What good is the "DHW Boiler Offset"?

keyote

boon i agree its disturbing it doesnt seem to want to mod down to maintain a steady temp.
I think youu just mistyped but to be clear i wasnt suggesting raising the boiler set point or offset but there is a dhw offset that can be adjusted.
you know it says the boiler will select a supply temp based on dhw setpoint maybe it just want to heat that boiler faster which would be efficient but the setpoints too low , but yeah even so it should do what its told. hey if you talk to tech support run that part number by them again curious what they say to your part numbers v the one they gave me.
well thanks for sharing

keyote

ntjohnny the KHn come with 1" copper ports and the smart 50 with 1 1/4" taps. I used a copper 1'sweat x 1-1/4' thread on the boiler because of this difference. if he had a flow issue or maybe the smart isnt absorbing fast enough

Boon

I've been through that page a gazillion times.

njtommy

Change DHW blr offst to 10 lets see what happens. Or if you can't do that change boiler Max DHW temp to 175

Control may not be able to respond fast enough

njtommy

Looking at the smart 50 specs the tank can and should able to handle way more btus then What he is giving it with 80k Max.

Boon

change DHW blr offst to 10 lets see what happens. Or if you can't do that change boiler Max DHW temp to 175

I did both. Set point 175 with 10 offset so that it would still shut off at 185. Took the tank temp down to 112. Pump speed 2. The outlet hit 183 - 8 degrees over set point - before it started creeping back toward 175. It finally came back down to 175 and then stayed between 175-177 until the call was satisfied. ∆T was between 16-17 in the beginning and 8-9 toward the end.

Did a second test. Only took the tank down to 134. Kept the pump on speed 2. The boiler water was already hot and the boiler was slow to modulate upward for the first few minutes of the call. Wasn't long before it went above 175 and maxed out at 181 before coming back down to 176-177 when the call was satisfied.

Nice! These settings effectively solve the short cycling problem but I'm still calling tech support tomorrow. Isn't something wrong if the boiler can't respond fast enough to factory-default settings and under normal operating conditions?

njtommy

Well your getting some where now and at least your not short cycling most importantly.

You can change your temp back up to 180. With the 10 degree offset you should be fine.

Gordy

Boon said:

I've been through that page a gazillion times.

I believe that Boon, but do you understand the parameters, and their effects?

keyote

i hate when youre cryptic master

Boon

The only parameter that I don't understand is the subject of this thread - DHW Boiler Offset. I understand that it is the number of degrees above the set point at which the boiler will shut off, but there must be something more to it.

Thinking out loud: If the boiler isn't that precise, why is the factory default 5? For that matter, why allow the setting to go lower than 5? Apparently I wouldn't stand a chance setting that offset to 1.

And no one has jumped in to say, "oh yeah my KH boiler does the same," or "no my KH performs better."

Maybe my expectations are too high. I'm the new guy on the boiler block so I can accept that, but others here have shared my expectations. Folks have said, [paraphrase] "yeah it should modulate."

Speaking of modulate, there is a Modulation Factor setting under the anti-cycling menu on which the installer can set how responsive the boiler will be. The default is 100 (most responsive) and mine is set to 100.

And I still haven't seen the display read "SETPOINT MET" which is either an indication of part of the problem or the manual isn't correct.

njtommy

I would call the KHN boilers still new to a lot of people and I'm sure their has been plenty installed, but I would think most people don't want to spend the money for the indirect tank and or rely on 1 gas appliance to give them heat and hot water. For a lot less money you can install a regular tank hot water heater. Although you can't beat the performance of an indirect tank in my opinion.

