Lochinvar Knight MRHL/ ARHL issues

baxuz

I'm a homeowner who's been struggling with the MRHL/ARHL issues on my Lochinvar Knight KBN-106 boiler since we got the house last year. I live in the Boston area so as the weather is getting colder and more demand for heat I've been having lockouts on a daily basis. Summer months are fine as there is only DHW demand lockouts only happen occasionally in the summer time.The boiler is connected to 4 circulation pumps. 3 for space heating and one for indirect hot water tank. I've drawn and attached a piping diagram for reference. I'm also including some photos and a video of one of the lockout situations.

At first the lockouts seemed random, but lately I've been trying to see under what conditions the lockout happen. I'm still not sure if its a specific thing but it seems that when there is a call for DHW it happens the most. What happens in a typical situation is that there will be a SH call and a DHW call comes. The boiler will switch over to DHW, but as its firing (typically at 100%) and trying to get up to the setpoint temperature, before it reached it will stop firing and there is some kind of noise in the boiler (can be heard on the video starts at 52 second mark) and the outlet temperature will spike up past the high limit and the lockout will occur. It seems that before the water reaches the setpoint (160) there is a surge of how water from somewhere that comes in and trips the high limit. Limits are set to 200 for ARHL and 210 for MRHL.

In certain occasions where there is no SH calls and boiler is in standby mode and the DHW call comes by itself it will modulate properly and not trim the ARHL/MRHL limits. However this is not always the case. At times it trips the high limit even if its only a DHW call. That's why I cant figure out the exact reason why it might be tripping since it seems somewhat random. I believe the main issue is that surge of how water coming form somewhere which increases the outlet temperature and trips the hi limit sensor.

Any ideas why this might be happening? Does the piping seem ok?

HERE IS THE LINK TO THE VIDEO streamable.com/sckml

ScottSecor

At first glance it appears that your firing at or nearly 160,000 BTU and the load is probably about one tenth of that. I did not notice if you were in heating mode or dhw mode in your video, but either way I think your grossly over-sized.

I'm only guessing here, but for DHW you might be able to get 100,000 BTU out of the boiler and into your tank (at best) with your setup. Again I'm guessing but I suspect each heating zone varies from roughly 10,000 to 15,000 BTU. If I'm correct, I think the short answer is you need to not allow the boiler to ramp up so fast and perhaps never let it fire at 100%. In heating mode, I suspect the boiler will cycle a lot less if you restrict the firing rate to say 40%. In DHW mode I'd start with say 60%.

Others will offer better more detailed advise.

baxuz

Thanks Scott. I will try lowering the DHW and boiler firing rates and see how it reacts.

Looking at the specs on the boiler the input rating is 105,000 BTU and the Squire 52 gallon tank has a minimum coil load of 133,000 BTU/hr. Do those numbers make any difference in your suggestion?

ScottSecor

@baxuz in your first post you wrote the boiler is a "KBN-160." Without looking up the model information I assumed it was 160,000 BTU input, I now realize you likely meant to type "KBN-106."

This does change my perspective slightly. The boiler is still heating up faster than the demand (at least in the video). I cannot see how decreasing the firing rate (at least temporarily) will hurt the system. I'll try to give you an simple analogy; your driving your car entering the freeway, you get up to the speed limit and are now at the same speed as the rest of cars, do you still press the gas pedal to the floor? No, hopefully you ease up on the gas pedal and "go with the flow." (pun intended). Remember the goal is to prevent the boiler from short cycling, while providing adequate heat and dhw.

Most indirect tanks for dhw are incapable of "consuming" large amounts of BTU's at normal operating temperatures. Perhaps when the tank is initially filled with say forty degree fresh water and the boiler is producing 180 degree water you will achieve the rated BTU's. However, in most homes where we designed the system, the boiler rarely needs to reach 150 or 160 and the indirect tank rarely goes below 120 degrees.

baxuz

Sorry about that. It is KBN-106. I've made the edit. I will follow your suggestion and still reduce the firing rate tonight when I get home. Should I still make it 40% and 60% as you originally suggested?

