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m2e (Mechanical) (OP)
1 Sep 09 12:11

I'm designing a heating system for a warehouse.  The air exchange is high because it stores chemical.  We wanted to put in a heat recovery unit to capture the exhaust heat, and put in an oil furnace for the heating.  The two units have separate ducts and they operate independently.  ie, the furnace is connected to a supply duct system and is controlled by t-stat, and the heat recovery unit is controlled by a timer and a user override switch.  When the two units are operating the furnace will blow warm air into the space and the HRU will supply tempered cold O/A.  We worried that when the outside O/A temp drops below 32, it'll blow freezing air into the room.  The equipment supplier suggested to tie the two ducts together, that is, connect the S/A duct from the furnace directly into the S/A duct of the HRU.  So the cold are from outside and the warm air from furnace will be mixed together before distributing into the room.  

In this case, the two fans will be blowing into the same duct.  Is that going to be ok?  Or would the static pressures of the fan be fighting against each other?  The HRU has high/low speed setting tied with dampers to supply fresh air to different zones of the building.

What do you think is the best solution?
Zesti (Mechanical)
2 Sep 09 2:50

I don't know in what climate you are or what the inside temperature is but temperatures on the supply-side of the heat-recovery unit might not drop below freezing.
Ask the supplier of the system for the temperatures at lowest expected outside air temperature and design inside temperature (return air).

The lowest possible outside temperature will only occur during the night and since your system is already on a timer I assume it does not have to run 24/7 anyway. What I would propose is having a control system which switches of the system if a lower temperature limit is reached.

willard3 (Mechanical)
2 Sep 09 14:58
Running two fans in parallel creates nasty control problems if the fans are not carefully matched and both started at the same time.

Consider also that if only one fan runs, it may windmill the other fan.

Check the fan types, fan curves and etc. Any fan that has a dip in its characteristic curve is way more of a problem than fans without a dip.
ChrisConley (Mechanical)
2 Sep 09 17:01
Much more typical to connect the S/A from the HRV to the R/A of the furnace.
m2e (Mechanical) (OP)
2 Sep 09 17:29
Thanks for the replies.  The area is in Alaska area, the it can get real cold.  In terms of control, the two units operates independently.  There are times when the furnace operate on its own (building unoccupied and in heating only), and there are times when the HRV operates on its own (summer time, when building is occupied).  

I like the idea of connecting the S/A of the HRV to R/A of furnace, but is there any arrangement that'll allow them to be able to operate independently?

The units, although has a timer for purge, have to operate when people come in for code requirement; therefore, we can't set a temperature limit for them.  

AbbyNormal (Mechanical)
3 Sep 09 6:47
why not duct the supply air from the HRV to the oil furnace return and interlock the blowers


The way we build has a far greater impact on our comfort, energy consumption and IAQ, than any HVAC system we install

m2e (Mechanical) (OP)
3 Sep 09 11:51
Can we do that if the two blowers have different flow capacity and static pressure?
AbbyNormal (Mechanical)
3 Sep 09 15:12
As long as the furnace fan moves more air than the HRV, yes

The way we build has a far greater impact on our comfort, energy consumption and IAQ, than any HVAC system we install

AbbyNormal (Mechanical)
3 Sep 09 15:31
air out of an HRV could be pretty cool up there not much above freezing.

You need to take a look at how much air you have moving for the furnace and how much of that air volume will come from the HRV.

Work out a mixed temperature of the amount of return the furnace will draw (supply volume- hrv volume) with the HRV volume of air.

Now I know you said oil, but with a low return temperature a gas furnace not designed to condense could turn into a condensing furnace. So you need to look at your anticipated mixed mixed air temperature and decide if you think you need a stainless steel HX on the furnace or if the furnace manufacturer figures the mixed air temp is too cold.

Depending on ariflow of that HRV maybe you need a duct heater to temper the air up a bit.

It is hard to tell from the info you have given so far.  

The way we build has a far greater impact on our comfort, energy consumption and IAQ, than any HVAC system we install

ChrisConley (Mechanical)
4 Sep 09 11:16
I'm in Canada, and nearly all new residential homes in my area are being built as myself, and AbbyNormal have descibed:

F/A from HRV ducted to R/A of furnace, fans interlocked to run together. I'd recommend a high efficience ECM motor on the furnace.

The alternate to a duct heater is to run a bypass duct from the S/A of the furnace back into the return air duct, downstream of the HRV.
AbbyNormal (Mechanical)
4 Sep 09 11:21
good point on byapssing the heated supply back to temper the HRV air Chris

The way we build has a far greater impact on our comfort, energy consumption and IAQ, than any HVAC system we install

cry22 (Mechanical)
4 Sep 09 16:48
AbbyNormal and Chris
One would think that up there, the use of heat exchangers pumping Glycol all over the place would be the way to go to pre-heat cold air and avoid the freezing problems.
I don't get a sens that glycol in HW system is widely used in you neck of the woods.
wonder why?
ChrisConley (Mechanical)
4 Sep 09 17:09
Didn't sound like he had a boiler in the system, just a furnace and an HRV. Most small residential projects don't use boilers up here... or anywhere else.

I use hot glycol (35-50%) on almost every commercial project I've ever worked on. What does your sense come from?

I'll give you that Abby doesn't use much glycol, or heating anymore.

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