Carrier running strip heaters often
My father in law just built a new house in South Dakota with a Carrier ground source heat pump. He is having some trouble with it, as the auxiliary strip heaters keep coming on and he is spending a lot more money heating than he expected.
I don't know all the details (I live far away), but here's the basics:
It's a Carrier 3 ton model 38-something
His water source is 52F groundwater, with waste dumped into a separate well.
The return water is 9 degrees cooler, indicating that heat exchange is occurring.
He has an insulated crawlspace of a few feet, with the ductwork running uninsulated under the house. He had some crawlspace vents that were accidentally left open, but they are closed now without a great improvement.
When outside temperatures are in the 30s and 40s, the crawlspace is a comfy 65F.
The pump is rated at 10 gallons per minute, but the installer put in a choke to 3 gal/min because he said the manf. called for it. (Why not open this up?)
So, the basic question is, why do the strip heaters keep coming on and running up the electric bill? Is too much heat being lost to under the house? He had a ductwork guy out, but he said the ductwork should not be insulated under the house. We were thinking maybe he was losing too much heat to the environment under the house.
It's a new house and well-insulated. He is disappointed in the whole project and isn't getting anywhere with the contractor or subcontractor. Any thoughts on this? Thank you, experts.
At a glance, 3 gpm seems a bit light for a 3 ton but 9° drop over the evap doesn't seem out of line.
In an insulated crawl, I wouldn't insulate the ducts. I'd say get a temp rise over the geo without electric heat running. I'd expect 30° diff between in & out typical. Could be with that little waterflow not getting enough heat out of the water though I'd expect colder water coming out of the machine.
Thank you for your reply. I'll be interested to see how this plays out.
Check the thermostat settings some they have a differential that can be set. If the room temperature drop ½ degree the unit will bring on 2nd stage, then make sure the auxiliary needs a 1 degree drop before bring it no. Some have an option of changing the time also so if it is 1 degree below set point and has been 1 hour then bring on the auxiliary, this allows the geo a chance to bring the home up to temperature before allowing the more expensive auxiliary to engauge.
Some stats also have a lock out feature that locks the compressor out at a certain temp.if it has an outside sensor. See what temp. setting you have your lockout on.
I agree it should be in your thermostat settings. If your water is entering at 52F what is it leaving at? You said 9 degrees cooler so 43F? I also think 3GPM is a little slow, you could probably run a higher GPM and only pull 4-5 degrees of heat out of the water. When we setup ground source heat pumps we size for the house total load and we lock out the heat strips. Try turning off the breaker to the heat strips and see if the ground source heat pump will still heat the home. If it still heats the home you know its thermostat settings. If the home isn't heating, either the system is not working or the load was done improperly. How many sq feet is the home and what insulation was used and what u-value windows? Lastly, what is the well temperature now? Has that been checked recently, maybe your well is to small and your extracting to much heat. Maybe it was originally 52 and now its 40?
I agree with skyheating. On a 3 ton system i would think you should be around 5 GPM. Also when we do a Geo System we size it to handle the whole homes heating/cooling needs without backup heat. We install electric backup heat strips in case of a unit compressor/pump failure and is locked out by the stat unless the homeowner manually turns the thermostat to emergency heat.
I personally have a 4 ton water furnace geothermal heat pump. I have mechanically disconnected the electric backup. I want to know if the heat pump portion is not functioning. I have needed to replace the coil multiple times under warranty. It would have taken longer to realize the problem each time if i had the auxillary electric enabled.
A couple of thopughts here.
IGSHPA calls for 3 gallons per minute per ton of load, not just 3 gallons period. Big mistake up front on the install with the flow orrifice. Many people throttle the flow back to increase the delta T, and that is fine it is part of tuneing the system after the install. It has been our experiance that excessive use of stage 3, the electric strips is either a botched load/duct calc, or a fubar thermostat setting.
Another misconception about the stage 3 heat is that it is a function of the design alot of times, not a mistake. OG installer bids 3 ton insted of 4 to reduce equipment costs and put the burden on the owner to pay the stage 3 electric costs.
