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Question for the engineers re makeup air units

ratioratio Posts: 1,822Member
Ok, this has been bugging me for a while now. More than a few of the MAUs I've started have been recirculating-style, 80/20 split usually, for building pressure control. They often (but not always) have a stat on the return air to drive the damper to 100% outside air if the return air temp falls below set point; and/or a similar stat on the outside air that again drives the unit to 100% outside air. My understanding is that this is supposed to be some kind of oh-shoot-it's-cold response intended to warm the place faster or keep it from freezing up—but does it actually do that? It will certainly increase fuel consumption, but will it warm the space faster bringing in 100% outside air vs recirculating? These units control to discharge air, so until the unit is making its maximum ΔT it's going to put out the same volume of air at the same temperature regardless of how much it recirculates. In fact, I believe that going to 100% outside air will potentially limit the space temperature as the unit may not be able to make it's highest discharge air temp when it's trying to reheat potentially sub-zero air.

Or am I wrong in my assumption of the purpose of the stats, or my analysis of what will happen?


  • EBEBRATT-EdEBEBRATT-Ed Posts: 4,977Member
    This is my take on make-up air systems and I am sure others will take issue with it. And I realize many are not installed this way

    I don't look at make-up air units to "heat a space" To me a space should have it's "own " heating system that will heat the space based on it's heat loss.

    The MU air unit supplies only "replacement" air.= the amount of air being exhausted.

    For instance a kitchen could have it's own heating system used to heat the kitchen during off hours when the kitchen equipment and exhaust hood is not used. When the kitchen hood is turned on to exhaust the MU air units starts and brings in the amount of outside air that pretty much matches the exhaust air. You might run the discharge air at 70 deg

    The 80/20 units are called that because these are direct fired to limit the CO in the air the outside air used cannot fall below 80% of the units capacity...they want to make sure the have enough dilution air for the CO. So your allowed to use up to 20% return air to save fuel.

    Not sure how the controls are set up I have not seen one in a while but I would think if the OA is cold and the gas is on Hi Fire and the discharge temp drops the OA damper could close down to 80% and take20% return.
  • ratioratio Posts: 1,822Member
    Right, they're not for space heating. (The general on this latest job is going to be sad, he wants the space warmed up & the humidity lowered…)

    All that makes sense re outside air and everything. What I can't wrap my head around is why drive to 100% outside air when the return &| outside air temp is low. Every one (that I've come across) that has a return or outside air thermostat does the same thing. I've got a hard time believing that the Old Guys who though up these units got it wrong but for the life of me I can't come up with a reason for it.

  • EBEBRATT-EdEBEBRATT-Ed Posts: 4,977Member
    All 100% MU air units drive to 100% OA whenever they run. The CFM they move is supposed to be matched with the proper btu input to "keep up" Amount of air they bring in is not based on OA temp

    80/20 can take a maximum of 20% return air for dilution air yada yada.

    Not 100% sure how they control those.
  • ratioratio Posts: 1,822Member
    These are 80/20 for building pressurization control, as building pressure rises bring in less outside air, up to 80% return air. That part makes sense. But I can't understand the reasoning for driving to 100% outside if it's cold out—it's more likely that the discharge air temp will drop due to the inability to heat the (presumably sub zero) outside air as warm as trying to reheat 20% outside air + 80% return air that will probably be somewhat to significantly warmer than the outside air.
  • EBEBRATT-EdEBEBRATT-Ed Posts: 4,977Member

    I apologize I was wrong and your right. 80/20 is minimum of 20% outside air I had it backwards.

    Well for building pressurization I think the controls look at building pressure and bring it whatever OA is needed to maintain the pressure. The MU air unit should be set up to be able to heat the replacement air up to a minimum of 55-60 deg I would thing and the building system can do the rest
  • Jamie HallJamie Hall Posts: 9,773Member
    Keep firmly in mind when you are looking at these things that they are not there for heating and, unless they are coupled with a sensible heat heat exchanger, don't even pretend to be efficient in that regard. They are there to maintain indoor air quality.

    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England.

    Hoffman Equipped System (all original except boiler), Weil-Mclain 580, 2.75 gph Carlin, Vapourstat 0.5 -- 6.0 ounces per square inch
  • ratioratio Posts: 1,822Member
    I'm not explaining my question correctly. What are the benefits of firing at 100% outside air when the outside air temperature is below (say) 58° when you are guaranteed not higher than 90° discharge air? (real numbers from current job) My opinion is that you are only guaranteed to increase fuel consumption & that net heat to the building will suffer once the firing rate of the unit can not meed the DAT setpoint due to a depressed inlet air temperatures, because you will be relieving warm air that could be recirculated.
  • ratioratio Posts: 1,822Member
    I suppose I am getting hung up on discharge air temperature, but even with that in mind, how would air quality be better served by going to 100% outside air just because it's cold, either in the space or outside?
  • Jamie HallJamie Hall Posts: 9,773Member
    Can't answer that one, @ratio -- unless for some reason someone thinks they need that much makeup air when it's real cold And I can't think why...

