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Community and Q&A

Water-source heat pump sizing

DavidAlex | Posted in Mechanicals on

My 2-ton Carrier water source heat pump needs replacing. It’s a condo, so I own the heat pump & handler, but not the water loop. I think I’m going to replace with another Carrier, but I’m considering downsizing to a 1.5-ton unit.

It’s an 800-square foot condo on two levels, with conditioned space on either side and above my apartment; fairly standard early 1980s insulation levels: existent, but not great. I have about 20 square feet of southern window, so it gets plenty of solar loading in the winter, but decent shading from a tree in summer. I live in DC where the 97.5% temp is 17 degrees.

The reason I was thinking to oversize was that it gets a little stale and humid between cycles in the summer. The reason I’m hesitating is because the chilling tower for the cooling loop can’t quite keep up sometimes, and I thought oversizing the pump might mitigate that, BUT I’ve also read that modern heat pumps can work with a pretty high source temp. Thoughts?

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  1. Expert Member
    Dana Dorsett | | #1

    Oversizing the equipment results in shorter on cycles and longer waits between cycles, which makes the stale-humid air problem worse, not better. When a system is running continuously it would not stagnate at high humidity, it would be constantly be drying out the air.

    Increasing the capacity of the heat pump does not increase the capacity of the water-coupling heat exchangers or cooling towers, it just hits their limits faster.

    Downsizing to 1.5 tons or even 1 ton on the heat pump will likely increase comfort (provided it keeps up with the heat load) since it would run longer cycles. But how well it would work well with your existing water-coupled heat exchange/cooling tower depends a lot on the system design. To be sure 2 tons of compressor for an 800' space with two of the walls common to other folks' conditioned space seems like a LOT of compressor for the actual loads.

    So many aspects of this are specific to YOUR system design, it's hard to be sure what all of the right solutions are. But the first place to start is to reduce the loads where you can.

    If you have 20 square feet of south facing window it sounds like you may have zero square feet of west facing window, which makes me wonder where the cooling load heat gains are coming from. Air sealing the place would reduce the latent loads (humidity) some, maybe even a lot. If the attic is just crummy R19s poorly installed between joists or rafters that could be a significant unwanted cooling load, and fixing it would reduce both heating & cooling loads.

  2. GBA Editor
    Martin Holladay | | #2

    The first step in determining the size of your replacement equipment is to perform an accurate Manual J calculation (a heating and cooling load calculation). You can't just assume that the original equipment was properly sized.

  3. DavidAlex | | #3

    Dana, understood re: oversizing and humidity. I think what I was getting at about the limited capacity of the cooling tower is that a higher-BTU system will be able to pull more BTUs of heat out of my apartment at higher water temps. For example, with a water loop temp of 110 degrees F flowing at 5 gpm and intake temp of 75 degrees, the 2-ton unit has a capacity of 17.5 MBtuh, whereas under the same conditions, the 1.5-ton unit has 15.1 MBtuh capacity.

    Lastly, what do you mean with respect to my cooling load heat gains and west facing windows? My heat gains are from internal loads, the unshaded portion of the south windows, and of course, air leaks (which I can definitely mitigate).

    Martin, I'm concerned that I don't have very good input data with which to produce a useful Manual J calculation. The apartment is partially below grade, I don't know the condition of the insulation, and I don't know the air change rate. I'm afraid a Manual J would just be garbage in/garbage out.

  4. Expert Member
    Dana Dorsett | | #4

    There is no WAY you have 2 tons of internal heat gain unless you have space heaters going, or 27 of your nearest & dearest friends over having a dance party! :-)

    No seriously- do you have a major heat source putting out 10,000BTU/hr into the condo or something? That would be something like an average of 3000 watts of electrical plug load inside the house. You might have short-term power use peaks that high, but even playing video games on the big-screen TV with all of the lights on in the other room you probably aren't averaging over half that. But if you DO have a lot of incandescent lighting, and old-school high- three high power DVRs and an oversized circa 1975 refrigerator with bad weatherstripping on the door you might be able to approach than number (but the fixes are obvious.)

    "Typical" cooling loads for single-family homes runs about a ton per 1000' of conditioned space, but in a condo with no late afternoon solar gains from west facing windows and only 20' of south facing glass (with partial shade at that!), in a condo that is partially below grad would come in at an even MORE favorable ratio.

    The high angle of the mid-day sun relative to the vertical glass reflects a large fraction of the incoming solar radiation off the exterior surface rather than into the building. But in the afternoon at much lower incident angles that doesn't happen, and the solar gain occurs after the sun has already heat-soaked the roof & walls all day and the walls are already at a higher temp, establishing the cooling base load. Those late-PM window gains spike the peak load on top of that background load. On a square foot basis west facing glass has something like 3x more affect on the cooling load peaks than south facing glass.

    Wall R-values are largely irrelevant unless there is literally no wall cavity insulation. But attic R can play a signficant factor in the cooling load, since roofing shingles are a big unglazed solar collector, and probably angled for high mid-summer gain if it has any pitch at all. (And a level-flat roof could be even worse.) How much insulation is in the attic, and what is the roof angle & roofing color? Roof gains are probably a significant hunk of the load, maybe even the lion's share if it has effectively no insulation and a low-angle south facing dark roof.

