What good is the "DHW Boiler Offset"?

Boon

I just watched my boiler (KHN085) slowly creep up to 185° during a call for DHW and it shut off. According to the parameters, it was supposed to. The setpoint is 180° and the DHW Boiler Offset is 5°. At 185 it should shut off. But, why? What is the point of shutting off when it could have modulated down and finished the call for heat?

At/around 180 the boiler started modulating down, 99%, 98%, 99%, ... 97%, 98%, ... but the temp slowly [3-5 minutes from the time it hit 180] crept up to 185. The call for HW was still active when the boiler shut off, and as soon as the anti-cycling block ended, which was 1 minute later, the boiler ignited and finished the DHW call. The pump is constant speed, btw.

Maybe I should be asking "What would make the boiler overshoot the setpoint?"

njtommy

It could be a couple of things. Not enough flow like pump speed and pipe size could have a factor in it.
How hot was the tank? what's the set point of the tank? What's the return water temp on the boiler for a call for DHW?

Zman

Do you have a copy of the manual that shows "DHW boiler offset" The version I found on line indicates that you should be setting the max DHW fan speed to resolve your issue.

Boon

Zman I completely discounted the fan limit parameter because the TT Smart 50 is rated 140k BtuH and the boiler output is much less. I interpreted the manual to mean that if the boiler were, say, 250k BtuH and the indirect was only 140 BtuH then I'd need to limit the fan. I reduced the max fan speed about 10% and the modulates to 91% maximum. That doesn't seem like the right solution.

As for some other questions, the thermostat on the indirect is set to the second "A" in "Scald Hazard" ... don't know the actual temp. The ΔT throughout the call is between 16 and 20. Curiously, "SETPOINT MET" never appears on the boiler status bar display.

I watched it closely this time. It spent about two minutes over the 180° setpoint, modulated down as low as 88%, and as soon as the output hit 185° it shut off. The last in/out was 166°/185° 19ΔT at 19:49 into the cycle.

njtommy

Does the boiler keep short cycling for calls of DHW? Was hot water in use or just bringing the tank up to temp from standby loss?

Boon

@njtommy it is bringing the tank up from usage but nothing excessive - a couple showers. I mostly drained the tank for another test and it actually took three cycles to finish the call; the last cycle was barely a few minutes. Something isn't right.

njtommy

What's the tank temp set point?
How's the system piped and what size along with what pump are you using?

Zman

The rate at which the indirect will absorb energy will change as the DHW temp and boiler temp change. The hotter the DHW water in the tank gets, the less energy is transferred. The 140,000 BTU figure is based on conditions that may not exist in your system.

Boon

nytommy >>> What's the tank temp set point? ... piped ... what size ...what pump

Don't know. I'm using the thermostat on the Smart 50 so the setting goes from "low" to "scald hazard" and there is no wet/dry well to get a actual temp. It's all 1" copper, DHW priority, and a Taco VT2218 speed 2.

Zman >>> The hotter the DHW water in the tank gets, the less energy is transferred. The 140,000 BTU figure is based on conditions that may not exist in your system

Totally makes sense but I can't shake the idea that the boiler had plenty of time & ability to modulate down and stay at setpoint. I gotta chew on this some more.

Brewbeer

Try running the circ at max speed, that might keep the boiler from hitting the high limit.

njtommy

For a call of DHW it may not like the 20f delta T. It wants to satisfy the DHW call as a fast as possible. with a 20f delta T it probably won't back down or drop firing rate till the delta T gets smaller.

Zman

I absolutely agree that it should modulate.
I am not very familiar with that controller.
Are you using the aquastat that come with the Smart tank or do you have a sensor in the tank?
Do you have a link to your specific boiler control?

Gordy

you can use the auxiliary connection next to the thermostat as a dry well for a tank sensor. It's the same port that the smart 80 identifies as the dry well.

Boon

This may not shock anyone but I had no clue that well was there. I know "read the manual" is thrown around here [deservedly-so] but dang-it I swear that isn't in the manual!

I'm using the smart 50's thermostat and it's wired to the KH's tank thermostat terminals.

I have run the pump on speed 3 and witnessed the same behavior with a smaller delta t (12-15) but only when the demand took a longer time to satisfy. At the time, I assumed another setting was interrupting the call or the call timed out but I don't have any space heat demands and the SH/DHW timer is set to the default 30 min.

