Wanting a high-efficiency boiler

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Old 10-30-08, 02:37 PM
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Wanting a high-efficiency boiler

Hi -
I need to replace my 95K BTU boiler and am considering a high efficiency condensing boiler.

I have a small old house and did a ManualJ on it using the SlantFin tool, arriving at a 34.9K heat loss. The house is 1320 sqft and I have also estimated the heat loss by rule of multiplying by 32 to arrive at 42.5K. I think my current boiler is oversized.

However the operating temp of my current system is 180F. The Monitor MZ25/MZ40 units seem to put out a max of 175F (monitorproducts.com).

Can anyone tell me how much of an issue this is going to be? They seem to produce more than enough BTU - at a slightly lower temp, would the heat just have to run a little longer to match operation with the old boiler?

Is it true that high-efficiency condensing boilers lose their efficiency when running above 130F? If so, am I stuck with a regular old boiler?
 
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Old 10-30-08, 03:00 PM
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What is the fuel?

With higher hot water temps, there may be little or no condensing. So, the added cost and complexity of a condensing boiler may not result in a payback.

Your existing boiler may be oversized as you suggest. But what about the radiators/convectors? If they are sized properly for 180-deg water, then they would be undersized for, say, 160 deg. (The radiators don't "know" the size of the boiler; they only "know" the temp of the water supplied to them.) I don't think the difference between 175 deg and 180 would be significant.
Doug
 
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Old 10-30-08, 04:48 PM
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Boiler temp

Boiler temperature really doesn't make much difference IF you have enough radiation, you can heat with 100º water. Now you know how much heat you need, you can find out the capacity of your emitters at various water temperatures. You might have enough emitter capacity to use 130 or 140º water.
 
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Old 10-30-08, 05:25 PM
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Remember that radiators, baseboard or whatever sized for 180F on a design day don't need 180F on non-design days. Design conditions by definition only occur about 35 hours each year.

[Writing from a house with fin-tube baseboards sized for 180F at 0F outdoors, which is currently meeting the heat loss of a 34F outdoor temperature with a supply temp around 90F. At 0F, the supply is 135F or so.]

For a heat loss of 35k BTU/hr (which itself is probably an overestimate of 10% or more), a modulating/condensing boiler like the Munchkin T50, Munchkin Contender MC50, Triangle Tube Prestige Solo 60, or similar small unit would do.

The Monitor units condense, but they have fixed input rates. Even the 25 is far too big.
 
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Old 10-30-08, 07:55 PM
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Dont mean to hijack the thread but I'm in a similar situation. My heat loss is about the same and want to put in BB in place of the forced hot air furnace. I'm limited to a oil fired unit, what would be a good choice seeing as even the smallest ones are double the size?
 
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Old 10-30-08, 11:07 PM
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Awesome. Sorry I forgot to mention it's a natural gas system, hot water and radiators.

Just for other guys I found this on calculating radiator emission which helped to understand a lot - haven't done the calc yet though.

Radiator Sizing Guide

Thanks everyone.
 
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Old 10-31-08, 03:52 AM
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Oil Fired Boiler

With oil fired equipment your choices are quite limited but I think Monitor makes a small oil fired. Other than that I'm afraid you're stuck with something 2-3 times as big as you need. As much as I hate to suggest it, is propane an option?
 
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Old 10-31-08, 05:28 AM
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I guess it is but at a very last resort. Propane is around twice as much as ng around here, plus I would have to add in the extra cost of having the tanks installed and piped. My house is pretty small, 1200sq ft +/-, so it would take a while to recoup the extra cost.
 
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Old 10-31-08, 06:56 AM
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Look at the Buderus GB125BE. Probably the closest you'll get for size and efficiency. The Logamatic control will do the best it can to run the oversized system efficiently.

Failing that, you could go with the smallest oil-fired boiler you can find, pipe it primary/secondary with variable speed injection and outdoor reset (and indoor feedback), and that would be a pretty efficient setup too. More cost up front, but probably savings over the long term make it worthwhile.
 
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Old 10-31-08, 08:52 AM
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jockor, the monitor MZ 25 would be a poor choice for you. It's a rugged boiler of excellent quality but it is large (94 MBH), doesn't modulate down, and it lacks any type of reset control for water temperatures. The 40 is even bigger although it does do 2 stage modulation, again with very simple controls that don't reset.

For what's on the market right now, I'd recommend you look at the Triangle Tube Prestige Solo 60. It'll modulate down to lower outputs to help match your heatload. With a measure heatload of 35 MBH, you probably have a true max sustained heatload of 25 MBH. I'd really stay clear of the Monitors for this low of a load.

Your water temperature will be only what it needs to be to heat the house. That's the beauty of a modern boiler with sophisticated controls that are dialed in.
 
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Old 10-31-08, 09:11 AM
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If I went with the Buderus, would I need to do the primary secondary piping and all the other stuff or would the logamatic take care of everything?
Originally Posted by xiphias View Post
Look at the Buderus GB125BE. Probably the closest you'll get for size and efficiency. The Logamatic control will do the best it can to run the oversized system efficiently.

Failing that, you could go with the smallest oil-fired boiler you can find, pipe it primary/secondary with variable speed injection and outdoor reset (and indoor feedback), and that would be a pretty efficient setup too. More cost up front, but probably savings over the long term make it worthwhile.
 
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Old 10-31-08, 09:35 AM
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Good question. The install manual shows typical piping. A bypass is suggested for large-volume systems.

The Logamatic would do the best job it could, and it's a very good control. Not sure if it would allow full outdoor reset without adding a mixing station, which would be similar to primary/secondary in effect. On page 9, the install manual has a table that says there are no minimum supply or return temperatures, but it does say that a mixing approach would help.

With cast iron radiators, there would almost definitely be benefit to running nearly full outdoor reset to just above room temperature. A knowledgeable installer or Buderus rep (or Buderus tech support...) would know about whether it can do this without the mixing station. Worth asking if you get serious about this boiler.
 
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Old 10-31-08, 05:50 PM
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Would I need a mixing set up anyway to deal with the low return temps as a result of the outdoor reset? Or is that not a issue with that unit.
 
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Old 11-01-08, 04:15 AM
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The install manual says there are no minimums, but that mixing will help. It also recommends mixing for low-temp systems, and there are a couple notes that appear to say going to low temp requires some additional provisions (e.g., notes 1-3).

A thermic bypass loop might satisfy the requirements. Or implementing the whole system as a mixed system.

[the manual http://buderus.net/LinkClick.aspx?fi...d=495&mid=1308 ]

I would consider full outdoor reset a low-temp system. A knowledgeable installer should be able to recommend the appropriate approach based on the characteristics of your rads, their output, and the supply water temps you'll use.

You could start on that calculation by figuring up the radiator output at various water temps, and comparing that to the heat loss at various outdoor temps. In slantfin-ware, you can get the heat loss by changing the outdoor design temp. Start at say 55F and go down in 5-10F increments. Plug it all into a spreadsheet.
 
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