Troubleshooting Mod/Con Gas boiler: Water temp runs up past Setpoint- Remedies?

jmon

My first post, am Home owner.
ISSUE: Under certain conditions, my mod/con boiler water temp. runs up beyond "Setpoint Offset" point. Offset pt. is now set at 14F above Setpoint (outdoor reset is used). If+14 is triggered, the system goes thru a block/purge stage until boiler temp cools down to offset differential point and begins to heat again.
Conditions this problem occurs: When the Family Room (FR) Zone is only one on, COUPLED with outdoor Temps above 44F. For example at outdr. temp of 46, the system setpoint is 117, so it will permit a runup to 130 before "tripping"/resetting at 131

The lowest power the boiler can run is at 10%. So, I'm gathering that when FR zone is only one on, water temp will gradually rise until a certain energy demand-equilibrium seems to be 133F in my home- it will sustain 133 at 10% for at least an hour and likely longer. (If I have a 2nd heating zone on, or 1 larger zone on, this problem does not occur, because of added demand it will run at more than 10% power)

Some system/home info. that may help:
BOILER: Lochinvar WHB 110N , 2022 version 110 watt input. Natural gas
2022 installed. 10/1 mod turn down, Used for space heating only, not DHW currently
Home: 3,000 sq ft. 2 story Colonial. Climate zone 5 (Southern New England)
3 zone system. FR is smallest zone

Questions:
1. Is this Offset point tripping/resetting harmful to boiler or does it just waste energy in the purge?
2. If I raise the "Setpoint Offset" higher than current +14 (factory default is +10), this would at least extend interval between system tripping/resetting. Is there a downside to doing this?
3. Other observations?

Thanks for your input/advice.

EBEBRATT-Ed

Is this offset point tripping resetting a (1) normal limit control or are you having to (2)reset a control or high limit to get the boiler running again?

If it is #1 I don't see the issue. If it is #2 it is an issue.

Teemok

Boiler piping picture and what type of heat emitters?

jmon

To Ed;
If offset point gets tripped, the boiler automatically goes thru its purge/post purge stages and will restart again when water temp is reduced to the Differential point (no action needed by me).
Is there a reason I shouldn't increase the Offset point to say +18 or even +20 so that this issue doesn't occur or occurs less often and at a longer interval?

jmon

To Teemok:
It has fin tube baseboard. This zone is a Family Room with 350sq ft. (approx.450sq ft. equiv. factoring cathedral ceilings) and has 27.2' of fins. Other 2 zones are much bigger.
A experienced professional installer piped it, so believe piping was done right. Can post a pic tomorrow if you think it will help.

pedmec

This is normal operation of a lochinvar boiler. It is part of there programming to prevent short cycling.

Teemok

What it sound like is, that zone can get rid of 13,500 btu when SWT is 180F but as you reset to lower SWT temp the output capacity of the zone drops and falls below the boiler minimum firing rate 11,000 so it has to cycle off. Shifting offset raises the calculated set point. There is step firing rate parameter that might help. In any case if a lower set points creates a low load on the boiler it will cycle and that's normal. You can set anti-cycle to wait to re-fire if it doesn't mess with the comfort of the space. It forces elongated cycles. No need for a picture if you're confident in it's arrangement.

pedmec

you can check and make sure that your boiler (primary) circulator is set for modulation with the boiler. if it doesn't then the boiler circulator is recirculating the heated water back thru the boiler which is rapidly increasing the temperature rise thru the boiler.

jmon

Reply to pedmec: I don't know how to check if circulator is set for modulation w/ boiler. However, except for "low load" conditions i described earlier, boiler modulates down to low level for hours...generally getting long cycles (avg. 4-6hrs/cycle) during awake hours-which is what i want. (So, i'm thinking this isn't an issue, right?)

hot_rod

Does it have step firing, and it set for those short cycling? Step firing and ODR should help with issues like this.

jmon

reply to Teemok: Excuse my limited knowledge of boiler systems... your last post that will help me learn:
I do get the minimum firing rate of 11K BTUs and that output capacity of small zone at lower SWT is apparently under the 11K level, creating this issue. How do you calculate that the zone can get rid of 13,500 BTUs at SWT of 180?

But, to my question of possibly further raising Offset point to delay or even eliminate this issue (depending on calculated SWT), you noted that "shifting the offset raises the calculated set point." In this system, with outdoor reset, the heating curve is set by programing min/max SWT to corresponding max/min outdoor temps (maybe that's how all mod/con's work?-this is only one I know).
Anyway, my understanding is adjusting the Offset point does not change the calculated set point-and this seems to be the case as when i adjusted the Offset from default of +10 to +14 (due to the run-up from low load condition), the Calculated set point for that outdoor temp does not change, as shown on screen.

If this makes sense to you, it would seem i can greatly improve this issue by raising Offset point to say +18 or even +20. Is there a risk to do this? (If it matters, my heating curve settings are SWT 165 at 10 outdoor; and SWT 90 at 70 outdr.)