Zman

It will work far better if the boiler knows the exact temp in the indirect rather than responding to an on/off call .
I have always just pulled the factory sensor out of the well and slid in the boiler sensor.

keyote

zman i was just about to ask if i could use the factory sensor well,sinse im pretty sure the other well i would have to put the t and P on top of a tee or something. so next question any recommendation for sensors for smart 50 with KHNs?

delta T

I'll second @Zman, the boiler should have come with a tank sensor, try using that instead. I am not a fan of the TT aquastats.

Do you have any way of independently verifying the temps on the supply and return out of the tank? an infrared thermometer works great if you use a piece of electrical tape on the pipe where you want to measure. I have a sneaking suspicion that it is not quite at a 20 degree delta....

Check the tank temperature on the domestic hot supply line coming out, before the mixing valve (if any), while someone is running full hot water and the tank is satisfied. I am going to bet it is close to 155 or so, and that near the end of the cycle the boiler water is having a hard time shedding btus, but the sensors the boiler is using for delta t are still telling it that the delta is wider than it really is.

Just a thought, need more data!

Boon

I ordered the Lochinvar DHW tank sensor Sunday night as soon as Gordy mentioned the Smart 50 had a well. I figured it would come to this. The sensor doesn't come with the boiler (it ships with the Squire indirects) and Lochinvar's manual say their sensor "is the only sensor suitable for use with the SMART SYSTEM control." pg 56.

My volt meter has a temp probe but readings are all over the place when I secure it to the pipes. I'll see about getting one of those infrared readers.

the sensors the boiler is using for delta t are still telling it that the delta is wider than it really is.

Those are the boiler's sensors!

njtommy

Im sure once the boiler can see tank temp you won't have any more problems. I really like these KHN boilers I've on installed two so far for smaller commercial buildings. Hopefully doing two more in the next month or so.

keyote

boon I am not quite to this stage yet [working on controls] but might as well get ahead of this.Is this the part you found http://www.supplyhouse.com/Lochinvar-TST2050-Immersion-Inlet-Sensor?gclid=CMzXjOqcwc8CFQlZhgodrOYDBQ, I would think a sensor that sat way down in the tank was needed?

Boon

Not that one, keyote. The KH manual says the sensor is part #100170544. The Squire replacement parts list says the sensor is #100208545. These new part numbers are difficult for me to track down. The legacy part number appears to be TST20015, which might be TST20015K now (K for kit). Pick one. I chose TST20015.

keyote

Thanks boon Ill check supply house and parts4heating

keyote

I see supply house has that one hmm have yoy talked to loch tech support about the number discrepancy
has it arrived yet does it fit in the TT tstsat well or are you going to use it in the auxillary port?

keyote

boon talking to loch tech i was told your #100170544. is only the part no itself but to find it to buy you need the kit number which will include ' a bag and instructions and ? a price he said the kit we need is tst30022 and its the same whether using the loch tanks or TT tanks. Im a bit skeptical this short little dry well sensor up in te top is going to give enough infor but he says if it didnt they would change it. I havnt read the book in a bit so ill have to go back

but now i find neither supply house or parts4heating list that part number. hmm so what gave you confidence in this TST20015? I emailed them for more clarification

Boon im looking for that part number in both manuals where did you see it?

keyote

you would think it could do the job from its boiler supply and return sensors

Gordy

Hat if you look at the smart 50 it's just a thermostat like a standard WH. So boon, or rather the KHN has no idea of tank temp via boiler comtrol, and the boiler is really just getting a call with out a definitive temp from the indirect to go by.

Boon I like ZMans idea better. Since you won't be using the aquastat just use that well. I can not find how deep the aux port goes into the tank. I would assume the well for the aquastat would be in a good location for tank depth to get a good reading.

njtommy

@Hatterasguy looking of the manual it hits around that it does control modulation around tank temp, but didn't seem to come right out and say it.

The sensor is not needed, but could help.

I would really like to see how the unit is piped.

Gordy

By using a tank sensor you then open up more control because the sensor is now plugged in to the "smart system logic" of the boiler.

Boon will know more precise tank temperature through the boiler logic, and be able to have a few more fine tuning options verses the factory tank thermostat of the smart 50.