    If the insulation is low density fiberglass less than a foot thick or with gaps/compressions it's definitely a place to start. Fiberglass is somewhat translucent to infra-red radiation, so the top 2" is actually less than worthless- it's working against you, since the temperature 2" into the fiberglass layer ends up being a few degrees above the attic air temperature, which means you're insulating against a higher temp with less insulation than is apparent. Overcapping it with even 3" of blown cellulose fixes that problem, restores the effectiveness of the fiberglass layer, and adds another R10-R11 to the stackup. Installing an aluminized fabric radiant barrier at the rafters (leaving enough room at the top & bottom for air to convect through the rafter bays- necessary to keep the roof deck dry enough in winter) stops the IR radiation temperature stacking in the fiberglass too, but it doesn't add any R-value, and has a very poor price/performance characteristic if you have even R25 of fiber insulation at the attic floor.

    I'm pretty sure you don't mean Mbtuh, but rather you mean Kbtuh. With 15,100 BTU/hr of cooling capacity SHOULD be able to cool an 800 square foot partially sub-grade condo with only 20 square feet of south window and no west facing window twice over. If it's not keeping up, squeezing another 2400 marginal BTU/hr of cooling performance out of the chiller tower by up-sizing the compressor isn't the solution, even if it somehow magically crosses the threshold of finally keeping up. It's usually easier to kill 2400 BTU/hr (700 watts) of peak load, and you should be able to reduce it even MORE than that if you can figure out where the heat gains are really coming from.

  5. acman0926 | | #5

    I have been doing ac n heating for twenty years. To keep this simple, normally a maual j is a good start. I would have to say leave tonnage the same, the flow needs to be the same. Go with a two stage heat pump and it will spend the majority of its life on low speed( about 50-60 % capacity) therefore it will not short cycle and most geo thermal systems have a variable speed blower to solve the dehumidification issues. If it needs it, which it doesnt sound like it will. Most of your two stage equipment is only sized in 2,3,4,5 anyhow so there will be no 1.5 option. The large heat gain may be from the boiler being near you generating domestic hot water as well as heat in winter. Look into Waterfurnace Envision series.

  6. DavidAlex | | #6

    Dana, wow, yeah, I really do think 2-tons is grossly oversized for cooling. Thinking back to last summer, I remember it being too cold and breezy when the system was running, and stagnant and stale when it was not running. To answer your roof/ceiling question, I'm not on the top floor. There is conditioned space above my apartment. There are a three can lights and (ahem) some unpatched drywall from running a new circuit that I'm sure leaks some air. But I suspect you're right -- a 12000 btu system would easily handle the cooling load on all but the hottest days on which I am also baking a pizza and/or having 10 friends over. But will it handle the heating load? A Manual J calc from returned a heating load of about 8000 btiu but I don't really know how accurate that is. Then again, we've muddled through the last 6 weeks or so of pretty chilly weather with a barely-functioning heat pump, and only rarely has the thermostat dropped below 66 degrees. (I'm fairly certain the refrigerant has been totally gone for the last three weeks, and minimal heat coming through the vents is just waste heat from the compressor before it shuts itself off.)

    Charles, I get what you're saying, but I don't think any thought went into the spec for the original equipment, so I don't really see any reason to defer to the previous installer's preferences. My apartment is very compact, 800 square feet in two stories, with duct run lengths to match. I actually think the ~700 cfm flow rate on the current unit is hurting, not helping. It puts out so much cold air all at once that it never really dehumidifies to my liking. And in heating mode, the air is moving so quickly that even 110 degree air feels cold against your skin. I think a smaller blower would be preferred -- I want it to run for longer at each cycle.

  7. Dana1 | | #7

    For a partially sub-grade 800' condo with conditioned space both above and on two sides of it a heat load of 8000 BTU/hr @ +17F might even be on the high side. The only exterior surface subject to significant heat loss is part of the north and south side walls (the below-grade losses being much lower.)

    If a 2 ton GSHP unit isn't keeping up with the cooling load the operating efficiency has to be pretty atrocious. A 1-ton (or maybe even a 3/4 ton) mini-duct cassette type mini-split such as the Fujitsu 12RLFCD or Mitsubishi KD12NA (splitting the output between the 2 levels) should be able to keep up with both the heating & cooling loads of this place, unless you have truly unusual heat gains & losses that somehow can't be remedied.

    Of course there are all of the aesthetic & permission issues involved with installing the outdoor unit of a mini split in a condo situation. But it sounds like there may be some serious design flaws with the existing system.

    Before spending any more money on the GSHP system, hire an energy nerd or engineer (whose only product for sale is the accuracy of their math) to perform a Manual-J on the place. If there is any low-hanging fruit to be had (if by some strange factor it's more than 12,000 BTU/hr @ 91F , the 1% outside design temp for D.C.) you should be able to figure most of that out by the details in a full Manual-J report.

