I would like to know what your experience is with split units vs. regular units. I have a 5 year old forced air oil furnace and my contractor wants to install a geo unit coil above heat exchanger of the furnace. I understand the split units are designed for that. I have a number of questions:

 

1)       Are split units less efficient than regular units when installed properly?

2)       What is the difference in efficiency between single stage units and 2 stage units? Is there any. I may only have a choice of a single stage system because my air handler engine is NOT a variable speed motor.

3)       Is it a good idea to put split unit in or should I just replace the unit. It is kind of nice to have oil for back up.

 

Splits tend to have slightly lower efficiency ratings, but the difference is small and may be lost in the extra blower power a package unit might need if the package unit isn't as centrally located relative to the ductwork. Bottom line, don't let efficiency considerations drive the split vs package decision.

Two stage units tend to be a bit more efficient in the lower stage, but the operating savings would take years to cover the additional upfront cost. We like them in the southeast because they tend to do a better job with humidity by running longer in the lower stage.

Know that a heat pump can't operate in concert with any 'upstream' source of additional heat (such as an oil or gas furnace) Controls will need to be arranged so that it is one or the other.

Be absolutely certain that the furnace blower can provide required airflow at the expected static pressure imposed by the addition of a split coil in the ductwork.

The contractor told me I would have two separate systems with separate thermostats. Geo would be set at desired temp and the oil furnace below the coil would be set at much lower temp to only kick in when the geo fails. Does that sound right? I don’t see a problem with but I am not an HVAC professional.

One more question:

What can I realistically expect for COP on a single state unit? Some folks tell me with an open loop system I can expect 3.5 not 4.

 

I see big problems:

I remember hearing in school that two separate thermostats in the same zone or space is a code no-no, and I doubt I'd ever want to install or own such a thing - too much chance of thermostats causing units to work against each other if mis-set by an unknowing user.


If the geo fell behind to the point where the oil furnace kicked on and the geo stayed running, the flood of warm air entering the indoor coil would cause a rapid increase in refrigerant pressure causing the system to quickly trip out on either high pressure or compressor overload - very undesirable outcomes. It may be that a single thermostat could be implemented that would keep the blower going while heating on geo only, and then cut the geo out and fire oil for stage 2 operation, but I think this would be a bit of a kludge and inefficient. I think the two-thermostat plan would be a complete disaster.

Refrigerant coils are routinely integrated into gas or oil fired forced hot air systems, but as far as I know that is for cooling only. A single thermostat controls the system and operates only the furnace for heating and only the compressor for cooling.

Carrier offers a Hybrid heat pump system that may combine both fossil heat and heat pump operation at the same time, but that would seem to me to require that the coil be upstream of the furnace and a special thermostat to control operation of both systems.

Despite claims of COPs in the 4-5 range, I think most pros here figure on a COP of 3 in heating dominated climates when evaluating alternatives. A two stage unit might bump that up about 0.5 but not enough to justify the cost on COP considerations alone. Again, the major attractions of 2 stage units are longer and quieter operation in cooling mode for better dehu.

Disclaimer - I have very little experience with combined fossil and and AC systems. I hope one of the pros from heating dominated climates with fossil experience chimes in and corrects any mistakes I have made here. (Paging Joe or Dewayne...)

Posted By Andrzej on 08 May 2010 08:21 PM

It is kind of nice to have oil for backup.

Having converted from oil to geo, I'm delighted NOT to have oil for backup.

Getting rid of the furnace, tank, odors and soot has been a major-ly
liberating experience. The smelly, grimy old furnace room suddenly
became a clean, comfortable workshop -- with the quiet new GSHP
taking up far less floor space than the old oil tank and furnace.

The backup heat strip (mounted inside the air handler cabinet) takes
zero space, requires zero maintenance, and (at $0.07/kWh) costs less
per BTU than heating oil. (1 US gal @ 85% efficiency = 35.5 kWh.)

A question for the HVAC pros: Are there any practical concerns with
long-term storage of heating oil? If used only for geo backup, a tank
of oil might last for many years. ...any sludge or stability issues?

Posted By Andrzej on 09 May 2010 08:48 AM

The contractor told me I would have two separate systems with separate thermostats. Geo would be set at desired temp and the oil furnace below the coil would be set at much lower temp to only kick in when the geo fails. Does that sound right? I don’t see a problem with but I am not an HVAC professional.

One more question:

What can I realistically expect for COP on a single state unit? Some folks tell me with an open loop system I can expect 3.5 not 4.