keyote

Boon since im not quite at this point yet im still a bit hazy but if you read hats comment above and where he quotes me and i see you subsequently tried it and it helped but you just commented you dont quite get it, and while i am certainly sympathetic that i too paid for what i thought would be tighter control all that said i think off the cuff the way to think about the dhw boiler offset is not so much that the boiler can modulate but that it needs a bit of time to get there in your last succesful experiment if i recall it did manage to hold a temp for a while but has the most trouble when ramping up or down to some extent this makes sense because its s/r sensors are on either side of the heat exchanger so it doesnt get a lot of notice, its not clear if it actually uses the specific boiler temperature as it moves through the call you thought it did not though it says it uses it to set initial boiler temp but beyond that point who knows so it may only use its own s/r temps to maintain the temp it decided on at first and by the time the return water temp changes it too quick to modulate so this wider margin of era gives it enough time to catch up to the water. That said this is going to become a lot more important on ch calls particularly on a system without a buffer like mine.
btw i did notice in something i read raising or lowering the dhw tank sensor will tweak it ways we can imagine but they will also do things like increase calls or lower useable capacity

keyote

Hat
am i misreading this or did you miss this post of boons

change DHW blr offst to 10 lets see what happens. Or if you can't do that change boiler Max DHW temp to 175


I did both. Set point 175 with 10 offset so that it would still shut off at 185. Took the tank temp down to 112. Pump speed 2. The outlet hit 183 - 8 degrees over set point - before it started creeping back toward 175. It finally came back down to 175 and then stayed between 175-177 until the call was satisfied. ∆T was between 16-17 in the beginning and 8-9 toward the end.

Did a second test. Only took the tank down to 134. Kept the pump on speed 2. The boiler water was already hot and the boiler was slow to modulate upward for the first few minutes of the call. Wasn't long before it went above 175 and maxed out at 181 before coming back down to 176-177 when the call was satisfied.

Nice! These settings effectively solve the short cycling problem but I'm still calling tech support tomorrow. Isn't something wrong if the boiler can't respond fast enough to factory-default settings and under normal operating conditions?

keyote

Boon i ordered that dhw sensor your guy seems to be correct khn parts dept says tech dept doesnt know new part numberes.
anyway have you any updates on that dhw situation or are you resigned to that last set up you said more or less worked?

rick in Alaska

Sounds like you are not getting rid of heat fast enough in to your indirect. I just pulled up my KHN service manual, and on page 23 there is a heading called"fan speed limiting for indirect DHW tank". If I understand it right, you can set a maximum fan speed for the burner, which will effectively lower the output to match what you can absorb in the indirect. It is in the parameter settings, which I think is called "DHW max rate " under the DHW settings.
I would assume you would still have the same offsets above and below setpoint though.
If you don't want the boiler to overshoot its setpoint, you can set the boiler offset temp to zero. The default is 5.
I think this is the info you are looking for.
Rick

Boon

<div class="Quote">[Keyote said]... have you any updates on that dhw situation or are you resigned to that last set up you said more or less worked?</div>
I thought I posted this comment a couple days ago - sorry. I've been back and forth with tech support a bit but nothing has changed. I saw the 175/10 settings also short cycle but only once. I'm curious to find out how yours performs.

Rick, thanks. I double-checked with tech support last week and they agreed that my 85k boiler shouldn't need a fan limit for a 140k input indirect tank, but - as Carl noted above and perhaps what you are suggesting - the conditions that allow that 140k input rating may not exist in my situation. I've been trying to reconcile the Smart 50's performance conditions with my conditions and I don't see enough differences [in cold water inlet, degree rise, circ speed] that would reduce the indirect's input rating by more than half.

I suspect you and Carl have given me the solution but I'm having a hard time accepting that this is how it is supposed to operate.

<div class="Quote">[Rick in Alaska said] ... If you don't want the boiler to overshoot its setpoint, you can set the boiler offset temp to zero.</div>
So I set my DHW Boiler Output to 175 and Offset to zero and when the outlet temp hit 175 the boiler shut down and the display said, "Setpoint Met." [Maybe the temp hit 176 but didn't update the display fast enough]. This experiment further confuses me.

keyote

thanks boon i just received what i expect was the same sensor you got from procure though it seems like it came direct from lochinvar.its only got a short pigtail which is fine i can heat shrink an extension but its got this bracket attached that seems like it designed to keep the sensor from going down into the tank more than about four inches, no instructions of course. this seems wrong is yours the same, the triangle tube aquastat was designed to sit down below the thermocline where i would think it would sense a need for heat before the top of the tank. dis this come up?Im tempted to cut that thing off and drop it down into tank 2/3

Gordy

Is this a new indirect Boon?

