Lochinvar high limit manual reset - keeps locking out

[Dwassner]

Little confused here. I recently turned the setpoint up from 170 to 190 with a 9 deg differential. I wanted the max temp through the red pex to be under 200. During the cold season 170 does not warm the house.

Since increasing the setpoint, I keep going into lockout with either the manual or auto high limit. I am only able to turn the limits up to 200 and 195 deg respectively.

What really confuses me though is that the lockouts only began after the outdoor temp went above the low outdoor reset, meaning that the boiler setpoint is now lower than before, yet it is throwing a high limit reset.

Can anybody offer any input?

thank you,
DW

 

[Dana]

A model name/number would help, different Lochinvar have different outdoor reset controls.

 

[Sponsor]

 

[Dwassner]

It is a knight WHN055.

 

[Dana]

I have not yet pored through the minutae of how to set up that boiler in the manual, for that boiler but you should.

It's not clear if there is a separate manual for Adjustable Manual Reset High Limit kit (which you apparently have?)

 

[Dwassner]

I have several pieces of literature that came with it and have read them all. They explain how to adjust both the manual and auto reset high limits, but the manual states that the automatic reset has a 200 deg max and the manual reset has a 210 deg max. Mine only allows me to go up to 200 and 195 respectively.

What I am unable to grasp is why the limit is even being reached. Over the last several days I have been turning the differential down 1 deg at a time, with a setpoint of 190. I am now at a 2 deg differential. Does this not mean that the boiler will reach a max of 192 degrees before shutting off? If so, then why are either of the reset high limits even being reached?

With a setpoint of 170, which was too low, which I ran for several years, the limit temp was never reached for obvious reasons, but I am unable to grasp why it is being reached now.

DW

 

[Dwassner]

I realize now that I have been referring to the offset in the recent posts, but saying "differential". Sorry for the mistake

 

[Dana]

The meaning of the offset or differential varies- there is no industry-wide standard definition. Sometimes it refers to how far below the setpoint the water temp has to be before it re-fires, sometimes it means how far above the setpoint it will run before turning off.

A safety high limit lockout normally can't be overridden by the other controls, even if the parameters of those controls allow programming it to a temperature higher than the safety. That may be what you're running into. The manual reset kit is probably somewhat generic across their product lineup, some of which may have a higher safety limit, but the safe operating limits of the WHN055 don't change when the kit is installed.

Try backing off the setpoint to 180F, with a 10F offset, or a 185F setpoint with a 5F offset and observe it's behavior. Bumping the temperature from 170F to 180F delivers more than a 12% increase in the heat emitted by the radiation, bumping it to 185F delivers more than a 15% boost. So if 170F was enough to just barely keep the house at 70F at Rochester's 99% outside design temp of +5F (a 65F difference), boosting it 10% would have you fully covered down to about -2F, and 185F would have you fully covered down to -5F before it started losing ground, assuming the boiler has non-condensing output at full fire to deliver 185F when it's that cold outside.

Bumping the boiler temp up doesn't help if there isn't enough burner. While it'll deliver about 51,000 BTU/hr at full fire in condensing mode, it'll only deliver ~48,000 BTU/hr at temperatures that high. If your heat load is 45,000 BTU/hr @ +5F you're covered, but at -5F you're not. If you're not sure what your actual heat load is, run a fuel-use based heat load calculation to get a pretty close ball-park number.

But you could also be radiation limited, in which case the boiler would always be able to hit it's setpoint temperatures without actually reaching full-fire. A bit of napkin math would be able to show if that's the case. If the radiation can't deliver at least 48,000 BTU/hr at an average water temp of 185F raising the boiler's temp to 190F is pointless.

 

[Dwassner]

thank you, Dana. that all makes sense. I calculated the radiation potential several years ago and don't recall that being an issue.

It appears that you are suggesting to start with a 5F difference between the setpoint + offset and the high limit. Is there any reason not to drop the offset to 2 deg? I just created another thread where I was basically asking how critical the offset setting on a mod-con is, since the unit decreases the firing rate before it hits the setpoint. It just changes the firing rate to hover around the setpoint.

The reason I ask about how critical the setpoint is is because I want to avoid short cycling. Perhaps I can decrease the offset setting and increase the differential to avoid this?

thanks again,
DW

 

[Dana]

Opening up the temperature swings increases the minimum burn times at water temperatures when the radiation can't deliver the minimum-fire output of the boiler, thus reducing short-cycling. With high-mass high water volume radiation it usually doesn't matter (especially at the fairly low minimum-fire output of the WHN055) but with fin-tube convectors it does. If most of your heat emitters are low mass you may want to run an experiment over the weekend and measure the burn times when the output temperature is cranked down to 125F and with low offset/differential settings. If the burn times are less than 180 seconds the short cycling potential at condensing temperatures is high.

 

[Dwassner]

I am not sure if my heat emitters are low mass. They are just standard lengths of fin tube baseboard radiators.

When you say "low offset/differential settings", you mean each of those settings being a few degrees apart from the setpoint temp?

