Thinking heat pump undersized

[Tankmister]

Hi there,

I'm new to this forum and this is my first post.

I have a new house and I acted as the general contractor during the build. We moved in in June of 2011. I decided to have a Luxaire Acclimate 2 stage heat pump and variable speed air handler installed. The reason I decided on the Luxaire unit was due to the fact that the guy selling the unit was very competent. He sized the unit to a 4 ton. I'm heating approx. 4650 sq ft with the heat pump, have 9 foot ceilings on the main floor, cathedrial ceilings in both my sunroom and master bedroom. I have in floor electric heating in all my tiled areas, which is maybe 40% of my total floor area of the main floor and upper floor. We also have three propane fireplaces installed as well.

My issue is that my aux. heat, which is 15kw, is cutting in way too often. It's consuming between 100-150kwh per day. I have an energy meter installed on my air handler/aux heater. I have spoken to the guy numerious times, and he has come back and installed a switch so I could turn off 10kw of heat. The problem is that the house cools off too much and even with the 5kw of heat going so now I leave it on.

He says he has done a heat lose calc and come up with 44,000 btu.

I live just outside of St. John's, Newfoundland, Canada. The temperature doesn't get too cold. The past few nights it has gotten down to 14f, but last night it was 25 and it was still using the full 15 kw of heat.

I've had my infloor turn off for a while to see if the heat pump could keep up and it can't so I've turned my in floor back on and it's not really helping.

When the guy's installed the system they installed a couple of remote sensors, one in my living room and one up stairs. We were finding the up stairs really cold so the guy came back and turned the sensor on upstairs. So now the termostat is reading that sensor. Were still finding the upstairs cooler than the main floor so we have been closing the grills on the main to try and force more air to the upper level. Now the guy is insisting on coming back to install manual dampers in the basement to close off the main floor ducts. I think it's a waste of time considering the grills are almost closed.

Could someone please give me your opinion on why my aux heat is kicking in so much? Is it an undersized heat pump or a balancing issue?

[openclasspro]

If you have a bigger heatpump,will be too big in ac season.

[openclasspro]

You want to balance your airflow at branch where it attaches to main,not @ diffuser.

[SkyHeating]

14f is pretty cold. I don't know your area but it seems like it could be undersized. Without knowing much more about the house like windows insulation etc, we can't tell you if it's undersized. For that many sq feet in Portland Or(lows in the high 20s less than a few days per year) we would be wavering between a 4 and 5 ton. Remember a heat pump puts out approximately 50-60% of its rated capacity at 17f. So if your heat load is 44,000 BTUs and your heat pump is rated for 48,000 your getting less than half of your needed heat at 14f.

[tigerdunes]

Tank

Have you actually seen the load calc in writing on software letterhead? If not, I would ask for it.

Several things

I would want to see an independent load calculation performed for comparison purposes. You can perform one yourself-see HVAc Calc above. I believe the charge is around $50 or pay another dealer to perform one. You might be undersized.

Refrigerant charge on system should be checked.

And make certain second stage heating on condenser is working properly.

Your 15 KW heat strip should be staged.

That would be a good starting point to get to the bottom of your high electric usage.

IMO

[BaldLoonie]

A heat pump should be sized for the cooling needs, backup for the heating needs. So a house in a cold climate (ours is colder) will need a lot of backup heat when it gets frosty. We size to -10° to be safe, I think our design temp is 0°. Around here, any house with a 4 ton pump will have AT LEAST a 20kw backup, an older system could be 25 and we have seen 30. Hopefully you really built the place well being new. Got foam insulation???

Anyway, I'd agree with openclass, if you tried to size a heat pump for the heating load or a good majority of it, you would be way oversized for cooling, even with a 2 stage. That technology is 75-80% capacity on low.

Somebody crunch a summer heat gain, that's what the HP should be sized for. If he thinks 44K for a loss, then he's sized your backup just fine.

[SkyHeating]

Anyway, I'd agree with openclass, if you tried to size a heat pump for the heating load or a good majority of it, you would be way oversized for cooling, even with a 2 stage. That technology is 75-80% capacity on low.

Somebody crunch a summer heat gain, that's what the HP should be sized for. If he thinks 44K for a loss, then he's sized your backup just fine.

while I agree somewhat I also slightly disagree. The Trane two stage units like the XL20i are 50% of capacity at low stage and a XL16i is 60%-70%. We have installed systems and locked them into first stage cooling to meet both loads accurately but I also know we don't have the same extremes in Portland that you do in Indiana. This is why I like the 20i so much is that it would be a perfect unit for this example. 5 tons of heating and only 2.5 tons of cooling requiring less backup heat and more efficiency while still dehumidifying in summer. Then there are the variable units like the iQ drive and green speed.

[Tankmister]

Thanks for the responses guys!

The house is 2x6 wall construction with R-20 batt insulation in the walls, ceiling is R-50 blown in and windows are low-e coated with argon gas.

The first step that the guy wants to do is the dampers in the basement. My fear is that the coldest part of the winter will be over and we'll run out of time in getting this resolved this year.

I haven't seen the load calcs myself and I was going to download the software to do my own check. I've also been in contact with my utility company to see if they can provide me some assistance.

Does anyone know how to tell if and when the second stage is cutting in?

[openclasspro]

Amp draw ,heat rise increase

[Tankmister]

I don't mean aux heat. I mean the second stage of the heat pump.

[openclasspro]

Heat rise

[tigerdunes]

Tank

Keep in mind that a true 15 KW heat strip yields about 51 KBTUs.

IMO

[Tankmister]

Yea, I know. And that's why I'm concerned. Last night it was 25f and 15kw (51,000 btu) of heat going along with the 48,000 btu from the heat pump. Maybe a little less due to the temperature. 99,000 btu of heat to counteract 44,000 btu's of heat loss. Something is wrong there.

