Need help with cycling issue- Monitor MZ25S Boiler

**BadgerBoilerMN** · 10-27-2011, 07:07 PM

I'm confused. Our hapless homeowner starts out with "I just finished installing a Monitor MZ25S Boiler" and ends up with "Lastly, don’t assume I am not doing things correctly and cutting corners. I have received a permit from my local building office; I am using the largest, most reputable, local plumbing supply business in my town and am working with a single sales person at the business; I am working with a local master plumber who has over twenty years boiler experience; and, tonight I have an appointment for an HVAC engineer to come troubleshoot my system and confirm whether a buffer tank is necessary for my system."

Which is it? If you "did it yourself" then you might have planned better...thus my time proven mantra. If you are hiring experienced plumbers or going around them direct to the wholesaler and you have an engineer, why come to a DIY site? I think the truth is in one of your meandering statements..."I couldn't justify hiring out the whole job. Ergo my decision in making this a DIY project with input as needed from pros. By the way, the 50k to 60k heating loss estimate came from one the plumbers (it does include both my upstairs and basement)."

It appears to me that you received excellent "free" advise and a fair price, from two experienced professionals - albeit without the benefit of a proper heat load - and chose to ignore it because you wanted to cut corners, lower your cost, get a better deal, etc. Had you really done your research and followed my various blogs, you would have started with a proper heat load analysis and wouldn't have been tempted by one of the 7 deadly sins, avoiding an obsolete, over-sized boiler and the understandably embarrassing position you are in at the moment. You are further mistaken by assuming that I have the time or inclination to belittle or bully you. I assure you I do not.

I consider your situation a teachable moment, as you are but one confused soul and many may benefit by your example and my sound advice.

I am really here to help those who will follow the wisdom gained of experience. I honor the spirit of the true DIYer; their courage, inquisitiveness and independent spirit. Unfortunately there is pervading prejudice among a fraction of the DIY crowd that frown upon any professional help or advice. I will not convince or persuade this lot, but hope to influence the more even minded that a little professional help up front, and perhaps at the end, will make for a safer, more efficient and longer lasting heating system.

Kudos to my esteemed colleagues, who tolerate my bad attitude and have patience when mine is gone.

**craigw78** · 10-28-2011, 10:00 AM

Hi Jim and BBP. Welcome to the party & thanks for the input. Its reassuring to hear that although my boiler and system design has obvious problems things are likely salvageable. I will let everyone know how my project works out. Hopefully i can address some of the issues this weekend. Although, with Halloween festivities it may be difficult to find too much time. BBP, i'm glad you still hang around and post.

Morgan, sorry to hear you suspect I am taking advantage of those helping me. Maybe one of the "pro only" sites would be a better fit for you. I don't mean that to be a judgmental comment, I also work in a field with a high number of DIY folks and believe me, I understand your frustration. The absolute last thing that would be healthy for me would be to spend my free time reviewing their misguided internet submissions. My posts would likely be interpreted by the recipient as much more pointed than yours are here. At times this has been a frustrating and humbling experience and I have started to make a much more significant effort to be patient with the frustrating DIY folks I encounter at work. Have a great weekend everyone and thanks again for your contributions.

**Dana** · 10-28-2011, 11:30 AM

Originally Posted by craigw78

Unfortunately my past usage would probably be less accurate than a size based room by room calc. For the last two winters I haven't spent much time at home and kept the heat 60 degrees or lower the majority of the time without really heating my basement at all. In addition, I recently replaced my windows and entry door as well as insulated my attic. I think I will talk to the heating engineer I had over and see about getting a heat loss calc. I think that will provide the most reliable data. Using the BT = t *(Qh - qload)/(500*dT) formula it looks like my buffer tank should be 80 gallons or more. I currently have a 40 gallon tank I am thinking of adding to see if it improves my situation and if it does I will either daisy chain in another tank if that is feasible or replace it with an 80 gallon tank. Pumping shouldn’t be too much of an issue since my system currently has a Grudfos 15-42 and a 15-58.

It's pretty easy to allow for keeping it at ~60F vs. 68-70F and not heating the basement, and that would put an upper bound on the whole thing. Estimating the air leakage without a blower door test (or even WITH one) is an inexact science, prone to huge errors, and air leakage may be currently dominating the picture. (And as previously discussed, it can probably be cut considerably for relatively short money.)

If you're going to fully condition the basement putting R10 of rigid (or spray) foam on the foundation and building an interior studwall snug up against it with UNFACED R11-R13 batts is cost-effective and quite snug and it would probably knock 15%, maybe even 20% off the whole house heat load compared to what you had before. Insulating and sealing the rim joist with 2" of closed cell foam sealed to the top of your rigid foam & top plate of the studwall would also be important. Rim joists & foundation sill often leak more air than all the windows combined(!).

