Knight, Munchkin or Weil McClain Mod Con?


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Old 09-15-08, 04:14 PM
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Knight, Munchkin or Weil McClain Mod Con?

I am converting to propane and want a high eff mod con boiler with an indirect water heater. I was leaning towards the weil mclain ultra but am wary of the cast aluminum heat exchanger even with its 15 year warranty. Am I just being irrational? The knight and munchkin (which are similar in price to the weil ultra) have stainless steel heat exchangers which I think may be more reliable. Anyone have a recommendation or any experience with these boilers? I need to make a descision soon bu do not want to be too hasty.

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Old 09-15-08, 05:50 PM
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Of those three, I'd probably pick the Knight. That said, the Knight and the Munchkin both use the exact same Giannoni heat exchanger. (Made in Italy.) The new Ultra3 has a lot more sensors for tuning the boiler control, though. If you have good-quality water and can keep tabs on the pH, it is worth a serious look.

In the larger scheme of things, the jury is still out on the long-term (decade+) durability of these boilers. However, the manufacturers have been building and improving them for nearly that long (or more), so the current models are at the least 2nd generation.

If you have an option for the Triangle Tube Prestige, that would be my preference. The Solo 60 or the Prestige 110 are very well regarded and have a 'self-cleaning' heat exchanger design. These can also be piped direct, rather than primary-secondary like required for the three you are considering. (See their respective install manuals for the how and why of primary/secondary. Basically it is to provide adequate flow through the heat exchanger, independent of system flow.)
 
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Old 09-15-08, 08:30 PM
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Thanks XIPHAS

Thanks for the response. I need a unit that provides a max output between 110 and 120k BTU/hr (I have gone through multple heat estimations to verify this) for my 3800sqft old farm house. That would require using a prestige 175 which is about $600 dollars more than the Knight. I have read good things about the prestige in the forums but precluded it because of price. Do you think the extra cost is justified?
 
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Old 09-15-08, 11:20 PM
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I think the extra cost is warranted, but I also think that if the heatloss is 120 that I would totally go with a 110.
 
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Old 09-16-08, 05:40 AM
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I will be using propane, According to the prestige 110 rating with propane the max BTU is 93k BTU far below my max estimate. The prestige 175 cost $4400 which is well above the weil and the knight. What do you think the prestige provides over the others (ex no secondary loop, better heat exchanger)?

Thanks
 
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Old 09-16-08, 06:22 AM
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I'm with Who. My inclination would be the 110 Prestige. Take a look at your heat loss, and run it with a common (not design) winter outdoor temperature (if your design is 0 or 5F, use a temperature say in the mid-20s to mid-30s). See how many BTU/hr you lose.

The bottom end modulation of the 110 is 30k BTU/hr. The bottom end of the 175 is 50k BTU/hr.

If your commonly experienced heat loss is <50k BTU/hr, then that argues pretty strongly for the 110. The 175 would sit on its bottom end (no pun intended) and never modulate. Instead, it would cycle on short bursts.

You could also tally up your estimated maximum radiation output using 180F supply water (e.g., for fin-tube baseboard, use 550 BTU/hr per foot of element). If the house heats fine with the existing installed radiation, there is definitely no point in getting a boiler with an output larger than that.

Also keep in mind that the Manual J (e.g., slantfinware) heat losses are commonly overstated anywhere from 10-30%. So your 110-120k may well be less than 100k. In which case the 93k output of a propane-fired Prestige 110 would be just perfect.

And also keep in mind that design conditions simply don't occur all that often, or for very long. Literally hours per year, and quite often don't even occur in a given year. Consider this. On average, in a 10 year period, there will be about 350 hours of design conditions. More than half of that will likely occur at night. Given the choice between a) throwing on an extra blanket or wearing a sweater should the space temperature drop a whopping 2-3F for a day, and b) paying for excess fuel and system inefficiency for the other 44,000 hours of the heating season over the same 10-year period, IMHO the blanket and sweater win every time.
 
