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Thread: Professional Opinions of Weil McClain Ultra UO-3 Oil??

  1. #1
    DIY Junior Member Greatwhitewing's Avatar
    Join Date
    Apr 2011
    MA, United Staes

    Default Professional Opinions of Weil McClain Ultra UO-3 Oil??

    In combo with Advanced Feature Module and Ultra Plus 40 Indirect HWH? Riello Burner.


    Have a quote in for this boiler that's in the game and the brochure looks really nice (sarcasm).

    Still waiting for more quotes for Viesmann, Buderus and Peerless.

  2. #2
    In the Trades Tom Sawyer's Avatar
    Join Date
    Nov 2010


    It is a nice piece of equipment but I prefer Buderus.

  3. #3
    In the trades Dana's Avatar
    Join Date
    Jan 2009


    There are many quality manufacturers out there- what makes or breaks it is the quality of the system designer/integrator, and the degree it's supported locally by both the installer & distributor. Weil-Mclain has a significant installed base and good distributor support in central MA/RI/CT, so it really comes down to the level & quality of system design & support you think you can get from the contractor.

    Dropping a 100K boiler like the UO-3 on a multi-zoned low-mass system without adding some buffering mass would be an issue, if unaddressed, since it is still ~3x oversized for your design day heating load, and WAY oversized for any individual zone (but aren't they all?). With the U0-3's 15 gallons of water and another 1-2 gallons per zone you're looking at well under less 200lbs of water in the boiler/radiation heating side of the system you're looking at about a ~1-minute minimum burn to get a 10F rise in water temp on the system.

    The UO-3 has 2x the mass of some of the smaller units out there, but 1.5x the burner, so even with 20F of hysteresis on the boiler controls it will still short cycle on zone calls. With 40 gallon electric HW tank (not electrically hooked up) utilized as a relatively cheap buffering mass for the heating zones you more than triple the min-burn, but the indirect needs to be plumbed & controlled without involving the buffer tank, something topologically similar to this:

    See: http://www.radiantandhydronics.com/Articles/Glitch_and_Fix/BNP_GUID_9-5-2006_A_10000000000000597932

    In this sort of configuration outdoor reset can be used to provide higher comfort levels, but forcing a higher hysteresis in to the boiler controls may be necessary to get decently long efficient burns out of it at the low end of the temperature curves. The total amount of water in the heating side of the system with a UO-3 + 40 gallon tank is now on the order of 500lbs. With 98KBTU/HR of boiler output it still takes ~3.5minutes of burn to raise that mass 10F (but that's way better than the ~1 minute you'd be looking at using just the mass of the boiler itself.) Observing system behavior at low load and tweaking as necessary would still be a necessary part of commissioning such a system. If burns are under 5 minutes at low load it's worth programming it for a higher hysteresis. It needs 10+ minute burns to reliably hit near it's AFUE numbers.

    Without the buffering mass and running 1-5 minute burns it would get 12-15% lower "as operated" efficiency in a multi-zoned system with a burner at 3x the whole-house design load. With the buffer there'll be a very modest increase in standby losses, but a huge savings on cycling losses. If you have the patience to wade through it the reasons are fairly well explained here:


    It's written from a larger system point of view, but the math is the same for a multi-zoned residential application. Every burn cycle throws away a fixed amount of heat in ignition and flue purges, and with short burns it's an ever increasing fraction of the total. For oil-burners it takes between 5-10 seconds for the burner to even reach it's combustion efficiency on startup, which for a 1-2 minute burn is 10% of the total burn time (or more), followed (or preceded) by a necessary flue purge (to guarantee there are no explosive mixtures in the flue on startup), which extracts heat directly from the heat exchangers in the boiler.

    For operation below 140F (where you'll probably be running, at least most of the time) multiple zoned system they're really recommending a primary/secondary approach. See page 16. With a 40 gallon tank placed anywhere on the boiler loop, or at the point of hydraulic separation (the "MAX. 12"" sections of pipe in the diagrams) the mass will participate fully on every heating burn. If you were never going to run the system water below 120F (likely, with fin-tube baseboard radiation) using a reverse indirect as the both a buffering tank/hydraulic separator simplifies the system topology & boiler control.

  4. #4
    In the trades Dana's Avatar
    Join Date
    Jan 2009


    As a further means of vetting contractors, those that offer/insist upon a room-by-room computerized heat loss analysis as part of the package should move to the head of the line, and any body doing a "...35 BTUs a foot times 1800'..." type calc (or none at all) can be ignored. This weeds out 80-90% of the hacks.

    Those that also give you the heads-up about short cycling and thermal mass issues with fin-tube heat emitters ahead of time go on the A-list, but ask the others on the list how they intend to address it. Those caught flat footed and ask for more time should still be considered, but those who are in deep denial and that it's really an issue can be moved out as well. After all t's YOU that will be paying the maintenance & fuel bills, not them.

    It truly is the system designer & installer, not the boiler manufacturer that makes the biggest difference. The first 90% of the problem is getting the burner sized correctly for the load. The second 90% of the problem is getting it to behave well when the load has been sliced & diced. Since "right-sizing" simply isn't possible with an oil-boiler for a decently insulated mid-sized house in this climate, designing the system in such a way as to deal with it that fact is critical. When heating oil was $1/gallon or less the hit in efficiency didn't matter much, but that's no longer the case. No matter how much smarts you put on the boiler controls, the fundamental physics of the problem isn't going to change. Unless the radiation itself is sufficiently massive (eg radiant floors in concrete slabs, or oversized hot water radiators), there's no substitute for adding thermal mass to deal with burners oversized for the loads, yet 8 out of 10 old-school boiler installers won't go there unless pushed.

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