Need help choosing Munchkin size

Constantin

The turn-down ratio on either appliance is 5-1, if I recall correctly (Hi Bill!) Thus, the Ultra will chugg along at 20kBTU continuously, while the Munchkin will modulate no lower than 28kBTU. The difference doesn't sound like much, but it could make the difference between a continously-on vs. a on-off/system in the shoulder season.

I've heard that the former condition (continuously on) is preferable to the latter one (on/off). From my standpoint as a former designer, it would certainly make sense to minimize the wear and tear on the ignition system, etc. to have the thing running all the time, rather than weather the stops and starts.

Furthermore, considering how little these appliances find themselves with design-day conditions (2% of the year, 7 days?), it may be best to focus on getting appliance installed that performs best in the remaining 98% of the time. This certainly pushes the balance towards the Ultra.

My only caveat with the Ultra per se, is the return water temp. I hope that your design allows for return water temperatures that are sufficiently low so that the Ultra will always hit that 94+% AFUE they claim, as higher return temps from things like the fan coil could reduce the thermal output significantly, if it isn't tempered by low return temp water like the radiant circuits.

Lastly, it could very well be that the family may be interested in a high-eff indirect water heater in the future. For that scenario, a 140M might be more appropriate, as it will recover the IDWH faster. However, here we're sizing the capacity of the boiler not for the heating load but the IDWH.

Jake Timmerman_3

I am currently doing a bid on a house that has a heat loss of 55212 btu/hr on the first floor, 18853 btu/hr in the basement, and 26770 btu/hr in the garage. The first floor is hydro-air, the basement and garage are both radiant. This is based on a -15 outside temp. The total heat loss is about 101,000 btu/hr. The cowboy in me wants to go with the Munchkin 80m because of the lower modulation btu, but my nerves tell me to go with the 140m. I wish there was a boiler in the middle, because I dont want them to have high gas bills because I oversized too much. Also I will have vision 1 outdoor reset on either one of the boilers. What one should I go with?

Mark Hunt

They modulate


so the 140 would become a 101k boiler when it needed to.

You could push it with the 80m, but the first time it hit design day and the temp in the house was less than 68, you'd be the first to hear about it.

Mark H

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hr

A 105,000

WM Ultra

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J.C.A._3

Look into....

The new Buderus offering. The just released GB series. (sorry Chuck !)

I don't have the cut sheet with me but I believe they have something very close to your needs/doubts. Depending on the load and the piping arrangements, it may be another thing to look into. Chris

Jake Timmerman_3

I might have to go with the Ultra. I have never used it before but it would be closer to my design load and around the same price as the 140m.

Scott Gregg

Peerleess Pinnacle/Munchkin 140

Use the 140. The extra BTUs won't hurt the bill. As stated, they modulate. Plus on a really nasty cold snap like we have now, your system will keep your phone from ringing and make the customer happy.

Uni R

Modulation

It modulates, but the Ultra 105 would modulate down to less than half the level of the 140M, which really should help the bill. If the heat loss is 101K, why oversize by almost 40%?

Uni R

Modulation

Constantin, the Munchkin only modulates down to 44K input (as opposed to 21K for the Ultra - it does have 5-1). That was why I wanted to point out the huge difference at the low end where these boilers spend far more of their time.

Constantin

That's an excellent point!

I thought they had already made the transition across their entire manufacturing line to the new (Duns?) valve that allows higher turn-down ratios. Perhaps their literature hasn't caught up to manufacturing yet?

If the 140M only makes it into the 40kBTU range as far as turndown is concerned, then the Ultra is likely to be a better choice for this application.

.

This entire discussion of WM Ultra 105 vs Munchkin 140 is based on taking the heat calc as gospel. A heat calc is better than no heat calc, but that's about it. The number is probably too high (allowing a Ultra 90 or Munchkin 80), but it could be too low in this case, in which case the Munchkin 140 would be a safer option. There is a huge error bar.

If it is an existing house, current gas consumption per degree day will give a much better handle on heat loss than heat calcs do.

Constantin

Which way does the error bar swing?

I have done a heat loss calc on my home and found it to be 10% higher than that of my contractor. I used HVAC-Calc, he uses Wrightsoft. Both are Manual-J compliant.

IIRC, Mark Eatherton did a heat loss calculation on his home and then intentionally undersized the boiler by 20%, yet has been comfortable all winter long.

The good people at Building Science Corp. did a study out in the Midwest where they intentionally undersized split HP's and AC units and found that the homes with the undersized units performed better and at lower cost than the ones with "proper" units.