    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England.

    Hoffman Equipped System (all original except boiler), Weil-Mclain 580, 2.75 gph Carlin, Vapourstat 0.5 -- 6.0 ounces per square inch
  • EBEBRATT-EdEBEBRATT-Ed Posts: 4,977Member
    Make up air is replacement air heater only and in most cases is not to provide building heat only to heat air brought in to replace what is exhausted or to pressurize building depending on the controls.

    This is an "engineered design" and as such most of us service/installation guys are rendered stupid. We are on a need to know basis and we don't need to know.

    But seriously I would need the original building drawings or design to get into the engineers brain and figure it out.

    usually MU air is used for kitchens, factories (fumes, smelly stuff etc being exhausted) process or something like that. Building pressurization to prevent negative pressure (doors won't open) (smelly elevator shafts) Sometimes building are sold or change of use and then their are issues.
  • ratioratio Posts: 1,822Member
    So it's not just me then!

    Ok, guess I'll go back to wondering about it. I kinda think it might be some kind of mistake to have it respond that way, but without some confirmation I'm not gonna say 'That's dumb!'

  • ratioratio Posts: 1,822Member
    @EBEBRATT-Ed, this is a factory control feature, not a building design decision. It appears to be a response to a concern that the building is (or is about to become) cold. I think it is based on a mistaken belief that burning the most fuel will keep the place the warmest, but I don't think that's the case if we are discharging air that is cooler than would be if we were recirculating some.

    It just bugs me to not know.

  • RPKRPK Posts: 56Member
    Ratio, I’ve never seen one with the control you’re describing. I’ve seen them with space sensors that reset the DAT to accomplish some space heating. The only thing I’ve ever seen controlling the OA/RA ratio is building pressure. What is the make/model of the unit you’re taking about? Or have you seen this from more than one manufacturer?
  • ratioratio Posts: 1,822Member
    It's a Rupp RAM-M 27 w/ 80/20 return for building pressurization control. Here's the schematic, TS-09 factory setting is IIRC -25° & TS-10 factory setting is 58°. While they have a design max of 90° DAT, they have a max ΔT of 115° so we can do that 90° for quite a while, but if the ambient drops below -25° our DAT will drop & I believe at that point, we're likely going to be relieving heated air that we could have kept in the space by recirculating & the net heat input to the space will be less than it could have, even though we'll be burning more fuel.

    NB: This unit has a larger ΔT than some of the others I've come across with a similar configuration.

  • ice4meice4me Posts: 19Member
    Does the return air flow through the burner of this unit? And is the return air actually being directly exchanged for outside air? If the answer is no to these questions, then it would make sense that the OA damper opens to 100% when the return air temp drops, since this will provide maximum output from the burner. Presumably the unit would be designed/spec'd such that the outlet temperature would be greater than the indoor space under worst case conditions.
  • EBEBRATT-EdEBEBRATT-Ed Posts: 4,977Member
    PS-0 runs the damper motor and is running the show to control building static. The gas follows to keep up. @-25 discharge of 90 should be fine. Even if you drop to -45 you could still discharge 70 degree air.

    the way I see it the unit is first & formost designed to control building static and heat replacement air. No for heating the building, not for comfort not for energy savings.

    What I can never understand is why they don't use a heat recovery ventilator in conjunction with a MU air unit. A heat recovery wheel will recover 80% of the exhausted heat
  • mikeg2015mikeg2015 Posts: 575Member
    The units might be direct fired. So they need to bring in fresh air when firing above a certain rate.

    Also might have a maximum temp rise value.

    OF the programmers, general and engineers failed to communicate the function or wrote the specs wrong. This is how EVERY project I’ve worked on has been. There little common sens among designers because it all about meeting some building code, misinterpreting or not completely following the full code with it’s exceptions, or just being cheap or lazy.

    I left my last job because I got tired of arguing with my boss who believed the engineers were smart and there fore knew all and were infallible. I showed him hard data why they were wrong. Oh well. His problem now.
  • ratioratio Posts: 1,822Member
    Right. Now, I'm pretty smart, so when I come across something that doesn't make sense my first impulse is to assume that I'm right; but I also know that there are lots of people smarter than me (including some engineers :naughty:) so my second impulse is to try and figure out if I'm wrong or not. I'm at that stage right now. I can't see a benefit to driving to 100% outside air as determined solely by return/outside air temp other than consuming more fuel.
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