For a low mass condensing burner like yours a 5 minutes+ burn would be great, but a 3 minute minimum burn time would be fine. Working backwards... With ~45 gallons of water in the system (40 gallon buffer + ~5 gallons in the PEX & boiler), you have ~375lbs of water (specific heat=1). With (90KBTU/hr=) 1500BTU/min of heat input, in 3 minutes minimum run time you'd raise the system temp by only 4F- the hysteresis in the boiler's controls are probably at least 3-5F, so I think you'll still be fine with 40 gallons. (If the hysteresis is adjustable on that boiler it would give you more flexibility.) With a real heat load the burn times will exceed the min, and assuming an average wintertime heat load of ~15K you'd be at about 3 burns per hour compared to your current 20 burns per hour. You're increasing the thermal mass by roughly an order of magnitude, so the number of burns drops by about one order of magnitude. Anything under 5 burns/hour is just fine- doubling the mass with another tank to make that 1.5-2 burns/hour buys you very little.

I have about 50 gallons of local mass on my system (and a modulating burner) with the boiler loop slaved to the ~7F delta-T built into the buffer's aquastat, and it set up to get ~6 minute minimum burns at an average modulated output of ~28KBTU/hr. Min-mod on the system is ~20K, and when the zones are all sipping off the buffer the burns get considerably longer. Theoretically they'd be infinitely long at 20KBTU/hr or more of heat load & perfectly balanced radiation, but in practice it varies. Were it a single zone it would approach the theoretical though. I'm maintaining the buffer at ~130F to be able to use it's internal HX for domestic hot water, and with all zones running it's temp drops to ~118F- good enough for DHW. In a long showers or tub zone with all zones calling for heat it would eventually drop below 105F shower temps, but I have a separate control to stop the flow to the main zone off when the buffer water hits 110F to avoid that possibility (it keeps my marriage happy! :-) ) (Like I said, it's kind of a kludge...)

**craigw78** · 10-31-2011, 08:54 AM

So I piped in a 40 gallon water heater as a buffer tank this weekend and still have cycling issues. My burner is still firing for only about 35 to 45 seconds before shutting off. I have a no contact thermometer that I used to take a bunch of readings on the piping. It appears the flow through my boiler is extremely slow causing the water to heat up to about 145 degrees which in turn causes the upper temp limit to trip and shut off the burner. Then the temp drops quickly as the return water passes through the heat exchanger. The return water is about 110 and the low temp boiler turn on is about 112.

So my cycling appears to look like this, my boiler fires when the return water at 110 sufficiently cools the aquastat probe to about 112. I think my boiler is only cycling about a gallon a minute, so the water slowly migrates through the heat exchanger and within 45 seconds or so reaches the 145 degree upper shutoff limit. Then, 110 degree water from my return flows through the heat exchanger cooling everything to about 112 within about two and a half minutes starting the cycle over again.

All that I can think of that would cause this is some kind of obstruction or malfunctioning valve. Insufficient pumping should not be an issue as I have a Grundfos 15-42 and a 15-58 piped into the system.

I am thinking I need to verify the suspected flow issue with a separate gauge incorporated into the 1 inch boiler piping. I have flow meters on my manifold, but they look fairly cheesy and I am not overly confident of their accuracy. If I have a low flow situation then I could temporarily remove my air eliminator, and Y-strainer and see if I still have low flow and if so then I would have to consider taking apart my boiler to attempt to find a clog or malfunctioning valve. What further confuses the issue is that my boiler has a flow detector which should preclude the boiler from firing if the flow rate is less than 3.5 gallons per minute. The bypass is closed and does not appeat to have any flow through it. I have tried closing my balancing valves until the low flow switch detects insufficient flow and it does shut down my boiler when the flow becomes too restricted.

Any thoughts would be greatly appreciated. My best guess at this point is that I have an incorrectly functioning actuator that operates the valve for the DHW. Maybe something is interfering with the valve keeping it from fully opening?

**craigw78** · 10-31-2011, 11:17 AM

I was just looking at a Lochinvar Knight manual and surprisingly (at least to me) the recommended pump size for a 20 degree temp rise is Grundfos 26-99 or Taco 013 or 011. This got me rethinking my situation and now I am wondering if my pumps are in fact too small? This just seems like an awful lot of head and volume for a small system, however in looking at the head loss chart for the Lochinvars, it looks like at least the smaller Lockinvar boilers have a very steep head loss curve. What are some of the common pumps used on small condensing boilers? I left a message with Monitor, but I don't know how likely it is that anyone will call me back. Any feedback would be very much appreciated.
Thanks

**Dana** · 11-02-2011, 09:10 AM

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:

(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** · 11-03-2011, 06:58 AM

"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** · 11-03-2011, 10:37 AM

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

Fair 'nuff.

**craigw78** · 11-03-2011, 01:55 PM

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.