Boon

Agree, Hatt. Yes Gordy new indirect new everything.

Keyote that is the same.sensor. The metal wire is for the Squire indirect. Just cut off the curly part, extend the leads, and drop it down the chute.

Eric_32

Ramp delay only works on heating, DHW does not work with ramp delay.

I use ramp delay on boilers with multiple zones... works great to help prevent short cycling smaller zones.

keyote

Thanks boon did they give you an ideal depth for the sensor to be hung? you used the the aquastat well ? is it dry or can the leads short out if not water tight?

Boon

did they give you an ideal depth for the sensor to be hung?... dry well?

No instructions. The factory sensor's depth seemed like a fine place to start. I marked the top of the lead on the factory sensor before I removed it from the well and then I installed the Lochinvar sensor to the same depth.

keyote

do you have any idea what the theory was to place the sensor so high in the lochinvar indirect? like you my thinking is the water sensor down low should get the boiler a head start on a full tank of hot water. i see in the TTsmart book its actually a 37" deep dry well who knew, also notice in the TT smart books this;

"When adjusting thermostat, be sure boiler limit
control is set a minimum of 20ºF higher."

Despite this I agree lochinvar seems to have an issue modulating that worries me more because of small heat zones than the DHW. Hoping I have better luck and its something they can adjust for you.

keyote

hey boon just poste some pics under gas heating if you want to check me out. was wondering if you knew what that brass fitting is tapped into the fresh air starter piece looks like it ought to have a hose or is it when they test the flue gas?

Boon

Here is how this ends:

Shortly after this thread went dormant, I tested the DHW boiler set point at 190 with a 5 degree offset. With the 190 setting - and no other operating condition changes - the boiler behaves flawlessly. As it approaches 190 it sharply reduces the firing rate so as not to exceed 190.

This week, and with much colder incoming water, I decided to test the DHW boiler set point back to the default settings of 180 with a 5 offset. And, just like it did originally, the boiler doesn't hesitate to overshoot the set point & promptly shuts off.

So I'm keeping the set point at 190 and considering this issue resolved even though I don't understand how those 10 degrees of set point change my boiler's attitude.

keyote

Hmm I thought my setpoint was 180, because I have never seen it exceed that. As you say it approaches 180 at the last minute or less of cycle, and seems to almost alway hit 180 just before it gets the tank to 140 then shuts down flawlessly every single time. Never had a DHW shutdown.
However I now see my conxus says the setpoints 185,So I guess wanted 186 plus a 10 offset to fall under the autohighlockout.The boiler doesnt modulate below 100% until it hits 180, but it manages to hold it steady there for a minute or so. One point is it doesn't sharply modulate, it barely modulates, never seen it mod below 96 usually less, and sometimes it just nails it and doesnt mod at all simply hits 180 and 140 at exact same time and shuts down. But my offsets always been 10 degrees. My high limits are auto 200 and manual 210.This set up has worked fine on med speed of a 3 speed wilo as recommended. I later switched it to high speed, and it worked just as well so its been there since. I think its the 10 degree offset not the set point, that makes the difference. Though i cant explain why since i dont recall ever exceeding 180.

Gordy

The control is most likley based on satisfying DHW recovery as quickly as possible. I really don't see this as a control "issue" since most indirect tank ratings are based on high supply temps. The hotter the supply the faster the recovery obviously. Btu needed is a btu supplied. Do you really save anything on higher supply temps by 10* with a faster recovery than lower by 10*. Especially since neither will allow condensing.

