Difficult high efficiency residential boiler selection

JohnNY · 11:12AM

Hello all,

For the second time this year I'm trying to select a boiler against a handful of challenging and limiting criteria made especially difficult by the fact that Weil-McLain calls a 4" polypropylene 90° sweep the equivalent of 22' of straight pipe, Lochinvar Knight (floor mount) rates the same fitting as 10' of pipe, and Peerless assigns it 5'.
It's a 6-story home so I've got about 72' from boiler to roof line, and my static water fill pressure will be around 35 PSI at 70°.
I'm burning roughly 300 MBH for space heating and an 80-gallon indirect.

I guess my biggest issue is the wide range of equivalent pipe lengths per same fitting on the flue. I would really like to install a WM Evergreen but my flue would be over extended due to the total developed length of my venting materials.

The math above all points to the Peerless Purefire as the boiler for this project but there are some features it doesn't seem to have such as the ability to limit my maximum fan speed while in central heating (CH) mode. I find that useful in many applications. Both Lochinvar and WM allow this flexibility. Also, Peerless does not seem to offer a pre-fabricated manifold kit like the others do. Adding a heat exchanger would solve some problems for the low pressure tolerance of the Knight boiler and some others but I don't have the space to add one in a clean way.
I'm looking for advice. Is there a boiler out there that runs at about 300 MBH, has a HX tolerance of at least 50 PSI, can vent 100' with three 90° fittings, and is suitable for a residential application with good control parameter options for flexibility?

GGross · 12:06PM

my assumption is this is probably too large for the space

Viessmann CI2 can meet the venting requirements. I do not think you can specifically control the viessmann max fan speed (viessmann likes to control their own burner) but you can limit the max output for heating mode which will also limit the max fan speed for heating mode. unless that was the goal in the first place. these are high mass units not water tube like most viessmann.

I think the boiler comes with an 80psi relief by default so specify your relief size when ordering, you can save some money by providing your own relief valve if it matters. These are light commercial focused so they are designed first for room air dependent combustion air, if you are direct venting with 2 pipes there is a combustion air inlet kit you should ask for. they will also provide a filter for the burner for room air operation if you want (it's like a high performance auto filter)

Jamie Hall · 12:23PM

I think the Weil-McClain equivalent length for the 90 is very conservative. However, all is not lost. There are two directions I would approach this one from. Consider that what they are interested in is not tape measures, but pressure drop in the ductwork. WM want to make sure that that pressure drop is not excessive, and I can't say I blame them for that.

So, instead of getting out the tape measure, actually see what pressure drop they can tolerate, and then find a qualified engineer (which I'm not, sorry — never was my field) to design an exhaust which meets that specification. I'm going to go out on a limb (scary) and say that either a larger diameter exhaust with the same geometry would to it, or even the use of long sweep elbows might be enough. (scribbles on the back of the envvelope… if they rate a 4 inch at 22 feet, a 6 inch should rate at just under 6 feet… but don't quote me on that).

JohnNY · 12:26PM

@GGross Great answer. Interesting boiler.
Thank you.

GGross · 12:39PM

here are some photos one of my customers sent me from a service visit. he buys those K&N filters from the auto store for all of his mod/cons. the HXC sort of looks like a water tube when you see how he cleans it but its not. its made of a bunch of stainless steel sheets with a criss-cross pattern pressed together

EBEBRATT-Ed · 3:17PM

@JohnNY

I suppose you have to follow the boiler venting requirements, but I do wonder what the polypropylene mfgs say about fitting resistance. Any difference between MFGs?

Can't vent sidewall I assume?

GGross · 3:22PM

https://forum.heatinghelp.com/discussion/comment/1900787#Comment_1900787

Not sure code in his region but in mine the inspectors will look at both (equipment, and material manufacturers) and apply the most stringent of the two

Difficult high efficiency residential boiler selection

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