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Jack_in_VA
Premium
join:2007-11-26
North, VA
kudos:1
Reviews:
·Millenicom
reply to walta

Re: [HVAC] Manual J and heat pump sizing

said by walta:

When you talk to the salesman ask him to show you where the power for the circulator pump is in there calculations. You will likely get a hem a haw because generally the pump supplied separately and left out of the calculations.

If you chose an open system you will likely need a larger pump. In a closed system the energy of the falling water is used to lift to lift water from the well. In an open system all that energy is wasted.

I would size the geo system for cooling and run the old propane as stage 3 heat or electric strips.

You can get manual J the find the outdoor temp where stage 3 will kick in. My guess is 2 tons can get you down to about 20 degrees, most years you will run less than 400 hours on stage 3.

Walta

A 1 HP pump is not going to be cheap to operate.


pende_tim
Premium
join:2004-01-04
Andover, NJ
kudos:1
Reviews:
·Comcast

1 recommendation

said by Jack_in_VA:

1 HP pump is not going to be cheap to operate

Running continuously that would be about 25KW per day. or at 50% duty cycle about $40.00/month minimum. Yea in the grand scheme of life that may not be a lot just for the pump but then add the operating cost for the Heat Pump....

I assume you have a pond to pump the excess water to?

You must live in one of the few states where open systems are permitted. I know a lot have closed that door.

Variable speed fans will not solve any humidity problems with a grossly oversized unit. If the fan is slowed down enough to increase dehumidification, freezing is very likely and efficiency will take a serious hit. The variable speed is good to get initial airflow that is cool and dry but there comes a point where the coils must balance the incoming expanding refrigerant and pull that heat from the return air. The problem becomes that there is a lot of heat removed at 5 tons and when badly over-sized the room cools quickly and you don't get multiple passes over the coils to extract humidity.

Yes you can run a dehumidifier, but that is not free. Most of the 70pint ones draw 700 watts or so. That could add another $30 to your energy bill.

Will this be cheaper than propane heat? Yes, but it may cost more than you are expecting.

Another consideration will be duct size. A 5 Ton unit will be moving a lot of air so the ducting for the existing furnace will probably need to be modified. Registers will be blowing more air which will not be as warm as the gas furnace so watch out for drafts. I would *guess* the old furnace moved 1500 CFM and the 5 Ton AC will want to see in the 2,000 CFM range. Return ducting will also need to be modified, don't forget.

I am not saying don't do this project, just manage your expectations.
--
The difference between genius and stupidity is that genius has its limits.


Tex
Dave's not here
Premium
join:2012-10-20
kudos:2
You took the words right out of my mouth. Everything you've posted is dead on.

The only thing I would add to your most excellent post, without getting into the nuts and bolts of psychrometrics (wet bulb, dry bulb, latent loads, sensible loads, grains of moisture, etc.), the OP is sizing the AC part of the system based on an outside design temperature of 89 degrees and, I assume, indoor 75 degrees and 50% relative humidity. At those designs, he's still oversized ~10,000 to ~20,000 BTUs for cooling, depending on the system. Imagine how oversized (both for the sensible & the latent load) he will be under part load conditions. Most likely, the AC wouldn't run more than a few minutes at a time, which isn't close to enough time, as you mentioned, for lowering the humidity in the living space. Cold is good. Cold and damp is not. If the OP does go this route, I highly suggest a whole house dehumidifier. And, lookie here, Climate Master sells one. Imagine that!

iknow_t

join:2012-05-03
reply to pende_tim
said by pende_tim:

said by Jack_in_VA:

1 HP pump is not going to be cheap to operate

Running continuously that would be about 25KW per day. or at 50% duty cycle about $40.00/month minimum. Yea in the grand scheme of life that may not be a lot just for the pump but then add the operating cost for the Heat Pump....

I assume you have a pond to pump the excess water to?

You must live in one of the few states where open systems are permitted. I know a lot have closed that door.

Variable speed fans will not solve any humidity problems with a grossly oversized unit. If the fan is slowed down enough to increase dehumidification, freezing is very likely and efficiency will take a serious hit. The variable speed is good to get initial airflow that is cool and dry but there comes a point where the coils must balance the incoming expanding refrigerant and pull that heat from the return air. The problem becomes that there is a lot of heat removed at 5 tons and when badly over-sized the room cools quickly and you don't get multiple passes over the coils to extract humidity.

Yes you can run a dehumidifier, but that is not free. Most of the 70pint ones draw 700 watts or so. That could add another $30 to your energy bill.

Will this be cheaper than propane heat? Yes, but it may cost more than you are expecting.