 

I have something like this setup, but with a boiler and baseboard heat, which is the original heating system for the house. It works, but as noted elsewhere, I'd rather have gotten rid of oil, but I'd already invested in an expensive boiler, and didn't want to lose that investment. I also have gotten my last few years's oil for free, as my clients switch from oil to other heat forms, I've offered to empty their tanks for free so they can dispose of them more easily. But for 'normal' people, I would just get rid of the oil and stick in electric strips - much easier, less to go wrong and a common configuration so maintenance is easy.

On the sludge issue - I go through oil very slowly and use a high-quality sludge cutter additive that seems to be working well to keep down the bacterial sludge that can build up. I would not just let the tanks sit for years without any turnover. That's asking for trouble.

Posted By engineer on 09 May 2010 09:34 AM
  It may be that a single thermostat could be implemented that would keep the blower going while heating on geo only, and then cut the geo out and fire oil for stage 2 operation,

Disclaimer - I have very little experience with combined fossil and and AC systems. I hope one of the pros from heating dominated climates with fossil experience chimes in and corrects any mistakes I have made here. (Paging Joe or Dewayne...)

While not common,  having a thermostat that controls a geo unit upstream of a fossil furnace is available and works fine.

As noted  only one system can operate at a time.

  The more preferrable method is to use electric strips since with this setup the geo works all of time even when the electric backup kicks in.

The OP will can decide which he wants to use since he knows all of the factors involved.

Thank you Dewayne.

You are welcome and

Thank you Curt for the wealth of knowledge you bring to this forum.

Posted By Looby on 09 May 2010 11:44 AM

A question for the HVAC pros: Are there any practical concerns with
long-term storage of heating oil? If used only for geo backup, a tank
of oil might last for many years. ...any sludge or stability issues?

As I understand it, gas, diesel and heat oil all have an expiration date.  Heating oil shelf life is 18 to 24 months without fuel additives.  Additives can extend the shelf life of heating oil for perhaps another year or two, but it has to be added as soon as soon as tank is refilled, if you wait a year or two to add fuel stabilizing additives, it does little to renew or extend the life of heating oil.  Even if you were able to store your home heating oil for 3 or 4 years, that then?  You can't exactly pour 200 gallons of home heating oil down the drain, it's still toxic to the environment.  You would have to take it over to the towns recycling center 10 or 15 gallons at a time to dispose of it properly.  I guess any tank disposal outfit would have to proper equipment and storage to extract stale home heating oil from a tank, what what's that going to cost you? I say your far better off to get rid of the oil heating system and tank when you switch to Geo, the environmental liability concern alone scares the hell out of me.   
    

I agree. Use it or loose it. Looks like maybe 2 years with stabilizers from googling.

The reason the contractor wants to add on a split to your oil furnace is to sell you a system that costs less up front than a full replacement and to try and gain a few more years of life out of that 5 year old oil furnace. The thought is "why throw away a five year old furnace?" However, I would NOT install the split on the oil furnace for a couple of reasons. I would ask that it be removed and kept in complete working condition and then made available to a charity that may have a needy family with a defective oil furnace that needs replaced but cannot afford the replacement. You will feel good and maybe even get a tax deduction for it!

Why to NOT install a split on a fossil fuel furnace?

Reason one, when your geothermal needs assistance (when the gas or oil needs to come on to keep the house warm) you have to shut off the geothermal and all it's benefits. That means the WHOLE heating load is met by the gas or oil. During the hours the gas or oil is running, the cost saving benfit of the geothermal is eliminated.

Reason two, the air delivery of the existing furnace is probably much less than what the geothermal requires. For instance, a 100,000 Btu oil or gas furnace blower may only deliver 1000 to 1200 cfm. A home that requires a PROPERLY sized 100,000 Btuh furnace will probably require a 5 ton geothermal unit which will require 2000 cfm of air to give you the efficiency, cost of operation savings and comfort you would expect. Also, a geothermal system that is "starved for air(not enought air flow) will have a number of service calls due to the stresses placed on it. You will have a very unhappy experience.

Reason three, if the dealer does install a geothermal unit that wil operate properly with the air delivery you already have it will most likely be too small for the heating season and you will spend too much time using the gas or oil and not see the savings you expect. The smaller unit may have a capacity balance point of say 35 to 40 degrees outside air temperature. That means that when ever the outdoor temperatue is below 35 or 40 degrees, the geothermnal shuts off and you are solely using the gas or oil.

Reason four, OIl does not have a long shelf life like L.P. Gas does. When you need it the old oil will cause you problems.

