Help determining delta T for storage tank

[98kx1927]

I'm trying to understand the lower bound for the storage tank temperature. I'm getting an air source heat pump installed now so I also know the duct system design specs.

giving me:

first floor:
45k BTU/hr design temp 5F
1250 CFM from duct system

second floor:
49k BTU/hr design temp 5F
1600 CFM from duct system

I figure there should be a way to see what the minimum temperature that I can run with these known values right? From that I can calculate how much storage I would need so that I can have 12hours between burns, for example.

I took delta t = BTU/CFM * 1.08 and came up with a 109F lower temp based on a 70F indoor temp. Then 180F-109F = 71F plugged that into 8 * 71 * 2000 = 1.1million / (49k+45K) = 12hours.

Is this right? I thought it looked right, but now that I type it out, I'm wondering.

thanks
dave

 

[98kx1927]

Any ideas on how to determine the lowest usable temperature?

 

[PassionForFire&Water]

8 * 71 * 2000 ?

8 should be 8.34 Lb/gallon for water ?
2,000 is the size of the tank ?

 

[Clarkbug]

Your lower limit on your storage temps is going to be dictated by your emitter, not the load within your house. (but you need to know that load) Since you are going to be putting in an ASHP, are you intending to put a coil in the duct? Or are you running baseboard/radiators/radiant?

So if you can help us with some more info, we can help you with a temp guideline.

 

[98kx1927]

Yes, I will be putting a coil in the duct. I've asked the HVAC installer to leave room for one when he installs the air handerl and he said I can get one and it will just slide right in. He is also running the piping to the unit in the attic so that it will be there when I got to hook up the coil later.

I would like to size the system have enough storage for two burns per day at design temp. I know that this will result in a large storage, but I'm OK with that.

thanks
dave

 

[Clarkbug]

Hi Dave.

Ok, you will need to figure out what the coil will be rated at for its output temp. If possible, size it for the lowest temperature water that you can, without making it a HUGE coil, or putting too much pressure drop in your air stream. The bad news is that most coils tend to start dropping in output at fairly high temps. (usually 140 or higher).

So what size is your ductwork, and have you picked a coil? Are you going to use a single coil, or two, one for each zone?

 

[98kx1927]

I'm planning on using one per zone, one upstairs and one downstairs. I don't know the sizes of the duct work, just the CFMs for every register and the return. It was the return CFM that I listed above. I've not looked at any coils yet. I figure that is all part of trying to figure out the system. The air handlers are: TAM7AOC48H31SC for the upstairs, and TAM7AOC42H31SC for downstairs. Not sure if that helps.

dave

 

[Clarkbug]

Ok, unfortunately the air handler model numbers dont help for coil sizing. Thats going to drive your temps, and in turn your storage size. We can make an assumption that you need a minimum of 140 degree water, and would charge your storage to 180. Adding your heat losses together, times 12 hours is 1,128,000 BTU. Using a delta T of 40, 8.34 lb/gallon, and 1.08 btu/lb-degF, the math there works out to about 3100 gallons of storage, assuming you were at your peak heat loss above for the entire time (unlikely). Also, when your boiler is firing, you will be directly supplying the load, and not just charging storage. So that takes away from the 12 hours you need to use storage.

If you assume your boiler fires for 3 hours to meet your load, that gets you closer to 2400 gallons of storage (assuming no losses from your pipes or storage tanks)

What are you looking at for a boiler?

 

[98kx1927]

I'm not really sure. I've kind of narrowed it down to a handful of different ones:
Central Boiler E-Classic,
Portage and Main Optimizer,
Garn

Not sure the costs for a complete system, but say I went with 2000 gallons storage. The Garn would be nice, but it doesn't fit within my current outbuilding. I really don't need another outbuilding. Maybe retrofit one of the bays in the detached 3 car garage, not sure I like that either. The other two can be placed outside and then I can use my outbuilding for the storage. It was originally a brick well house, not it isn't used for anything. I can't put the storage in the house because there just isn't room.

Will a larger heat exchanger allow for lower working water temperatures?

dave

 

[Armaton]

Dave, are you certain that your heating need is correct? Seems very high for Texas. I live in Michigan, and my 3500+sf home is only 35kbtu at design temp. I purchased my WATX from OFS and they gave me a performance sheet with it, just had to ask them for it. It had data for btu output at a certain water temp, and CFM through it. All you need to know is the plenum sizes then you can figure which WATX size you need, then see if that exchanger will fill the bill.

 

[Clarkbug]

Dave,

If you are going with the E-Classic or the Portage and Main, you wont need to add additional storage. Those units will cycle their burn to meet you needs, and they have some storage built into them. Up here in NY I was able to fill an e-classic twice a day no problem. You shouldnt have any issues at all with any of those three. The Garn would be my choice, and there is plenty of storage in it for what you would need.

I also cant find those air handling units when I google the models you gave. What manufacturer are they?

And to answer your question, a larger Hx will allow you to have a closer temp between your loops, but lower working temps will be a function of the coil in your ductwork.

 

[98kx1927]

They are trane: http://www.trane.com/residential/products/air-handlers/hyperion-xl-air-handlers

I currently live in TX, but this house is in VA. I'm in the process of fixing it up so that I can live in it. HVAC was the first thing on the todo list. I'm as confident in the heat loss numbers as I can be until I actually heat it for one winter. Three different HVAC companies bid on the job, they all did manual-j calculations and came within 5% of each other.

 

[Clarkbug]

Thanks for that link. The Hyperion are the top of the line guys. Is your installer putting an electric coil in the units also, or will hydronic be the only heat?

 

[98kx1927]

It includes a two stage 15kw electric backup heat for each unit. I plan to use the wood boiler for my primary source of heat, but have the ASHP as backup. Hopefully the wood system will work well enough to not have to use the ASHP. I'm currently getting the electric service upgraded to handle the addition 160A needed for the ASHP and backup heat

It looks like I just put the largest heat exchanger in the plenum that will fit. Looks like a 20"x20" will fit. Then using a 10 degree water delta t I need 10gpm (yes all the 10's confused me a bit, not sure why everything worked out to be 10) of water flow to provide 49Kbtu. That is doable for the 1" pex I have to use, since it is only about 50 feet of tubing. So that is the water heat source.

As for the air source, at 1600 cfm I need to support a 33F temperature rise, so if I have 70F degree air coming in and 103F air going out, that should do it. So I "think" this means that I can run 120F water - 10F for heat entering the air via the exchanger = 110F which is > 103F minimum. So I "think" this means I can run my water temp down to 120F and be OK.

And I looks like I need a taco 0011 type pump to support the head pressure and achieve the necessary flow rate.

dave

 

[Clarkbug]

You will need to find your coils before you move to the rest of the sizing/figuring. Their output ratings at various temperatures will be the determining factor for you.