Heat Recovery versus Heating Loads

[mnc007]

Hi,
What is your oppinion on the effect of heat recovery (in 100 % fresh AHU), on the Heating Capacity Calculation? Should some or all be considered? Normally I don t use it to low the Heating capacity, but... The only reason i can find against including it as a saving, is during the system star up, once the interior conditions are below the design. Thanks, Regards

 

[mauricestoker]

I usually size for full capacity without consideration of recovery. If the energy recovery goes down, the capacity will be needed. It's much more expensive to have to upsize the hydronic piping once installed, and the down time can be a real aggravation to the customer.

 

[cdxx139]

Agreed, and some wheel use full bypass for frost protection, in which you will need full heating capacity during those few times.

Although the current energy codes in these parts requires you to size the unit with energy recovery included. (downsized), which I do for cooling, but use the above frost excuse as reason to not take the credit on the heating side.

knowledge is power

 

[mnc007]

Thanks! In my case the design exterior Temp is 0ºC so the frost excuse is not really usefull. one more though... The worst case scenario for dimensioning with heat recovery would be:

1 - System start up in the deisgn day;

2 - Building is onocupied and with no other internal gains;

It depends on the type of buiding but the odds are pretty low...

 

[317069]

Heat recovery is a good idea to save energy and protect your unit.
for the system start up, I belive it is better to intreduce an amount of fresh air with let say 3C into the coil than intreducing the same amount of the fresh air with 0C directly to a unit coil.

in your case I guess that each 1m3/hr(or 0.58cfm) of the fresh air needs about 7Kcal/hr(28 Btu/hr)then you can know how much energy you need for your fresh air and how much the recovery could give you, compare with recovery unit cost then deside.
Do not forget about cooling too. your are using electricity for cooling.

 

[MechEngNCPE]

One point, one should never use any internal loads on heating design. Calculate it as if no lights are on, no people in the building, no misc. electrical loads, etc.

I would not include the heat recovery, like mentioned before, if the recovery goes down, what would happen? Size the coil for the full 100%OA from 0 to whatever your lat is.

 

[317069]

ASHRAE and other organizations have issued professional procedures and standards to calculate heating load for buildings.
I agree that we should not consider heat recovery in calculation but let us think about it shortly
heat recovery unit usually raise the OA temp from 0F to about 40F in my knowledge(also we could find for 0C)
if we need to keep 70F forexample then without HRV we need
Q1= K.(70-0)=70K, but with HRV
Q2= K.(70-40)=30K, which k is a constant related to cfm and is the same in both cases, we see that Q1 is much mor than Q2.
if the recovery goes down no disaster would happen because it is a part of the system, simply we call maintanance team and fix the problem, what if the RTU or the furnace itself goes down?

 

[Waramanga]

Why wouldnt you take the heat recovery into account?? I would think that up to some percentage of outside air you would be advised not o include it simply because the marginal cost of the boiler capacity could be neglible. On the other hand if you double your hot water circuit half the advantage of the recovery system could disapear (ie the lower capacity pays for the heat exchanger). It would be a case-by-case exercise I think.....

 

[mauricestoker]

Good point. That is why I usually design for a baseline, full capacity, and then look at the costs of energy recovery. If a facility can afford to operate at higher or lower enivronmental conditions and no damage to the faility, then the energy recovery would also count as reduced upfront costs. I guess I've never had a customer that would put in writing that they would accept lower/higher temperatures in the event the energy recovery failed.