  8. charlie_sullivan | | #8

    If you have historical data on your monthly electric bills, back when the heat pump was working fine, you could correlate that with monthly heating degree day data and back out the ratio between your heating requirement and the capability of the 2-ton unit. That could be a more accurate way to size the new system than your

    We are at a funny point where you can buy great technology in air-source mini-splits at low cost, whereas the ground-source units are typically older technology and more expensive. The net result is that even though the water source fundamentally gives you a huge advantage, in practice the actual performance can be similar. It can end up that the main advantage of the ground-source system is the ability to supply hot water with a desuperheater, which nobody bothers with in a minisplit. Does your system have a desuperheater (does it help supply not hot water too?)

    But better technology in ground-source systems is available. Usually, the best technology is only available in the larger systems, which is unfortunate for you. I second the suggestion to look at Waterfurnace. Their highest-performance 7 series only comes in giant units, but their 5 series includes a 1-ton unit (that would probably work for you), a 1.5 ton (that would surely be big enough) and a 2 ton. For 1 ton, you don't get any good options, but at 1.5 ton you can opt for a variable-speed high efficiency "ECM" fan which would help you on the days when you the humidity is high but not the cooling load. And at 2 tons, you can opt for a "dual capacity" unit.

    The "low" mode on the 2-ton dual capacity is about the same as the capacity of the 1.5 ton. So you would end up with abou the same dehumidification performance with either of those, but the 2-ton running on low gives higher efficiency, so either of those choices would work about was well as far as comfort, and you could decide just based on whether the 20% improvement in efficiency is worth the extra cost. Just don't get the single-stage 2-ton, as that would give you worse comfort with no efficiency advantage.

    The other option would be to buy a 1-ton unit, and with the money you save, add a mini-split in addition. But you could wait and only do that if you find the 1-ton unit doesn't have enough capacity.

  9. Expert Member
    Dana Dorsett | | #9

    Report back in July with the update!

  10. DavidAlex | | #10

    I'm happy to report that the 15000 btu heat pump is working great and easily keeping up with the cooling load, even through one of the warmer late springs we've seen in DC. We're up to 20 days of at least 90 degree high temperatures. Normal is something like 12 YTD. I do believe the HP is dealing with the latent load better than than my old 24000 btu unit. The apartment does not ever feel stuffy between cycles, and I don't need to set the thermostat to an artificially low temperature just to keep the humidity down.

  11. charlie_sullivan | | #11

    Glad to hear it. Thanks for reporting back.

  12. Expert Member
    Dana Dorsett | | #12

    Success! Sometimes smaller really IS better, eh?

    Ovesized cooling equipment very often leads to cycles to short & intermittent for good latent cooling. Looks like you are one more data-point demonstrating the principle.

  13. DavidAlex | | #13

    Update for anyone still following. With our very warm December here in the east, it took awhile to get cold enough to adequately test the heat pump's heating performance, but this week has seen to nights get down close to the winter design temperature for the area. (17 degrees for DC.) The heat pump kept up perfectly fine, maintaining 70 on the thermostat without noticeably long run times. The only (minor) complaint I have is that the register temps are on the low side -- they only get to mid-80s. The 2-ton pump I had before would blow around 110 when it was functioning properly. On one hand, I would like the higher temps, but on the other hand, I believe the lower outlet temp is more energy efficient. The Carrier unit I have supposedly has different fan settings, but the manual does not give much information on how to reduce the fan speed.

  14. DavidAlex | | #14

    Oh yeah, one more data point for ignoring the rule of thumb about replacing old equipment with the same size: I recently found the paperwork for purchase and installation the old 2-ton unit. (It was only 6 years old when it failed -- but that's another story.) From what I can gather doing a little forensic HVAC analysis, the first guy who came out determined the previous HP was dead and spec'ed a 18000 btu replacement. When they came back to install it, they brought a 24000 btu unit. I don't know what happened in the intervening weeks, but I believe this casts additional doubt that much effort went into choosing 24000 as the "correct" size.

  15. Expert Member
    Dana Dorsett | | #15

    It's true that lower air temps is more efficient due to the lower temperature difference between the earth/water loop temp and the coil temp- it's a lower hill to climb.

    It's also true than anything under ~100F is less comfortable for humans, a common issue with air-output heat pumps of all types. (Ductless mini-splits are by-design set up for 110F+ exit air temps, which probably limits their absolute efficiency.) On new water-source & earth-source heat pump systems it's standard practice to put the output registers where they are less likely to be blowing directly at/on the human occupants.

  16. DavidAlex | | #16

    I've thought about calling Carrier to see if I can reduce the fan speed (thereby increasing outlet temp) but my solution, insofar as one is needed, is to lower the thermstat to a sweater-compatible temp. The sweater keeps me comfortable in the 85-degree drafts.

  17. DavidAlex | | #17

    For anyone still following along, I went with a standard one-stage Carrier WSHP in the 1.25-ton size. It was installed a couple days ago. I can't speak to its performance in extreme hot and cold (being that it's April) but it's much queiter than my old 2-ton unit. There were no cost savings among 1.25/1.5/2.0 ton sizes. I just wanted the smaller unit for its quieter fan and better ability to to handle the latent cooling load.

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