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Old 09-16-08, 11:49 AM
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What is your estimate base upon? If it is a Manual J calculation, keep in mind that the true heat load is often only 65% of that calculation.

My Manual J is 56K. I estimated my true load was around 46K, it turned out to be 38K.
 
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Old 09-16-08, 03:32 PM
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I used the sland fin software which determined the load for my 3800sqft farm house at 123K BTU/hr with 0F which is the design point for rochester ny. I alos used a wel mclain boiler estimator which arrived at 115K. Also used a rule of thumb proviede by a HVAC contractor wich sizes a unit by muliplying by 30BTU/ft2to arrive at 115K. I would be willing to rough it on the very cold days which we get about two to three weeks of during the year. However, my wife home schools and I am sure shhe would not be as inclined. So I want to assure her and the childrens comfort.
The prestige 175 only modulates down to 49k BTU/hr. The knight and the ultra go down to 30k BTU/hr. Why is the prestige viewed so well? I am leaning towards the knight because th 150 model is 300 dollars less than the ultra and 700 dollars below the similar sized prestige.
 
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Old 09-16-08, 07:01 PM
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I ran the slantfin calculation for 20F outdoors as suggested and the estimated load is 90k. This is about the peak load for the propane 110 presitge. However, my concern is the weeks of the winter where we are below 20F.
 
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Old 09-16-08, 08:58 PM
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If you are using the SlantFin program and your home isn't drafty, I would expect the max heatload to be 70% of 123 MBH, which is 86 MBH and probably less than that.

What water temperature do you calculate to be running at design temps?

My preference on the Prestige (I have one in my basement) is that the HX is a very open design. It is also downfired so a lot of the crap in the gas ends up going out the condensate path instead of fouling up the HX surfaces over time. It also flows much easier and can be piped direct (saves you electricity). The MCBA is a nice straightforward outdoor reset controller.

My Prestige takes 14 watts in standby mode and 98 watts when heating at minimum RPM (which is the norm for a firing cycle). Look ate the boiler pump requisites for a Munchkin or a Knight. Then also think that boiler stays on the whole time there is a heat call, even when the water reaches target temp and the system cycles the burner off and on.
 
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Old 09-17-08, 06:20 PM
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I was fortunate to find outdoor temperature data for my area (Rochester NY). About 10% of the heating season (5080 hours) the ambient temperature is at or below 20F. This is the break point where the prestige 110 LP would be unable to sustain and indoor temperature of 65F (Again based on the slant fin program) . If the program is conservative this may be okay. However, I am unsure about the insulation and air infiltration into the building since I just bought it. All I know is that in previous heating seasons 1650 gallons of oil were used. This corresponds to 227 MMBtu for the seaon. For my area based on degree days this translates to about 150K BTU/hr input based on using degree days (6720 DD, 68 DT). This causes me concern that the slant fin program may be under predicting my load requirements.
You have me sold on the prestige but on the sizing I am unsure and most likely will tend to a larger unit. Thanks for all the help.
 

Last edited by AC4Steve; 09-17-08 at 06:48 PM.
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Old 09-17-08, 06:46 PM
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For mod conds I believe efficiency is equivalent over the whole range of turndown (5-1) - am I wrong. If the min capacity is above the heating load required for 5% of the season how much will this impact seasonal efficiency? It is not possilbe to have a mod cond which turns doown very low so there must always be a portion of the heating season where it will have to cycle on of due to min capacity.
 
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Old 09-17-08, 10:40 PM
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AC4Steve, the difference in price between a 175 and a 110 will buy a lot of caulk and insulation.

Seriously though... the Manual J seems to have been designed to provide enough excess BTUs for recovery at crazy temps.

One other thing to keep in mind is that a modcon lowers your infiltration. You aren't sucking combustion air in through cracks... Personally, that close to a 110, I would make it work.

Modcons are most efficient in the shoulder seasons. The complete opposite of oil. I was hung up on turndown ratios and minimum modulation levels. The Prestige actually from what I can gather is just under a 3:1 ratio, but it does work extremely well on an intermittant fire basis. Mine only stays on and modulating below about 10F.
 
 

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