Harvey Ramer

What type of indirect tank do you have?

keyote

Yes it seems like its trying to reach the setpoint temp asap,it ramps right up to 100% mod, then the temp continues to climb to the setpoint temp,while the tank temp also climbs, the DT and flue gas ratio seem to steady once the mod is 100%. I do find it curious its timing is so impecable, it rarely has to wait long for the tank to hit 140 once its hit 180, but it can do it and on average it will mod down a couple degrees to hold 180 until the tank tops off.
I have a TT smart 50 i think boon has the same. One factor is my near pipe is very tight and i think his is quite long.

Harvey Ramer

OK, so it holds 8 gallons of water on the boiler side of the tank. What boiler do you have and how many gallons does it hold? How many gpm are you moving on a call for DHW? How many seconds or minutes does it take till the boiler over shoots the DHW boiler outlet temp setting?

ced48

I reading your initial post, and not much more, I must say, that it is my understanding that Lochinvar boilers will only modulate in the space heating mode, and only with an outdoor temperature sensor in place. Maybe the KHN's are different than the wall hung-

njtommy

As long as the boiler is not short cycling on a call for DHW I think your fine.

keyote

Answering for myself [we both have the same boiler khn 085 and tank but a bit differrent piping] as you say 8 in the tank jacket and almost none in the rest maybe another gallon or two between the near pipe and boiler. I havnt timed it but id say five minutes.I am not sure what the hi speed on a wilo 3spd pump puts out i want to say 20 gpm @ 0 head
well in the sense that they are trying to heat the tank asap they dont modulate,they shoot right up to 100%, but in as much as they try to not overshoot, they mod down a degree or two, to hold the 180 once hit, until the tank hits 140.

Mad Dog_2

Just a little "light" discussion on a Sat nite in front of the fire.....You created SOME place here Holohan...you done reeeeel.good. Mad Dog

Gordy

Hatterasguy said:

Gordy said:

The control is most likley based on satisfying DHW recovery as quickly as possible. I really don't see this as a control "issue" since most indirect tank ratings are based on high supply temps. The hotter the supply the faster the recovery obviously. Btu needed is a btu supplied. Do you really save anything on higher supply temps by 10* with a faster recovery than lower by 10*. Especially since neither will allow condensing.

It is a control issue.

The boiler modulates perfectly with a DHW setting of 190F and cannot modulate properly with a DHW setting of 180F. There is no other option.

Correct so what's that saying loch wants......

Gordy

Mad Dog said:

Just a little "light" discussion on a Sat nite in front of the fire.....You created SOME place here Holohan...you done reeeeel.good. Mad Dog

Miss me fireplace.....Nothin like a fire, good reading, and some spirits to warm the bones

Harvey Ramer

Run another one of those tests, that causes the boiler to stop due to overshoot. Monitor the boiler inlet temp while you are doing it and see if there is a sharp spike in return water temp that corresponds with the boiler overshoot.

Harvey Ramer

Sounds like it, but I'm still curious.

Boon

@Harvey Ramer I will review a couple videos I took previously because I'm not in a position to do a live test. I'm pretty sure the supply & return temps only bump up a degree at a time in lock-step and very gradually.

For those of you in front of a toasty fireplace this evening:

Hat>> it's a control issue
Gordy>>> Correct so what's that saying loch wants......

Gordy, Loch may want a 190 set point and if they do then it would be nice if they weren't so passive-aggressive about it. If they want 190 then (1) they shouldn't give me a default set point of 180; (2) they shouldn't even give me an option to select a different set point.

Harvey Ramer

While you are at it, have you calculated how many gpm you are moving on a DHW call? Also is the DHW piped as a zone off of the boiler loop? Is the setting in the boiler set to the proper DHW piping configuration? Just trying to make sure you don't have a situation where the boiler would be using the system sensor to measure output while the DHW is being pulled off of the boiler loop.

Note, the safeties operate off of the internal sensors. However, if a system sensor is being used, and the DHW is set up wrong, the boiler would still attempt to target DHW Supply temp from the system sensor.