See service manual for more details.

keyote

So the boiler temp water is up at 185 which is higher than the what? even if there were a temp sensor in the dhw could not the boiler water get hotter than say a 180 dhw setpoint before the tank actually reached 180. wouldnt it almost have to exceed the dhw setpoint to transfewr the heat

now the offset is another question its purpose i believe is to prevent short cycling by giving a bit of slack when working close to the setpoint. if you dhw is calling for heat still although the dhw is now up to 180 and the rwt is rising because tank is almost hot enough but not quite so its absorbing heat slower then you are almost sure to overshoot the boiler setpoint.you can widen the offset or its time or lower the dhw temp or raise the setpoint but if youre trying to get the dhw to have the sanme temp as the boiler setpoint it like musk landing one of those rockets on a barge

Gordy

If your domestic tank set point is 180 you just voided the warranty.

keyote

Ok gordy but we dont know what the actual TTaquastat set points are low med hi? but we know the call was still open while the boiler was giving it 180 plus and twice the boiler tries to mod down but goes back up a percent so i would guess its still got a call then too.I used 180 dhw to illustrate but i would think even a lower dhw setpoint might make the boiler still try to hover close to its own setpoint to get those last btus rammed in to satisfy the call.Im just saying if the two are even close there will be a balancing act, where the boiler cant mod finely enough to get an efficient heat exchange. because the heat exchange is so slight the return water comes back almost as hot as it left.

Boon

Well, I installed the sensor and the boiler behaves the same. No difference. I was hopeful, too, that with the tank sensor installed it would treat a DHW call a little differently. No such luck. I documented a short cycle on video (Boiler Pr0n!) including the settings and will contact tech support tomorrow.

@keyote >>> im looking for that part number in both manuals where did you see it?

I found the TST20015 part number on an [old] Squire indirect revision A replacement parts list. I looked there because the manual says the sensor ships with the indirects. The current replacement parts list is Rev B and only has the new part numbers, which were next to impossible for me to track down or cross reference until I found Scott at Procure Inc HVAC, who confirmed that part numbers #100170544 and #100208545 and TST20015K cross reference to the same part, and he gets an additional shout-out for having a website with both the new & old part numbers loaded.

@Gordy >>> I can not find how deep the aux port goes into the tank

On the Smart 50, it is 36 to 38" from the top of the well to the bottom. Thanks again for the clue about using that well.

@njtommy I have about 35 equivalent feet of 1" copper coming out of the boiler, through the pump, into the bottom of the tank, out the top, and back into the boiler. See attached. Nothing scandalous, I'm sure.

Gordy

So what is your tank temp?

Brewbeer

Set the circ speed to 4 and give it a try. The higher flow rate will give a lower delta and may help modulate it down.

Gordy

keyote

can the ramp delay be used on the dhw cycles?if so you could prevent it from going 100% So i was surprised the rwt wasnt that high so it doesnt seem to be caused by it not being able to mod low enough to keep the dt from narrowing in fact it hovers in the high 90swhile the rwt is down in the 160s. gotta be ramp delay

Thanks for the lead on the parts, ill talk to this guy scott, im confused why lochinvar tech support is giving me that TST30022 as the dhw sensor kit.

this short cycling even on the dhw is depressing pretty much paid double to avoid that on the CH and its doing it on dhw, hopefully its a matter of tuning but i would think dhw would be a default set.

ohh hey boon have you signed up for the Lochinvar University spoke training they have a couple dozen videos on the KHN its just a email deal,only watched a couple, months ago but im due to re read the manuals and all the videos since ill be commissioning in a few days

Gordy

What are your settings for the parameters I posted in the service manual?

keyote

gordy i think if you watch the video and freeze it in a couple places you can get his parameter settings

keyote

it seems like another approach is just raising the dhw boiler offset higher 5 degrees is default but 20 degrees is max

Boon

@Gordy >>> So what is your tank temp?

140. After it is satisfied I get a reading of 141 on the pipe before the mixing valve.

According to the manual the ramp delay doesn't apply to DHW. And I thought about raising the set point another 3-5 degrees but that really isn't the point ... if the boiler is supposed to maintain its set points and it doesn't ... that's a problem.

@Brewbeer I get the same result no matter the circ speed.

I'll let y'all know what tech support says.

njtommy

I really think you have a flow problem as in not enough flow for the smart 50 tank and the way it's designed and the boiler seeing 20 delta T.
Not getting the btus out of the boiler fast enough.