I guess all I'm saying is that the people who wrote the Manual-J probably may have left some margin for error in there. While I wouldn't risk running a T80 in this home the way Mark did in his, the setup certainly seems amenable to a 100kBTU boiler.

You could guarantee top-notch comfort upstairs by assigning the AH to be a priority zone (treat it like a WH) and leave the radiant zones as secondaries. However, my personal preference would be a 3-way motorized mixing valve for the radiant zones with all heating zones set up as secondaries. That helps the boiler condense and that'll lower the bills better than having the AH running separately from the other heating zones.

.

I quote from my previous post: "The number is probably too high".

So why not a WM Ultra 90, or a Munchkin 80? Why obsess on a WM Ultra 105?

The fact is, given the uncertainties, the whole range from 140 to 80 (or so) is possible, just as the OP thought. It is not particularly an improvement to try to hit the calculated heat loss on the nose.

Constantin

Allow me to quibble...

If I were a heating contractor and I were to quote a system, I would use a Manual-J compliant heat loss/gain program to determine the heat gain/loss and the system I would install would be sized to meet those specs.

I wouldn't care if the fine people at ACCA were wrong and added too much fudge factor. The fact is, if something goes south, it would be me with the problem. When the time comes to defend your position in court or whatever, the minimal savings you hoped a smaller unit would entail for the homeowner evaporate rather quickly.

Plus, with the wide range of modulation in an Ultra, the minimum firing rates are very, very close whether you use a Munchkin T80 (~16kBTU), a Ultra 90 (~18kBTU) or a Ultra 105 (~21kBTU). IMHO, being able to modulate down another 3-5kBTU isn't worth the potential trouble it can get the contractor into.

As stated above, what makes the 140M less than ideal for this application is that it's modulation turndown range is only 3-1, apparently. I thought they had made the switch for all Munchkins, but their current web literature does not support that.

During the shoulder seasons, a 5-1 unit should have an advantage over a comparable 3-1 in terms of energy efficiency for the same reason that modulating units have an advantage over non-modulating ones. However, I imagine that the advantage is somewhat less dramatic than the one between a 5-1 and a on-off controlled burner..

Anyway, I imagine that most customers will expect the whole house to be comfortable, all year round. So if the customer insists that all three zones have to be able to fire at the same time, then a 105 pretty much becomes the minimum you can install and cover your rear end. If the customer is OK with having a cold garage on the coldest of days, now that becomes another story... but I would get it in writing!

Steve Wilson_2

Munchkin Sizing

I thought that boilers needed to be sized to The IBR Output rating? The output of the Munchkin 140 is 112Mbtu. Not a bad match to your heat loss of 101Mbtu.

Mike T., Swampeast MO

Cowboy

With a DOE rating of 74 mbh and a calculated loss of 101 mbh, I'd say you're really pushing the limits of "calculations are overstated". I'd NEVER spec. that boiler without the utter approval of the homeowner.

Too bad the next size has nearly double the output.

Jake Timmerman_3

Why is the IBR rating so much lower than the DOE heating cap. Were do they get this number from? I forgot to mention that there will be an IDWH in the system.

Constantin

The IBR rating is...

.... Net IBR = DoE out x 85%.

The missing 15% is what they assume pipe losses, etc. will amount to... If the heating system in the house has been installed correctly (i.e. no pipes in exterior walls, unconditioned spaces, etc.) you ought to be able to match your heat loss to the DoE output... where is the heat loss from the piping going to go?

The indirect water heater is yet another interesting variable that you're throwing in for good measure. Assuming this family doesn't have high-flow bathrooms, etc., you should be OK even with a 105 Ultra, the main thing here being to understand their shower/tub habits and sizing an appropriate buffer (i.e. IDWH) to meet their needs.

Our boiler puts out 116kBTU net, yet thanks to our large storage tank I expect the system to work despite having multiple bathrooms to feed... and lest we forget, pretty much any boiler has a better recovery rate when coupled with a quality IDWH than a standard 50 Gallon gas (or someone help you) electric water heater.

Standard gas water heaters are close to 60% efficient, so a 50kBTU input rating only translates into ~30kBTU of heat making it into the water. Contrast that with a T50 Munchkin coupled with a super-stor that'll make water at 90+% efficiency all day.

Ted_9

Why not try the GB142 by Buderus.

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Constantin

Why not?

Well, for one, are they available yet? It was simply amazing how much the launch date kept moving along... However, should the Buderus GB142/30 be available, it would be a fine choice.

The Vitodens WB2 8-32 would also be a good choice. While its modulation range is only 4-1, it offers a thick, high-grade stainless heat exchanger and it runs on far less electrical energy than any of its competitors.