Lochinvar Knight MRHL/ ARHL issues

Discussion in 'Boiler Forum' started by Sadmir, Dec 3, 2019.

  1. Sadmir

    Sadmir New Member

    Joined:
    Dec 3, 2019
    Location:
    boston
    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 http://streamable.com/sckml

    HERE IS THE BOILER PIPING DIAGRAM
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    Last edited: Dec 3, 2019
  2. Dana

    Dana In the trades

    Joined:
    Jan 14, 2009
    Location:
    01609
    I'm not clear which model KBN you have. A KBN160 is not listed in any of the online manuals- it jumps from KBN151 to KBN211. Or was that a typo- and actually a KBN106? see page 7.

    It's not clear where the Grundfos driving the primary(boiler) loop is located (is it internal to the boiler on this series?) or the tees that form the hydraulic separator. If the closely spaced tees are too far apart from one another or are too skinny, starting/stopping of the secondary pumps could interact too much with the primary loop flow, causing sudden slow-down and subsequent spike in the output temperature.

    [​IMG]

    Also note on page 36 where it points out"

    1. Boiler system piping: Boiler system piping MUST be sized per the pipe requirements listed in Table 6A. Reducing the pipe size can restrict the flow rate through the boiler, causing inadvertent high limit shutdowns and poor system performance. Flow rates are based on 20 feet of piping, 4 - 90° elbows, and 2 - fully ported ball valves.

    Does that all check out?

    In the video it looks like it's running 10F - 15F delta-Ts (which is fine) right up to a few seconds before the lockout the delta-T suddenly spikes to over 20F (the big change all at the outlet end), as if the boiler loop pump had suddenly stopped pumping. I suspect insufficient hydraulic separation as the proximate cause, possibly compounded by oversizing of the boiler for the zone radiation.

    FWIW: The very smallest boiler in that series, the KBN081 would cover the loads for 19 out of 20 houses in the Boston area (with margin!), and anything bigger is unlikely to modulate very much in heating mode. The KBN series only has about a 5:1 turn down ratio, which means you might not have enough zone radiation to emit the full min-fire output of the smaller zones (or even bigger zones.) The minimum fire output of the KBN106 is about 19-20,000 BTU/hr, which is fully half the design heat load of may 2400' antique bungalow + 1600' of insulated basement in Worcester- I only have one zone that can even emit that much heat at condensing temperatures (a hydro-air handler). What type, and how much radiation do you have, zone by zone?
     
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  4. Sadmir

    Sadmir New Member

    Joined:
    Dec 3, 2019
    Location:
    boston
    @Dana The boiler is a KBN-106. Sorry for the typo. The boiler pump is outside on the inlet side of the primary loop. I checked the spacing of the tees and they are under 4x the diameter. Basically next to each other. And the piping seems ok per the criteria you mention above. I ended up turning down the firing rate of the boiler and not getting as many lockouts now, however on SH calls there is a lot of short cycling.

    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
     
  5. Dana

    Dana In the trades

    Joined:
    Jan 14, 2009
    Location:
    01609
    The boiler pump should NOT be shutting off while the burner is still active, especially if there is still an active call for heat. (Definitely not normal operation.) I'm not sure if there is a post-purge or other programmable setting that could affect that. Will Lochinvar tech support talk to you, or do they only talk to installers/contractors? (I've never called them myself.) Read through the manual and verify that the primary pump is wired correctly.

    Can you describe the short cycling? (Length of burns, time between burns, etc.) Turning down the max firing rate on the boiler usually either reduces short cycling, or has no effect, depending on whether there is sufficient zone radiation on the zone(s) calling for heat to emit the full minimum-fire output or not.

    Have you measured up your zone radiation?
     
  6. Sadmir

    Sadmir New Member

    Joined:
    Dec 3, 2019
    Location:
    boston
    Issue ended up being the pump. It was replaced and its working perfect now.
     
  7. Dana

    Dana In the trades

    Joined:
    Jan 14, 2009
    Location:
    01609
    Great!
     
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