Ironman

A couple of questions:
1. Is the outdoor reset curve properly setup to match your emitters and heat loss?
2. What's the maximum SWT set to? It should not be above 180*.
3. Why is there a 26-99 pump on the boiler loop? The manual calls for a 15-58 on medium speed. Over-pumping the boiler can cause the delta T to be too low which may be causing your issue.
4. The secondary loops also look over-pumped. An NRF 22 should be more than sufficient for a 3/4" line. Again, over-pumping can cause or contribute to your issue as it will effect the control's ability to modulate the burner down.
5. Are you sure there's no air restricting flow through the boiler?
6. Why is the indirect piped off of the secondary instead of directly off of the boiler as the manual calls for? It can be done your way if the pumps are properly setup to run that way, if not, there could be no flow through the boiler during a domestic call and that would cause the fault.

baxuz

@ScottSecor I turned down the firing rates and not getting as many lockouts but a lot of short cycling now. In monitoring the boiler what I think is happening is that the boiler pump is shutting off before the burner shuts off. I'm posting another video. You can see the boiler firing rate modulating up according to the ramp delay and everything working fine until it starts to get close to the set point (in this instance 127*). However, when it gets up to about 122* you can hear the boiler pump shut off. The zone circulators are still running fine. So what I think happens is that the boiler pump shuts of but the burner keeps firing. The water in the primary is not moving and it heats up quickly in the boiler and and it shuts of the boiler and then you see the outlet temp spike. Reducing the firing rate has helped with the lockouts, but now its short-cycling because of this problem. This is the sequence that keeps happening boiler pump goes off ... boiler still keeps firing ... boiler heats up water that's not circulating in the primary loop ... water gets too hot ... boiler shuts off abruptly .... pump comes on after a little bit ...it cools down the water below the set point and boiler fires up again and it starts all over

Why would the boiler pump be shutting off early? That doesn't seem like normal operation.

here is the video
https://youtu.be/0KDl2D08wY0

Ironman

That's not normal.
I'd wire the pump hot so it runs continuously and see what happens. If it stays on, the board is bad; if not, the pump is bad.

SweatHog

New here. I've had this same issue. Incoming gas pressure was within specs @ 6.9 w.c. but dropped to less than .95 when boiler ignition was triggered. Factory told me it was possibly a combustion issue but I don't have a combustion meter and no one in my area works on Lochinvars. The saving grace is that the boiler lockout times out after a period and because the lockout is mostly triggered by DHW calls, the house stays warm. But, Ironman, I did't know outdoor reset affects DHW calls. I always thought that, unlike space heating, DHW fired at what ever temp you set the boiler at, regardless of outside temperature.

ScottSecor

I agree with @Ironman. Pump should not shut off while boiler is firing, I would bet this will cause boiler damage if left like that.

I suggest calling a local professional from this site and having them go over these issues. I realize we do not discuss prices here, but in my mind if the boiler overheats and the heat exchanger fails as a result, how much money was saved in the long run by trying to fix it yourself? You mentioned that the system has been problematic since you got the house last year, I'd be concerned about the long term effects of a short cycling condensing boiler that we suspect the primary (boiler) circulator pump turns off while the burner is firing.

I'm guessing that reducing the firing rate is a step in the right direction, however there are obviously other issues that need to be addressed asap.

ScottSecor

@SweatHog you don't have to be a Lochinvar guru to understand that a roughly 6.0" w.c. drop on a low pressure gas supply pipe is a serious problem. Here in NJ we most often design for a .3" w.c. pressure drop on low pressure gas lines, yes that's "point three" inches w.c. So, when the incoming natural gas pressure to the boiler reads 6.9" with the burner off, the incoming gas pressure should drop no lower than 6.6" at high fire (low fire will be about 6.7" typically).

What area are you from?

Tinman

Have you checked voltage to the boiler pump once it shuts off prematurely?

baxuz

I ended up calling Lochinvar and they thought it could be one of two things. Control board or the pump. He suggested I wire the pump as a system pump and see if it works. I switched it and the results were the same. The pump still shuts off randomly during a call for SH or DHW. I even changed the settings so the pump will run continuously but result was still the same. I've got someone coming to look at it tomorrow so hopefully they will be able to resolve the issue. Thank you guys for your insight. I will post an update once the tech has looked at it tomorrow.

Ironman

The pump amp draw is limited to 2 amps to prevent damaging the micro relays on the board. The way you're piped requires that both the boiler and domestic circulators run simultaneously on a domestic call. Check to see that the circs aren't drawing more than 2 amps combined during a domestic call.

baxuz

So I got the tech out here today and it turns out it was a bad pump. The controller was sending the signal just fine however the pump would cut out randomly. He changed the pump and everything is back to normal working 100% percent perfect so far.

Le John

is a variable speed Grundfoss in the Knights?

Tinman

@baxuz That's why I mentioned checking voltage. Can't
assume problem is elsewhere.