Which method for preheating OA for make-up air unit?

[Matt78]

I have narrowed it down to two options for preheating the incoming OA for our MAU's in an existing system which currently pulls in 100% outside air but are unable to do so in the colder months due them being heat pumps. Usually they will trip out on a low temp fault and need to be reset.

Option 1 - Add a mixing damper in the plenum after the OA damper and modulate that to mix more plenum air when the OA temps get colder.

Option 2 - Add electric pre-heaters to condition the OA

There are no exhaust systems integrated with these MAU's so a energy recovery wheel would not be an option.

Any input or additional options appreciated.

[pecmsg]

Why 100% OA?

Generally with 100% OA there are no mixing dampers.

Steam is my first choice, I've used HW but it a complicated control system for freeze protection.

[Matt78]

Why 100% OA?

Generally with 100% OA there are no mixing dampers.

Steam is my first choice, I've used HW but it a complicated control system for freeze protection.

I believe when the building was first put up the mixing dampers may have been used and balanced and set to maintain static pressure. But this 40 years later and about 1/2 of the units have mixing dampers and the other that have been added on over the years do not. Steam is not an option as it is not in place.

[DPinst]

Depending on how much CFM you require. Electric pre heating could get very expensive. Could you not use a Heat pump to pre heat the air with electric back up heat? Installing a small hydronic system to provide the necessary heat to temper the air.

[pecmsg]

That would be one huge heat pump to maintain 50* Delta T then what do you do during defrost?

[DPinst]

Yes pecmsg that could be a very large heat pump.The OP does not give much information as to CFM that is being supplied or what he has available for an energy source.He

says an option is electric pre-heaters , Those are going to be some pretty big heating elements.

There are definitely some cost efficient ways to pre heat the air but it sounds like his options are limited.

Even a basic gas fired rooftop make up air has got to be more cost effective that a large KW duct heater.

[Nauger]

What's the application? 100% Oa but no exhaust. What is "cold" where you are?

[hvacker]

I've had good luck with direct fired nat gas section using modulation like Maxitrol.

[Matt78]

Yes pecmsg that could be a very large heat pump.The OP does not give much information as to CFM that is being supplied or what he has available for an energy source.He

says an option is electric pre-heaters , Those are going to be some pretty big heating elements.

There are definitely some cost efficient ways to pre heat the air but it sounds like his options are limited.

Even a basic gas fired rooftop make up air has got to be more cost effective that a large KW duct heater.

CFM ranges of the units range from 1200-2000. The few electric preheaters that were installed already range from 10-25kw with most not running at more than 50% on the coldest days or night which is around 0dF here.

All we need to do realistically is preheat the air up to about 45dF which is the minimum rated intake air for the heat pump and then that can take over to reheat the air again.

rooftops/gas fired is not an option here for multiple reasons.

The purpose of these units is ventilation air for the population. These units blow into the plenum and the other units pull off of that to provide fresh air to the floor space.

That's why my original thought of why there are mixing dampers on some of the original units. These could have been modulated at one point so that in the winter it would take in less OA and mix more plenum air effectively raising the intake temperature and not needing a preheater. The downfall is you bring in less outside air.

[BBeerme]

What kind of population and floor space needs all of that fresh air?

I'm guessing a bit of engineering and re-ducting will have a short pay back then continue to pay nice dividends for a long while.

CFM ranges of the units range from 1200-2000. The few electric preheaters that were installed already range from 10-25kw with most not running at more than 50% on the coldest days or night which is around 0dF here.

All we need to do realistically is preheat the air up to about 45dF which is the minimum rated intake air for the heat pump and then that can take over to reheat the air again.

rooftops/gas fired is not an option here for multiple reasons.

The purpose of these units is ventilation air for the population. These units blow into the plenum and the other units pull off of that to provide fresh air to the floor space.

That's why my original thought of why there are mixing dampers on some of the original units. These could have been modulated at one point so that in the winter it would take in less OA and mix more plenum air effectively raising the intake temperature and not needing a preheater. The downfall is you bring in less outside air.

[Matt78]

What kind of population and floor space needs all of that fresh air?

I'm guessing a bit of engineering and re-ducting will have a short pay back then continue to pay nice dividends for a long while.

Ventilation and make-up air technically, these buildings are upwards of 450,000 sqft each. Using rooftops in the fashion i would want to would eliminate 20 heat pumps and replace them with 6 RTU's at one building, but this is not an option unfortunately.

[BBeerme]

If it's make up air, then you cannot restrict the 100% outside air.

What is inside the building that needs all of that fresh air?

Ventilation and make-up air technically, these buildings are upwards of 450,000 sqft each. Using rooftops in the fashion i would want to would eliminate 20 heat pumps and replace them with 6 RTU's at one building, but this is not an option unfortunately.

[HVAC-matt]

By the way you said "population" are you talking about a prison? Just curious


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[BBeerme]

Yeah, I thought that too. The next thought was chickens.

We still don't have a clue if all of that fresh air is required. Could've just been a low bid to replace a bunch of swamp coolers. And they didn't want to penetrate the roof for a return plenum.

By the way you said "population" are you talking about a prison? Just curious


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[jwabel]

"Transpired solar collectors" aka "solar ventilation pre-heaters" represent an option I've not seen in this discussion.

Solarwall, the granddaddy of the technology is a choice but if accessing the lowest cost at the highest efficiency in this technology is desired I'd suggest looking at fabric transpired solar collectors especially if the application is for a rooftop installation. The geotextile collector was patented by the "National Renewable Energy Laboratories" but has been used mostly in the agri-sector where costs are much more acute than in commercial applications.

If you want more info on this subject just ask

[heatingman]

It seems someone already installed electric pre-heat successfully on other units that work, I would just go with that.


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