An acceptable stage 3 usage is when the design calls for a given number of design days that the stage 3 will be required in a given heating season is cheaper than upsizeing the eqiupment to meet a 1/2 ton or so of additional load. This design formula is for experianced installers with educated customers and has been discussed prior to installation of the system.
My first question in this scenario is allways " What did the design call for in stage 3?" the answer to that question usually tells you what you need to know about your installer or the person who did the load calcs for a given project.
Probably should have put in natural gas if gas was available. The strip heating is to keep the coils from freezing over (since with a heat pump you are cooling the outside/ground water). The extra auxiliary heat boost is also because South Dakota is pretty cold and heat pumps are not very good at heating. I don't know the size of your house but 3 tons is pretty small.
Originally Posted by Deadwood
How come your crawlspace is so warm? Thermal mass or is heat escaping from the house and entering the crawlspace? If the crawlspace is suppose to be unconditioned I would insulate the ducts. Also, if it was unconditioned I would vent it, I believe the venting is suppose to reduce mold and moisture.
Without knowing your unit, 10 gpm maybe above what your unit can handle, although based on 3 gpm @ 9 F difference we only get about 13,500 btu of heating.
I noticed you did mention you feel too much heat is escaping to under the house, I would see about getting some type of air/radiant barrier installed or check to see if you have one.
Edited: Woah, such an old post... owner is probably not around the forum or has solved his problems now.
Last edited by hcong; 06-04-2012 at 03:20 AM.
Reason: Noticed how old the post is...
You do realise the strip heat we are talking about here does NOT prevent the coil from freezeing? It is a part of the design referred to commonly as stage 3 heat for supplemental as a function of the design.
Originally Posted by hcong
To make a broad sweeping statement about a ground source heat pump really is letting what you don't know shine in the worst possible way. Ground source heat pumps are very good at heating and cooling when they are designed correctly and the controls function as specified.
Further a 3 ton load could be for a smaller house or for a very large one with a super insululated envelope, we really have no idea which it is.
Why would the coil freeze due to cooling of the return water in heating mode when it goes to another class 3 re-injection well?
My apologies, I should not have said they should have gotten gas furnaces. The geothermal and water source heat pumps have good EERs and COPs, much better than any air ones. I meant that natural gas is often cheaper than electricity and if you wanted to stay with 3 tons of cooling, they still could have gotten a bigger furnace. There are pretty efficient furnaces right now with 95% plus AFUE. But realizing this is the "geothermal" forum, I'll admit defeat and say go geothermal .
Originally Posted by waterpirate
I haven't been to their house so yes a 3 ton might be perfect, but even in your last post you mentioned that the contractor could have installed strip heating to make up for the heating being undersized. Also, if the crawlspace is being heated and the owner is noticing this then maybe this house isn't super well insulated. 3 tons might be perfect for the cooling load.
I did read that the water is pumped into another well, but I was advising the owner not to remove the strip heating because it is there for a reason. Sure if they find another way to compensate for the heating then getting rid of the strip heating would be okay.
I thought part of the purpose of this forum is to give views and opinions based on our experience. I admit I am probably wrong a lot and there are probably infinite better answers than mine, but still you post what you know and you hope that it helps.
A real first name would soften this up a bit. My name is Eric. Regardless of whether this is the geo forum or not everything starts with education and understanding, my personal mantra and our companies by the way.
Geothermal is only one small facet of the HVAC world but it is becoming the one with the most dis-information, incorrect information, and out and out bull.
Geothermal is not for everybody contrary to popular marketing. It is a decision that must be grounded in reality and economics. Different areas of the country pay wildly differing amounts for fossil fuels and electricity.
The costs to construct the heat exchanger in different areas of the country is also a wildly fluctuating cost. All these costs as well as the design must be considered before any wide sweeping statements about anything can be made in regard to costs, or which is better.
I have told more than one customer that IMHO geothermal is not a good fit for you, even when it meant that as a result money was not going to be funneled into my wallet.