I strongly recommed you replace the whole system and go with a packaged geothernmal unit, properly sized with all necessary duct system modificatins to handle the correct amount of air flow and use electric resistance auxillary. Unlike fossil fuel furnaces that have to be on the "inlet side" of a geothermal air coil, electric resistance heaters can and usually are on the "outlet side" of thge geothermal air coil. In this position, they can operate at the same time as the geothermal unit. This means that when the geothermal unit needs auxiliary, the geothermal stays on and the electric resistance "ONLY MAKES UP THE DIFFERENCE BETWEEN WHAT THE HOUSE NEEDS AND THE GEOTHERMAL UNIT PRODUCES. This amount of auxiliary heat costs MUCH less than the fossil fuel auxiliary.

Now that I've said that, there is one othe caveat to consider. Most of my customers are in northern clmates and served by rural electirc utilities. Some of these customers are subject to lengthy power outages during the winter. The issue is that a geothermal unit will require at least a 12kW or 15kW back up generator to start them. Those are not cheap generators. However, a gas or oil furnace will run very nicely on a 1.5 or 2 kW generator available at your local Home improvement store. In those cases, we install a split geothermal system on an L.P. or Natural gas furnace, sizing the furnace blower for the entire system, but we also install electric resistance duct heaters to be used when the geothermal unit needs auxilliary and the power is on. However, if there is a power outage in the heating season, the customer can start the generator and run the gas furnace to keep themselves warm until the power is restored. IN essence, teh customer has three sources of heat available; geothermal, electric resistance and fossil fuel. Each can be used when needed.

I hope this helps you make an informed decision.

One more reason to not go with a split is the warranty issue, your furnace is out of warranty on parts but not heat exchanger more than likely, if you put in a complete system the whole thing then falls under the warranty.

you also asked about two stage, this good for cooling season, you will have better control of humidity with the stages, and in winter it makes it more efficient too.
we have also installed the retrofit ECM motors on existing sytems single stage or not, true for those of us who like to see the blower stage up for second stage, with a relay you can achieve this.

Super informative thread GEO types... One thing I see though... is huge money involved in some of the above described systems at install...

Posted By adkjacUpstateNY on 15 May 2010 06:09 PM
Super informative thread GEO types... One thing I see though... is huge money involved in some of the above described systems at install...

not to many inexpensive geo systems. what costs even more in many instances is burning oil.....
j

Ahh... burning no oil now... never will again!~

burning wood...

going passive solar

going active solar

going very super insulated and super undersizing back up propane heat source and moving toward air sourced tiny mini splits

aj

Cellulose is inexpensive and so are super thick exterior walls.

Northern NY

lowest cost oil today is 2.42/gal.
KWH is at .14ish

so for here electric back up heat for a heat pump equals
twice the cost of oil.... $5.00/gal.

That is why we go NG if available, propane for clean no smell, oil or wood to lower the cost
and GEO to lower the cost if able to pay the added install cost and justify such cost.

NG is $1/therm today.  100,000btuNG/3400btu/KWH=27KWH
27KWH/3.5COP=7.7KWH  $1/7.7=.13KWH

Marcellus Shale deposit being hydrofractured will keep NG
equal to cost of GEO.

So that means if a homeowner goes green and installs solar PV panels to
offset GEO KWHs then with a cost of maybe $25,000 for each system
they could reduce their annual CO2 output substantially and reduce
their energy costs to near net zero.

If just going for cost and not believing in climate change then
Natural Gas wins hands down, with half to a third the install cost
depending on whether AC is installed.

Superinsulate,add PV, and passive solar, and solar hot water,
and possibly air to water altherma with wood cold night back up.
IMHO.

Southeastern PA:

Winter rate: $0.07 per kWh -- for all-electric homes only.
With propane or oil backup, the rate more than doubles.


...BTW, how's the propane central A/C workin' out for ya?

Looby

Looby... propane, electricity... very expensive here with poorly insulated homes. Burning wood/pellets is the only lower cost way to go with a poorly insulated home here and that's what many here do. Oops... Natural Gas.... that is by far the best, we just don't have it far from city centers. We are at this time sending in applications to get NG extended to many of the streets where I live.

Check out the specs I just posted here.... for a green retrofit....

http://greenbuildingtalk.com/Forums...fault.aspx

Looby....

your thoughts....

Daiken Altherma with wood back up for coldest hours of operation assstance.

I have an idea used as such that the seasonal COP may come close to 4?