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I much appreciate the practical feedback from folks who have experience with heat pumps. I totally believe what you are saying -- heat pumps are problematic in colder environments, and in your experience heat pumps do not deliver what I'm looking for. A real heater would be way better.
Unfortunately, there are impediments to a real heater in the new location where my equipment will have to live, so I am still working through all the issues. I would appreciate it if anybody could validate my current thinking on this or explain to me why I am crazy
When I bought my home I inherited a very old below-ground 15,000 gallon oil tank. Since the only thing burning oil is my pool heater, I just get it topped off once a year -- so from a nuisance POV it's pretty darn good. I really wish I could leave well enough alone, but I *have* to get all the pool equipment out of the lower level of my home, and the ancient Lochinvar oil heater won't live outside & wouldn't survive the move. Everybody I've called says that nobody makes an oil-fired pool heater any more. And I'd have to install a new oil tank anyway, because of construction affecting the original location.
The natural gas folks aren't very interested in supplying my site (difficult trenching, with the house 1500 feet back from the main road, up a rocky wooded hill and through the woods). It's possible that with the right contractor I might be able to get a 2-stage system run all the way down to the road, in which case there is a chance they would bring a gas line in for me. I haven't got that worked out yet, though, and I've heard only negative responses to date.
Propane has been suggested, and I do already have a modest size propane tank for my backup generator -- but calculations suggest that I would require frequent refills for the largest tank they could get within 600 feet of my pool (best case would likely require weekly refills) and the 600 feet of propane line would also be a bit of a problem. A 1000 gallon tank would last 10 days if I ran a 400K BTU heater at full burn, or about a month if I ran at 100K BTU/hr average -- but those things are really monsters and it would be quite difficult to find a place for it without clearing forest and doing a lot of excavation to grade.
However, I have *plenty* of electrical power available right where it's needed.
SOooo... I've been doing more research, and it really seems like the central fact is that BTU/hr are BTU/hr no matter what system is delivering them. If I can actually deliver the required BTU/hr around the clock during the months I want to keep the pool open, I will be fine, and otherwise not. To calculate my "real" BTU requirements, I figure that my current oil burner setup is rated at 100K BTU/hr but is very old and likely running at about 70% efficiency. It runs 24x7 when first heating the pool, and takes more than a week if I open the pool in March but only 3 days if I open it in May or June. In November (when the pool is already hot but the air is cold) it's running about 70% of the time. I am OK with all of this, which would suggest that my actual "rubber vs road" needs could be met if I could deliver an honest 70K or 80K BTU/hr into the pool, so long as I could deliver that 24x7 from April to November.
And there's the rub: credible experienced people tell me that when the weather cools off, heat pumps will not deliver the BTU/hr that I would need.
Since I don't yet have a good alternative, I've looked more deeply at heat pumps in hopes of understanding where the issues lie. I found several reasons why people often don't get good results with pool heat pumps.
1. Heat pumps are often selected in order to reduce operating costs, and as I look around it appears that most people are focused on saving money up front as well. Those who list their pool sizes and heat pump models online have nearly always installed smaller heat pumps than would be needed to achieve good performance in my area. Admittedly the price difference can be pretty significant for the bigger units!
2. The largest pool heat pump currently available is only vendor-rated at 150 BTU/hr with ambient air at 80F, where 400K BTU/hr gas heaters can easily be found -- so there is much less available margin for heating to begin with, even before the heat delivery ability is degraded with colder air temperatures.
3. Heat pumps that are "heat-only" are designed to stop operating when the temperature gets a bit below 50 F because they will frost up if operated much colder than that. The *average* November temperature in Bethesda, MD is about 57 F, but of course there are many times when it's well below 50F, so that would be a problem for sure. Fortunately, heat pumps that are "HC" rated (both heating and cooling) can work at much lower temperatures, because they sense the low side temperature and just run backwards for a short defrost cycle when needed. This class of heat pump (like the residential units I have heating my home) can continue to deliver heat when the ambient temperature is as low as 10 F, though their heat delivery capacity is severely degraded as the temperature gradient gets larger.
4. The BTU delivery of heat pumps falls off steeply as the air temperature becomes colder. In trying to quantify this, I found a great site:
http://www.ahridirectory.org/AHRIDirect ... earch.aspx
These guys run an independent testing laboratory that tests and certifies thousands of air conditioning, heating, and refrigeration units, including most of the commercially available pool heat pumps. They measure the actual BTU delivered both at 80F and at 50F, and the database is searchable. I did a quick search for pool heat pumps that can deliver more than 90,000 BTU/h at cold air temperatures of 50F. It turns out there are actually a number of pool heat pumps that can push 90 BTU/h when the outside temperature is 50F, and there's even one (tested at 170K BTU when the ambient temp is 80F) that tested at 107 BTU with air temperature of 50F. That thing is a monster: the Raypak PS10353TI-E-HC.
http://www.raypak.com/product.aspx?id=E ... 7A1A43FB67
It appears to be aimed at hotels or apartment buildings, and it's not cheap, but then *none* of my options are cheap. The test results only show the BTU rating at 80F and 50F, but the carnot curves suggest another 40% loss of capacity in the next 15 degrees of temperature change, so that would be around 54 BTU with an air temperature of 35F. OK, that's clearly not enough to maintain the temperature of a hot 35,000 gal pool, but it's only 15 BTU short of what I believe is needed. Maybe a second-stage resistive heater could make that up if push comes to shove. But for the most part, with average Bethesda temperatures of 57 F for the month of November it sort of seems like I should be right where I need to be. In fact, an honest (tested-certified) 105 BTU/hr is probably 30% more than I am able to deliver today with my oil burner.
When I was looking at this particular model I found that RayPak has an estimation tool that knows seasonal temperatures for many cities and will show you during which months they think you will be able to maintain a specified target temperature, given a series of inputs to address equation variables (which model heat pump, how much of the time the pool is covered, wind speed, etc.).
They don't have Bethesda, MD, but they do have Baltimore, and their calculator shows that this unit should be sufficient to achieve my temperatures from April through November. Bethesda is typically 2 degrees warmer and a bit more humid than Baltimore, both of which go in my favor.
Of course the coefficient of performance falls off as the temperature gradient widens and the ambient temp gets lower. This raises the cost of heating, but frankly here in Bethesda even if I got down to a COP of 1 (equivalent to pure resistive electrical heating) my costs per BTU would still be slightly less than for propane or fuel oil, so this is not the most important factor in my personal calculation. As it happens, the tested COP for the units I was considering remains at 4 even at a temperature of 50F, so it seems like if the tested values are borne out in a real-world installation, I will have noticeably lower operating costs compared with oil or propane, which is a bonus.
I looked in vain for a hybrid unit with internal resistive heating to assist the heat pump. Residential DHW heat pumps all have these because the heat pumps are underpowered for quick recovery (physical size being the critical factor) -- but I didn't find any pool heat pumps that had backup heating. However, I did find resistive pool heaters that presumably could be plumbed into the circuit for backup, if the control issues could be worked out.
So that's where I am: I have mostly convinced myself that I will be able to get most of what I want with a heat pump of sufficient size that it still delivers enough BTU/hr even when the weather turns nippy.
Now I'm anxious to see whether folks with real knowledge and experience think there are reasons why this is still a bad idea
Thanks!
-- Craig