Need help with cycling issue- Monitor MZ25S Boiler

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Dana

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If you don't have sufficient flow on the boiler, it overheats quickly, since it has very low thermal mass. At 1500BTU/minute and a 0.75minutes for a delta-T of ~30-35F implies at most 4.5 gallons of water are involved in that burn. To run your boiler steady-state at ~1500BTU/minute output with at delta-T of 20F in to out on the boiler you need a flow of (1500/20=) 75lbs of water per minute, which is about (75/8.34=) 9 gallons per minute through the system. Getting the pump sizing/flow volume right (assuming you're doing it with a single pump rather than primary/secondary) depends on the amount of head represented by both the boiler and the radiation, not just the boiler with a presumption of low-head radiation. (This too is a Hydronics 101 type of design issue.)

A 9gpm flow rate is quite a bit to be pushing through the radiation loop- 2-10x more flow than you'd likely ever need for the radiation, since at any flow rate the radiation can't deliver the full 1500BTU/minute at temperatures the boiler will tolerate. If you plumbed the system primary/secondary, with the tank in series with the boiler loop (or better yet, as the point of hydraulic separation between the loops) the head seen by the primary loop pump would only be the boiler (the head of the tank is very low- it's just a fat-spot in a short section of 3/4" piping from a head & flow point of view). The thermal mass of the tank would then be involved in every burn and it would take over 5 minutes of burn for the return water to rise from ~110F to ~145F where the boiler shuts down. A separate pump then drives the secondary (radiation) loop, at whatever flow it needs:

PME_0907_Feat2Fig10Lg.jpg


(Using a HW tank as the buffer you'd install Tees at the tank's cold & hot water feeds, since you don't have 4 ports to plumb to as in a purpose-built tank or hydraulic separator.)

You may have air in the system reducing flow too, not just obstructions & crud, malfunctioning valves & undersized pumps. (Fully air purging radiant can be a real PITA sometimes.)

But this is turning into design-by-debug-via-web-forum. A more reliable approach is to pay a hydronic designer to re-design the system, even if you end up doing all of the sweat-equity plumbing & wiring. It feels like a fairly straightforward project, I can't imagine it adding up to more than a day's worth of design time unless you're adding a lot of radiation to be able to meet your calculated heat load or micro-zoning or something.
 

BadgerBoilerMN

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"turning into design-by-debug-via-web-forum. A more reliable approach is to pay a hydronic designer to re-design the system"

Dana; didn't you mean, continues to be.

It is ironic than I spent 10 years designing, installing, troubleshooting and eventually manufacturing a non-modulating, high efficiency condensing boiler and our main problems were in misapplication, over-sizing and a general lack of hydronic knowledge in the industry and beyond. Manufacturers cannot take homeowner calls and stay in business. They correctly refer homeowners to a competent local professional.

I worked on troublesome condensing boiler when I first became an installing dealer in the 80's. In this particular case the homeowner bought the unit direct from the local "wholesaler" and he and his handyman installed it. To make a long story short, I was called out by the factory, submitted a report and handed him the bill. I noted several installation flaws - several potentially hazardous - and didn't hear from him again.

I did find out later, that he had the local propane dealer out to "fix the boiler" and once he had the propane pressure cranked up enough the boiler reportedly worked great...except the water heater on the other side of the house was then over-fired and burned the place to the ground.

On some queries you have to ask yourself; am I really helping?
 

Dana

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"On some queries you have to ask yourself; am I really helping?"

Fair 'nuff.
 

craigw78

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Thanks everyone for your feedback and insight. After speaking with the HVAC designer I consulted after I purchased the boiler it appears the best thing to do is replace the boiler and go a different route. Dana's calculations are exactly right and unfortunately it’s not really practical to add much more thermal load onto my boiler at the present time or install any other cost effective remedies. So, rather trying to make the system work, struggling with a work around, and spending more money on the current system I am going to go with another heat source design my designer is currently planning out. Thanks again everyone & have a great holiday season.
 
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