Teemok

The output of fin tube at 180f with normal flow is just above 500btu/ft from manufactures charts. So 27ft x 500.
Sorry, I confused the offset you were talking about. Some boilers have a shift of the OD sensor they call offset.
Reading the service manual I see what you are doing. You can't hurt anything by raising the offset. You are in effect negating the reset curve. Curve says X but you are letting in run up to Y so the curve is less accurate. I would change my curve settings.
90F SWT@ 70F OD is very low for baseboard. The differential (amount below set point to turn back on) is 20F That would be a SWT of 70F. Too close a room set point for good comfort. I wouldn't run below 115F SWT absolute minimum. That's 95F return water. Nice and efficient. 120F SWT might be better.
There may be charts and definitely some math to calculate your baseboard output at SWT 120F. I'm betting it's still below 11,000 btu so cycling is expected.
At one point elongated calls for heat are not worth the electrical energy if the call can't really be satisfied. Pumps running, loss in pipes and boiler cycling with sub 90F water moving. This is assuming thermostat controlled zones. Some designs are constant circulation so that's different.
Don't have such a low set point that the zone takes too long to satisfy. You want the thermostat to stop calling. Don't chase pennies to the limits of the rational.
All the other options, lock out anti cycle, step firing, what ever, are all kinds of mitigations after a rational curve is set.

hot_rod

I feel the "perfect" hydronic system would run 24/7. IF the boiler could modulate down to the lowest load, on the mildest heating season day. It would be nice to never shut down the burner or the pump. This would be comfortable, efficient and greatly reduce the wear on every component in a mod con boiler. No fan cycling, no gas valve cycles, relays, ignitors, etc.

I want a mod con that can ramp down to 1000 BTU/hr

With ODR you can get very close to constant running. the challenge is the weather conditions. Warm winter days may put you below the min fire rate of the boiler.

Here is how I run my shop on constant circulation. It cost me 24 W to run my circ. My thermostat connects to the 3 way zone valve. The end switch calls on the boiler.

I have the same boiler and I enabled the ramp delay to keep it at the lowest firing for a few minutes. I believe the boost function will over ride that if the boiler cannot catch the load after a certain time limit.

It takes some trial and error, but this boiler have a very well thought out control, with lots of options to dial it in to most any heating system.

Teemok

I've set up constant circulation attempting design for constant boiler calls. It's not something I'd offer a customer again without charging a lot of money for "dialing it in". The efficiency and comfort differences just aren't worth it in large temperature swings moderate CA. Works great in continuous cold climates on a structures with uniform loss characteristics with dual small boilers. Boiler nerds, striving for the ideal.

hot_rod

Constant circulation, not necessary constant boiler firing. The constant heat input is common on district systems in Europe. Via a actuated mixing valve you pull just what the building needs at any point in time. TRVs on panel rads or radiant modulate the heat output precisely.
No worry about boiler cycling what you have a massive thermal flywheel, from power plant waste heat or bio-fueled district systems. The pumps do shut down in summer months. Some via a central thermostat in the home. The new Alphas can exercise themselves during long off seasons.

Ideally a system that requires months or tweaking is for some of the owners that come here with an interest in leaning and applying the control adjustment. Willing to try options to get the best comfort and efficiency.


Maybe 10- 15% or installers even know how to access and apply some of the fine tuning functions on mod cons?
Even "smart" and wi fi thermostats overwhelm HO and pros. As we see here almost weekly
.

jmon

Teemok and Hot Rod, appreciate you both staying with this post and offering your expertise- very helpful.
Teemok, thanks for your explanations with a clarity I can understand and your practical views that non heating pros can relate to!

Hot Rod, it's great that you have the same system as i do. I know little about HVAC and nothing about pumps or piping, so some of your posts were well beyond me. But, i've grasped some of it and am trying to use OD reset to create long cycle time which it sounds like you do and I've read the service manual with some comprehension.

I have a different but related question that am interested in opinions:
> Should I add DHW to this unit? (currently I have a standalone 50 gal hot water tank, natural gas.). I'm thinking i have enough capacity with a 110K unit. What are +/- in your view? If yes, would you do it now or wait till current hot water tank gets old? Does it complicate a regular home owner from understanding the behavior of the unit?

Thanks

hot_rod

May as well wait till the tank water heater leaks or fails to add an indirect to the boiler. An indirect is a nice way to generate DHW. It will be a bit more $$ to install compared to a fire tank type WH, however.

If you are comfortable getting into the control menu, there are a number of useful settings. If you ball it up, it allows you to go back to factory default settings with one keystroke.

Go through the manual, right down the settings you want to try. So you can remember whare you are to make minor adjustments.

There really is no way for us to tell you the "perfect"settings. It something you need to adjust to your liking, local weather, etc.

Most of those optional settings already have factory defaults programed in as a starting point, you just need to massage them a bit.