By doing this experiment at 125F, I am not quite sure what I am testing for. Sorry. Seems like this would tell me how capable the boiler is at maintaining a tight range? Is there a reason I should try this at such a low temp of 125?

 

[Dana]

The 125F test is to see if it can operate at 95% efficiency in in your system without short-cycling, since short-cycling is one of your concerns. To get that type of efficiency out of it, the entering water at the boiler has to be under 120F, with an average water temp in the fin-tube of about 120F:

At an average water temperature of 120F (125F out of the boiler, 115F return) you'll be well into the condensing range and getting ~95% combustion efficiency out of it, but the baseboard itself is only emitting ~200 BTU/hr per running foot of length. At it's minimum firing rate the output of the WHN055 is about 10,500 BTU/hr, so at those low very efficient temperatures it takes 10,500/200= 53 feet of baseboard to emit that much heat with NO cycling.

If there is less baseboard than that and the setpoint if 125F more heat is going into the system than is being emitted, so the water temperature will rise until the boiler detects that it's above the setpoint, and turns the burner off, then waits until it falls into range again. Depending on the differential and offset numbers, just how much above the setpoint it turns off, and how far below it will fall before re-firing. (Just how the manufacturer defines those terms will determine which is the more important term to tweak.)

Fin tube baseboard has very little water volume, and thus low thermal mass. There is at least some amount of water in the boiler and distribution plumbing, at 8.34lbs per gallon it will add up to something. The reason it matters is this:

Say there is 4 gallons of water in the system, including all baseboard, the boiler, the distribution plumbing, that's ~33lbs. If the boiler is putting in 10,500 BTU/hr and the baseboard is only emitting 8500 BTU/hr of heat in to the rooms you have 2000 BTU/hr (33 BTU per minute) of "extra" heat going into that 33lbs.

A BTU is the amount of heat it takes to raise a pound of water one degree F. If the temperature swing is programmed so tight that it can only rise 2F before the burner shuts down it takes only (2F x 33lbs)/33BTU per minute= 2 minutes, which is a very short burn. But if it's set up for 5F temperature swings it'll take 5 minutes, which is fine.

When the return water temps coming back to the boiler are above 125-130F the most you'll get out of it is 86-87% efficiency, so it's nice to be able to run in the lower temperature ranges whenever possible, which is one of the major reasons to use an outdoor reset approach rather than manually setting the temperatures.

This is spelled out in some detail in this bit o' bloggery. The simple napkin math model isn't the whole story (reality is a bit more complicated than that), but it doesn't lie either.

 

[Dwassner]

Thank you, Dana.

I have not yet done the test above as I am still not past the point of avoiding the high reset limit lockouts.

I am unable to make any sense of this for the following reason:

I have been incrementally decreasing the setpoint 1F at a time until I get to the point of avoiding the lockout. I am now at 180F with a 1F offset!
This makes no sense to me, as I can visibly watch the unit fluctuate firing rate to maintain within just a few degrees under the setpoint, or exactly at the setpoint, and it rarely goes above it. It does this until the space-heat demand is satisfied. I see NO REASON as to why the inlet/outlet sensors are hitting 195F as the settings are a considerable distance from that temp.

The boiler pump and dhw pump are both programmed to run, I believe 20 seconds after the space-heat demand is satisfied.

please help!

 

[Dana]

Got an online reference point documentation/manuals for the manual reset kit? It's a bit like shooting in the dark without it.

How does the system behave when you open up the offset to 10F or higher, with the temperature set to 180F?

Have you tried measuring the in & out temperatures more directly (with meat thermometers taped to the plumbing, over-wrapped with an inch or more of fiberglass insulation)??

 

[Dwassner]

I don't have any documentation on the manual reset kit. Was not aware of any.

I haven't tried increasing the offset as it seemed like it would do nothing at best for the issue and if anything make it worse.

I have not tried measuring the in/out temps as they appear to be inside the boiler construction itself. There is also a 3rd sensor, system temp. I can see the reading on all 3 simultaneously on the display. The outlet temp, which is inside the boiler unit, never appears to go above 3F of the system temp. The system temp sensor is roughly 10ft downstream of the outlet sensor.

FWIW, as soon as the system even begins to decrease firing rate, the display shows all 3 temps dropping. They appear to be quite real time.

If the setpoint is 185F, offset is 1F, outlet temp is at most 3F more than setpoint, then I don't understand how anywhere in the system can reach a temp above 189F, especially since the pump continues to run for 20 seconds after shutdown, which would allow the heat exchange to be cooled with incoming water. I don't understand where the additional 6F are coming from to cause the high limit to lockout...

I am not a professional at this, but do feel like I have a decent grasp at the concepts at hand. With that being said, I suspect that nothing is wrong/damaged with the system, but that there is a setting that I am not using correctly.

Have you ever plugged a laptop into the connection on the face of the display? I am curious as to if I could see a chart that shows the issue at hand.

thanks again, Dana for all the input here.

