapest ones have the bigger differential swing (with no anticipator algorithm, just a 1F to 1.5F delta-T). Manufacturers more recently have taken to what's seen as a simpler (=cheaper) solution to implement, which is to limit the number of calls for heat per hour. That sometimes helps when you want a very narrow temperature swing without a lot of cycles per hour, but even on the most limited number of calls per hour it's not always doing what you want it to. With at tweaked-in mod-con boiler what you would like out of a T-stat is a minimum call-time, not a maximum number of calls per hour, but I don't know of any that actually work that way. Exactly which Honeywell do you have?

With 95' of baseboard you should be able to run the boiler with 120F output and a 10F programmed delta-T, or even 115F and a narrower delta-T just fine. It might not even cycle much during calls for heat at 105F output, but unless the curves compensate for the non-linearities of fin-tube baseboard you'll be forever fine-tweaking it without finding a satisfactory setting.


If your 99% design load were say, 30-35,000 BTU/hr @ 0F outdoors, the load at +40-45F would usually be slightly less than the minmum modulating range of the boiler at low-fire . It sounds like that may be your rough ball-park. It only takes ~140F AWT to deliver 30K out of 95' of baseboard, so if you set it up to deliver 145F @ 0F dropping linearly with outdoor temp to 115F @ 40F it'll probably keep up. From there, drop the temp at the 0F end of the curve 5F at a time until it doesn't keep up overnight on colder nights then walk it back a degree or two at a time. You can then try stepping down the 40F temp a degree or two at a time to 11oF to see if it begins to cycle during continuous calls for heat (probably won't), but below that you'd really want heat emitters with more linear output characteristics.

If you have a heating history on this place with exact fuel usage and meter reading dates to determine the BTU per heating degree-day it's possible to set the curve pretty accurately from the get-go using the approximate efficiency of the boiler as the calibrated measuring instrument.
 

Leon82

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You came up with this number in another thread.

So at +5F, a 60F delta below the balance point the load would be 60F x 591 BTU = 35,478 BTU/hr

my thermostat is a rth6350d1000/e

the reset is currently at 140 at 7 degrees and 73 at 68 deg.
the sh2 curve is at 105 and flat which thru the priority in the software prevents the boiler from going under 105 degrees.
I did this when we had a 50 degree day and noticed the boiler was calling for 90 degree water.

Its not what its meant for but it allowed me to keep the curve that worked under 40 degrees
 

Dana

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I'm not surprised that there are programmable differential T-stats out there, but I don't keep close tabs on them. Clearly being able to program the differential on the thermostat should solve the too-many-calls for heat cycling problem when the load is at or below the the min-fire output of the boiler.

So the load falls ~591 BTU/hr per degree F. At 95% efficiency the WH055 puts out about 10,450 BTU/hr at min fire, so the thing will of-necessity be cycling when the temp is: 65F - (10,450/591)= 47F .
 

Dana

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Also 35,478 BTU/hr in to 95' of baseboard is 373 BTU/hr per running foot, which takes ~150F AWT. Since the fuel-use calc is usually an upper bound you may be fine with 140F output @ +7F, but don't be surprised if you need to bump up to 145F (or even 150F) @ +7F to keep it from losing a little bit of ground when it's actually that cold.
 

Leon82

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The robertshaw 5110 helped.
I set the differential to two and it worked great as far as stopping the short calls for heat. The ODR is reading 0 degrees and the house was 66 degrees when I woke up,modulating all night. I may bump it to 145 High temp it, is soposed to remain cold today and tonight.

I raised the minimum temp to 110 because the house was acually more consistent with the warmer water.
 

Dana

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Sounds like you're on the right track!

(It hit -15F at my house Sunday AM, the coldest temp there in over 20 years by a few degrees.)
 
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