Heat pump questions

[John Harlow]

I just purchased a home with a 1.5 year old dual Lennox heat pump system (upstairs and downstairs). I'm not sure the most efficient way to run this for each season; for example in the winter should I mostly run the lower unit and let the heat rise (which keeps the whole house warm), or mostly the upper unit and stay in the upper section of the house (I work from home and am home all day)?

I'd also like to install setback thermostats, the existing ones are simple digital controls. Can anyone suggest a good 2 zone controller? I installed one in my previous house 11 years ago, it had a simple daily / weekend schedule - have they gotten "smarter" lately?

Thirdly, is it worth my time investigating a GWHP or GLHP replacement for this system? Would I just be replacing the compressors or are they so radically different they are a total system swap?

[Travis Jordan]

Geothermal systems require re-engineering of your existing system - not a trival matter. And they are have high life-cycle costs and are usually hard to justify on an economic basis.

Thermostat functionality really hasn't changed much in the past 11 years. The user interface has improved, however. Take a look at this for an example of the state of the art:

As to the cost of heating during the winter....it is logical that running just the upper unit (and letting the lower level of the house cool down) would be the most energy efficient. ANY setback saves you money.

[Astro]

Depends on what you're after, your budget, your location, system size needs, etc.

Some geothermal systems can be used with existing air handlers, though it doesn't seem they like doing that because of "system matching" issues. You'd have to ask each contractor whether they'd do that or not.

If you live in a mild climate, it might not be worth switching over depending on the existing system's efficiency. If it's a harsh climate, then you're likely going to be pushing the limits of it's ability to supply warm air and will switch to electrical backup. You might want to try it this winter to see how cost effectively it runs in your specific climate, in your home. Keep an eye on the thermostat to see when it has to switch over to electric heat.

It's my understanding that you shouldn't play too many setback games with heat pumps either, or you'll be forcing it to switch to electric heat because the heat pump won't be able to handle the temperature swings efficiently. Again, you'll want to examine how the system works in your specific situation. It should only take a few experiements to see how effective the setback actually is before even buying the thermostat.

It will be interesting to hear more about your system and what you ultimately do. Good luck!

[John Harlow]

Interesting, but it doesn't seem to do anything more than the one I installed 11 years ago. It's a little prettier though ;)

As Astro pointed out, it might not be true if the resistor heat is called into action. I'll need to see if the heat pump specific setbacks are smart enough to know this.

Thanks for the info.

[John Harlow]

I'll have to check with the local contractors. What got me interested is my water is supplied by well, and I may be switching over to municipal - leaving me with this basically free water supply. I've been discouraged with the reports of deposits in open loop systems though.

I will glance at it when it starts getting really cold here (central VA).

Great point. I would HOPE the 2 stage setback would be smart enough to know resistive heat would wipe out any efficiencies gained by setting the temperature back.

Thanks! If I end up doing something interesting, I'll be sure to post.

It would be a fun project to monitor indoor / outdoor temps and compressor / em heat resistor amperage on my PC. I could do some interesting efficiency calculations and keep an eye on trends.

Does anyone know of any wireless Current Transformers (CTs) I can hook to my PC?

[PrecisionMachinisT]

The electric backups are known as "heat strips"......

A thermostat can be added outside by a tech to eliminate them coming on until a preset outdoor temperature is reached, but if the outdoor coils ice up and the heat strips are not put into operation there will be cold air blowing inside during any defrost cycles--in essence the heat pump acts as an air conditioner, rejecting heat from inside the home during defrost in order to defrost the coils and regain airflow through the outdoor coils should they become iced up.

What you need to look at is an operating chart showing compressor current draw at various temperatures inside and out in order to make wise choices as far as thermostat setbacks are concerned--realizing that if you set back too far, there will be prolonged warmup periods where the electric strips will cycle on depending on your particular thermostat and other arcane system details, this sometimes partially negating the overall efficiency of the unit.

A typical operating chart for a package type heat pump can be found at this link :

The heating capacities start at around page 11 of the .pdf document, but yours may be somewhat different unless it happens to be of the same make / model / manufacturer and heating efficiency rating.

[Astro]

What's the water capacity of your well? I was looking at doing the same as I've got two wells on my property, but once I learned the capacity of the well (1.25gpm :-( ), I scratched that Idea and went with a direct exchange geothermal (to be installed soon).

But if you have got a solid 5-10 gpm from your well, and local code allowed installing an injection well (or you've got a big pond, etc.) then open loop could be pretty cost effective. Certainly lots cheaper than a conventional well field. Around here, it costs $6-$8/foot to drill, plus casings (real expensive now) so a conventional gshp can cost $10-$15k just for the drilling!

As you noted, there are problems with open loop, but it sounds like you're tech savvy. With maintenance and a decent heat exchanger on the water side, it could prove to be a pretty good setup.

Since your in VA, have you ever talked with Scott Meenan?

He's "out of favor" with the HVAC crowd, but you can check out his website and judge for yourself. I won't say more than that or I'm likely to get myself into trouble...

cheers,

[HeatMan]

He's out of favor because he's an idiot....

[PrecisionMachinisT]

If you already have old wells that are fairly deep on the property then you can use them as vertical loops--simply run 1-1/2 in poly pipe to the bottom and back up again.....

If that doesnt provide enough capacity then you can pick up the rest of your heat by burying some additional loops in the ground as in the more traditional trenched systems.

[Mike Fritz]

As far as using the two zones independently, I would set them the same, assuming you are using both spaces about the same. For example, if you and your family are using the upstairs and downstairs during the same time, set both thermostats the same. Any heat that rises to the second floor will add to the heat already there. Thus, that thermostat won't kick on as often. There wouldn't be any point in setting it any lower.

Now if you and your family are spending most of your time on one floor or the other, set the other thermostat back. If anything, set the thermostats a few degrees lower, so you can quickly recover. (For example, if you set the thermostats to 70 when you are home, have them set to 65 when you're away.)

--Mike

[Astro]

That was an option. The one well is 900ft (!) and would have had some good capacity. Unfortunately, as I understood it, using a single deep well rather than several shallower ones can be problematic because of pressure drop through the single pipe. A bigger pipe, like you mention, could probably reduce this issue. However, I was also a bit hesitant to deactivating that well because it's my backup, and periodically I need to use it.

[PrecisionMachinisT]

If the static water level is near the surface, then run the discharge pipe to near the bottom, and place a second pump at a depth of a hunderd feet or so, just under the max drawdown level when in use......or, simply use the existing pump and discharge back into the top of it.

That well should easily provide about 3 tons capacity, if was pressed into being giving out ballpark figures.........

[HvacTech2]

Hi PrecisionMachinisT, hope you are having a nice day

On 15-Nov-04 At About 09:06:04, PrecisionMachinisT wrote to PrecisionMachinisT Subject: Re: Heat pump questions

P> From: "PrecisionMachinisT"

P> The electric backups are known as "heat strips"......

P> A thermostat can be added outside by a tech to eliminate them coming P> on until a preset outdoor temperature is reached, but if the outdoor P> coils ice up and the heat strips are not put into operation there P> will be cold air blowing inside during any defrost cycles

Any system I have ever worked on had either a separate set of heat strips or a separate wire that bypassed the outdoor stat to energize the heat strips during defrost.

-=> HvacTech2