Another consideration will be duct size. A 5 Ton unit will be moving a lot of air so the ducting for the existing furnace will probably need to be modified. Registers will be blowing more air which will not be as warm as the gas furnace so watch out for drafts. I would *guess* the old furnace moved 1500 CFM and the 5 Ton AC will want to see in the 2,000 CFM range. Return ducting will also need to be modified, don't forget.

I am not saying don't do this project, just manage your expectations.

add in a 2 speed compressor and TEV instead of capillary tubes..


Ken
Premium,MVM
join:2003-06-16
Markle, IN
reply to pende_tim
said by pende_tim:

Running continuously that would be about 25KW per day. or at 50% duty cycle about $40.00/month minimum. Yea in the grand scheme of life that may not be a lot just for the pump but then add the operating cost for the Heat Pump....

Yes there will definitely be an added cost for pumping water, but I simply can't afford to install a closed loop at this time. My hope is in a couple years to install a horizontal closed loop and lower my pumping costs. Originally we planned to wait until then to put in the entire system, but the cost of propane this winter is forcing me take action now even if I can't afford to do the entire system at once.

said by pende_tim:

I assume you have a pond to pump the excess water to?
You must live in one of the few states where open systems are permitted. I know a lot have closed that door.

Yes we have a small pond to dump into. It's not big enough to put a pond loop in, but a good place to dump water into. Of course some of the water I'm dumping is going to evaporate, but a good amount will eventually soak back down into the ground. Our aquifer in the area is basically infinite for my small usage. I could pump 25gpm for years on end and never run the well dry.

said by pende_tim:

Another consideration will be duct size. A 5 Ton unit will be moving a lot of air so the ducting for the existing furnace will probably need to be modified. Registers will be blowing more air which will not be as warm as the gas furnace so watch out for drafts. I would *guess* the old furnace moved 1500 CFM and the 5 Ton AC will want to see in the 2,000 CFM range. Return ducting will also need to be modified, don't forget.

I didn't even mention that, but I already did the duct design and have to make a few changes. I need some more returns and a few more supplies. The returns will be easy where I need to put them, but I need to add 2 supplies on the 2nd floor and they will be challenging.


Ken
Premium,MVM
join:2003-06-16
Markle, IN

1 edit
reply to Tex
said by Tex:

OP is sizing the AC part of the system based on an outside design temperature of 89 degrees and, I assume, indoor 75 degrees and 50% relative humidity. At those designs, he's still oversized ~10,000 to ~20,000 BTUs for cooling, depending on the system. Imagine how oversized (both for the sensible & the latent load) he will be under part load conditions.

I guess it just bothers me to think about spending $6,000 on a geothermal heat pump and have to use electric heat strips all the time. I just assumed after I got my manual J results it would be obvious what size unit I needed, but the 3, 4, & 5 ton units all have their pro's and con's. I understand many people just size for cooling and rely on heat strips to make up the difference, but during my research I have found many people who don't even have heat strips in their unit. They somehow have sized a unit to work for heating and cooling, which is what I would like to do.

I've also just become aware of variable speed compressors. While more expensive they would seem to solve all of my problems. Another thing I have seen is the ability of some geothermal units to dehumidify by cooling the air and then reheating it as it leaves. I also have started to consider putting in 2 units, a 2 ton for the 2nd floor, and 3 ton for 1st floor and basement.


pende_tim
Premium
join:2004-01-04
Andover, NJ
kudos:1
Reviews:
·Comcast

1 edit
said by Ken:

thing I have seen is the ability of some geothermal units to dehumidify by cooling the air and then reheating it as it leaves. I also have started to consider putting in 2 units, a 2 ton for the 2nd floor, and 3 ton for 1st floor and basement.

If you put in 2 units, it may be possible to install dampers and use one to do the entire house for cooling and then use both for heating on 2 zones.

As far as sizing the units based on future "improvements", be cautious here. I do not doubt that you have every intention of doing the work, but it has been said many times that you should not size HAVC based on future work.

Suppose you spend $20,000 on a new HVAC system(s) based on the future state of the basement. Now lets suppose that something happens that you don't do this project. What do you now do with the undersized unit you just installed?

--
The difference between genius and stupidity is that genius has its limits.


Jack_in_VA
Premium
join:2007-11-26
North, VA
kudos:1
Reviews:
·Millenicom
reply to iknow_t
said by iknow_t:

add in a 2 speed compressor and TEV instead of capillary tubes..

Most all new systems now have 2 stage condensers and variable speed air handlers.


Ken
Premium,MVM
join:2003-06-16
Markle, IN
reply to pende_tim
said by pende_tim:

If you put in 2 units, it may be possible to install dampers and use one to do the entire house for cooling and then use both for heating on 2 zones.

That's what I was thinking. Just one unit would do a/c all summer. Then in the winter both could run to provide heat. I'll have to change a bunch of ductwork in the basement, but it's all totally exposed right now.