[SkyHeating]

Yea, I know. And that's why I'm concerned. Last night it was 25f and 15kw (51,000 btu) of heat going along with the 48,000 btu from the heat pump. Maybe a little less due to the temperature. 99,000 btu of heat to counteract 44,000 btu's of heat loss. Something is wrong there.

Do you have the model numbers of the heat pump and air handler. Remember 48,000 is only at 47f not 25f. You are maybe outputting 30k from the heat pump, also don't forget that if your system runs for 30 out of 60 minutes your still only putting in 40k BTUs of heat per hour not the 99k that you mentioned. How many minutes is your system running out of every hour?

[Tankmister]

I'm at work now and don't have the model #'s with me. I'll post them when I get home.

The system is running continuously, only stopping for a defrost cycle. Which last night was about once per hour.

How does the Luxaire Acclimate heat pump stack up against a Trane heat pump?

[tipsrfine]

I would check to make sure the heat pump is charged & functioning. I didn't notice if the ductwork is in an unconditioned area? Are you running those fireplaces a lot? Sealed combustion? Other than these, I would look to your homes thermal envelope; insulation without air sealing, insulation improperly installed etc.... Rim joist/sill plates with just fiberglass insulation is not doing anything. Cathedral ceilings...how were those insulated? Recessed canned lighting, home theater speakers in walls and any other ceiling penetrations are big air leakers if not air sealed properly.

[Tankmister]

The ductwork is in the basement and it's exposed. The basement is on it's own zone as well. The basement zone never calls for heat due to the exposed ductwork. There is no issue with the building envelope. All recepticals are R2000 boxes, vapor barrier used throughout the envelope. Recessed lighting are the proper sealed cans and the joists are sealed with SM insulation and spray foam. The fireplaces aren't used very much as I have a 9 month and a 5 year old and I'm afraid of them getting burned.

[BaldLoonie]

Roughly figuring, at 25 out your heat pump is maybe 32,000 BTUs.

I'd have to guess that your heat loss is well above that 44K figure! That's awfully low for a house of that size!

As for the discussion of what's low on an unloading scroll: depending upon coil and blower, this UPG product is around 74-75% on low. I've seen units, 16i and other brands, where low is 80% of high, looking at cooling capacity. This is why I can't overly get excited about the 2 stage scrolls.

As for going bigger, a 5 ton unit in this line at 17° puts out 6,000 BTU more than the 4 ton. That's not much more than 1 space heater plugged in. Would that greatly reduce the need for backup or lower the balance point more than a couple degrees? No.

As for how the Lux compares to a Trane: the 4 ton Trane is 77% capacity on low so even stronger than the Lux. At 17° out, the 4 ton 16i is putting out 30,000 BTU so maybe a few BTUs higher than the Lux but nothing major.

[Tankmister]

Do you have the model numbers of the heat pump and air handler. Remember 48,000 is only at 47f not 25f. You are maybe outputting 30k from the heat pump, also don't forget that if your system runs for 30 out of 60 minutes your still only putting in 40k BTUs of heat per hour not the 99k that you mentioned. How many minutes is your system running out of every hour?

I checked the model numbers last night. The pump is HL8B048F1C. The air handler is AVL48D3XH21CA.

[skippedover]

The load calculation is accurate if the information used for the calculation was accurate. It all starts with the tightness of the home, then moves to the weather data for your city. From there it all comes down to the actual characteristics of the home; air leak rate, R-factors and/or U-factors, roof type and color, window types, floor coverings, etc.

You've stated that you trusted the contractor who did a full load calc and I'll assume he's done many other installations as well??? (Bad thing to assume, I know.) So I'll give him the benefit of doubt and state that it's very likely the HP is properly sized. But that's only on facet of the possible issues. Is the house universally cold or are there both some warm rooms and other rooms that are cold? The operation of the aux heaters would be both normal and expected at low temperatures. The 48,000 Btu nominal rating on your HP is achieved at 47°F and that capacity drops off fairly quickly as it gets colder outdoors.

As to zone controls, those can boost the comfort in many instances by allowing the full capacity to concentrate on one area, then move to a different area of the home. At the end of the night, the total Btu's are the same but just allowing the system to concentrate on an area that needs more than its share (lot of windows, sky-lights, etc.) can often relieve an otherwise uncomfortable situation. This type of approach is called diversification of the load and works well, particularly for cooling where the sun is an issue but can also help with heating.

Given that the HP is anticipated to need help at 14°F, if the house is still cold, I'd suggest that the aux heaters are not doing the job. A 10-kw heater should, in theory, just make up for the lack of Btu's form the HP, with both operating continuously. That's a little close for most installers. But if the actual low temp load on the house is 44,000 Btu's, then 15-kw should do the entire job without the HP. If not, and if your airflow is correct, then the house needs more than 44,000 Btu's and in that case, then the load analysis is suspect. Or the house wasn't built to the tightness anticipated when the analysis was done.

[Tankmister]

The contractor had done another quote for a friend of mine and both his and mine work out to be 9.5 btu loss per hour per sq foot. So I'm curious if that is his rule of thumb or is it a coincidence.

The house on the main floor and basement are good. We do find that the upstairs is colder, especially our girls rooms. Keeping in mind that their rooms are on the north west side of the house.

We have the thermostat set on 22c and if it's really cold out the temperature drops in the house by about 1-1.5 degrees, then 5kw of heat cuts in. Some time later another 5kw will kick in and if that doesn't do it, all three heaters turn on. It doesn't take very long for the house to heat up. It does seem though that once all three elements are in use, it takes a long time for them to turn off. 2-3 hours at times. Which is driving up my power bill.