DW

 

[Dana]

Without documentation, if you don't experiment with the offset number and observe the behavior it's hard to know for sure what that parameter is really doing. Changing it a full order of magnitude (from 1F to 10F) should make it easier to spot what it's doing.

A post-purge of 20 seconds of heat out of the heat exchanger is still pretty short, and may be related this symptom. If that parameter is programmable, give it 50-100 seconds, to see if it affects anything.

Lochinvar recommends always plumbing it as primary/secondary, with a hydraulic separation external to the boiler (see page 38 of the manual.) I don't believe there are by pass branches such as hydraulic separators etc. The stated reasons for that recommendation are:

It is recommended that near boiler piping systems utilize Primary/Secondary configurations as shown in FIG.’s 6-3 through 6-9 only. The use of other near boiler piping configurations could result in improper building and system flow rates leading to inadvertent boiler high limit shutdowns and poor system performance if proper pump sizing and system flow rates are not adhered to.

(emphasis mine)

When facing the boiler there are two pipes coming out the bottom on the left side of the unit. The one closest to the wall is the output, the one closest to the front is the return/input, which would be the place to take the measurements.

 

[Dwassner]

I will increase the post purge time and see what happens.

The unit is not set up as primary/secondary. The 1" copper exits the boiler output and branches to each zone at 3/4". It is only a 1300 sq ft single story ranch and the boiler is located centrally in the basement. I only mention this because several years ago when researching how long each loop could be, I recall being well under the length to be concerned about flow restriction.

A few months ago I did turn the pump speed from high to medium to see how this impacted the performance. The delta T increased maybe 12 deg, which I believed would cause the unit to run more efficiently. It was not until a while later than I changed the setpoints.

I can change the pump speed back to see if there is a difference, but thought it would not be beneficial because of 1) the lower delta T (efficiency) and 2) a higher inlet temp which would again push me toward the high limit lockout.

DW

 

[Dana]

Lochinvar's definitions of differential and offset can be found on p.18 here.

SH Offset

The SH offset sets how many degrees above set point the temperature has to go before the boiler will shut off. This parameter can only be changed by the installer by accessing parameter B4. The temperature range of this parameter is 0°F to 86°F. The default value is 10°F.

SH Differential

The SH differential sets how many degrees below the turn off temperature the temperature has to go before the boiler will turn on. This parameter can only be changed by the installer by accessing parameter B5. The temperature range of this parameter is 0°F to 86°F. The default value is 20°F.

Setting the Offset to 0F, and the Differential to 20-30F, it SHOULD be OK to run at 180-190F setpoint,as long as the flow through the heat exchanger is fully up to spec. DO bump the pump's speed setting higher!

 

[Dwassner]

that is what I would have thought as well.

A question about increasing the post purge: the pump has the ability to keep flowing through a closed zone valve, correct? If this is not the case, then once the spaceheat demands are met, the zone closes but the pump continues to run the 100 second post-purge

 

[Dana]

A closed valve is a closed valve, and the pump won't pump through it.

If this system is set up with only a single pump and some zone valves, pumping-direct through the boiler rather than primary/secondary, there is NO post-purge occurring at the end of a call for heat, but there would be if the boiler reaches the high-limit while the call for heat was still ongoing.

That is one of several reasons why a local pumping loop (called the "primary") setting the flow through the boiler, and another loop & pump driving the zone flows, with a hydraulic separator of some type where the loops intersect can be a good idea. When the radiation flow requirements are a reasonable match to the boiler's flow requirements pumping direct (no primary/secondary) is often possible, but has to be designed to work OK at all temperatures and flows. Delta-Ts across the boiler of 50F or higher can damage many boilers, and flows lower than the specified minimums for the boiler can lead to flash-boil on the heat exchanger, lowering efficiency and sometimes lifespan.

The hydraulic separator doesn't need to be a pre-engineered component- closely spaced tees will do, but the tees have to be less than 4 pipe diameters apart to minimize interactions between primary and secondary flows.

This is a simplified diagram of primary/secondary plumbing. The indirect water heater has it's own pump, since it's flow requirements are generally higher than radiation typically needs. If the radiation was broken down into zones with zone valves, the zone valves would be down stream of the radiation pump, and would not affect the flow of the primary loop driving water through the boiler.

 

[Dwassner]

hmm. I am glad I asked. I actually had a Lochinvar tech at my house to inspect the boiler just after it was installed and he stated that the post purge pumping would be able to pump through the valves being closed, that it was not an issue.

My indirect tank does have its own pump on its own primary loop with no zone valve

This is now a little overwhelming. I have used the boiler for almost 6 years so it is hard to justify reinstalling it...

What if I replaced the zone valves with pumps? Then they would continue to pump post-purge and I would still be able to utilize zones.

I think I will manually lock the valves open for now and increase the setpoint to see if the lockout continues. I would think that if the pump was going to fail from this, it would have already. There are only two zones and their room temps are usually the same. The issue would get worse if I wanted to run a loop in the basement.