Conversion from oil to heat pump - need advice

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Oct 10, 2013
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Hi all,

I have an older 35K in-ground concrete pool in Bethesda, Maryland, and *all* of the pool equipment is in our main utility room in the lower level of the house. While this is very convenient when the weather is bad, I have nightmares involving 35,000 gallons of water in the rec room. We're often away on travel, which makes this more worrisome... so we have decided to move everything up and out to an area behind the pool, and modernize a few things while we're at it.

The pool heater is an old Lochinvar oil burner hot water heater that was converted from home hot water -- it was part of the original pool installation, and still works perfectly even though quite old. It's rated at 100,000 BTU and has a recovery rating of 125 gallons/hour (temperature raised 100F).

This is the very last oil burning device remaining in the home, and I would like to get rid of the oil tank entirely. We do not have a gas line and we are in the middle of 6 acres, so don't have that as an option. I could have a propane tank installed (big pain) or I could convert to a heat pump.

I like to keep the pool open well into November, and when the nights get cold I bring the pool up to 90 F -- which means the heater is running most of the time. However, it does manage to keep up and I've proven that it can maintain the temp up to 95 or so even when there is snowfall.

So here's my question: given that I am satisfied with the heating performance of my current ancient and obviously inefficient oil heater, will I be equally satisfied with the performance of a heat pump with the same BTU rating? Will I be switching the heat pump to "emergency heat" mode as soon as the weather gets cold, and if so, will that be a problem? The BTU rating on an oil burner heater is a deterministic indicator of exactly how much heat the thing will add to the water flowing through it, 24x7 x365, as long as I give it oil -- but a BTU rating on a heat pump can't possibly have the same meaning, because there have to be environmental dependencies. Basically I'd like to keep swimming in a toasty pool in the snow :)

Can anybody help me think about this and make a good choice of equipment? Thanks in advance!!

-- Craig
 
Using a heat pump to swim in Bethesda, Maryland during the winter, extremely unlikely. Heat pumps, as you mentioned, work off of the environment and if there is no warm in the air, no heat will be produced. So if you enjoy the heat you get from the old oiler, keep it in play.
 
"Keep the oil burner..."

Thanks, Paul.

I'm afraid that's what I'm going to hear from everybody :( -- but I have a lot of reasons to want to get rid of the oil burner, so I want to be sure I have investigated every other option first.

Our heat pumps sure keep our *house* toasty warm all winter long, and in the past 30 years we have only once or twice switched on the "emergency heat" mode that uses backup heating elements, and that was only for an hour or so. Because the specific heat of vaporization and condensation is so high, heat pumps can pump heat from one place to another against a pretty steep gradient. Our freezer uses a heat pump to pump heat out of the freezer and into the room, and it makes the floor toasty warm and that gives me hope ...

Looking at specifications, it seems that older air-source heat pumps mostly work only down to 40 F, but newer units work pretty well down to about 10 F. There is even one air-source heat pump (the Acadia from Hallowell) that is rated down to minus 30 F !!!

So I know an air-source heat pump will produce "some" heat for the pool even when the outside temperatures are low, but I just don't know how much and how fast.

Supposedly a "typical" air-source heat pump still produces 60% of its maximum heat when the environmental temperature gets down to 32 F, and at that point it has a coefficient of performance around 2 when heating domestic hot water at 150 F. So if a properly-sized heat pump system can produce *enough* heat when working against a gradient from 32F to 90F, then it should be a viable alternative.

So I guess maybe the real question is whether any reasonable commercial pool heat pump will be able to keep up with heat losses from the (covered) pool when working against that gradient. Since my old oil burner has a recovery rate of 125 Gal/hour (i.e., it can raise the temperature of 125 gallons by 100 degrees in an hour) and it is able to keep up, that seems to be a good target for a heat pump -- but I haven't found anybody who knows in a practical sense whether that is a reasonable expectation for a heat pump when ambient temperatures will be between 30 and 60 F. I've seen an all-in-one heat pump-type domestic hot water heater advertise a "first hour" rating of 80 gallons, but I bet that wouldn't hold up if the thing were set outside at 32 F :)

Perhaps a ground-source heat pump would also be an alternative -- from what I read, they have a better coefficient of performance and are not affected by freezing air temperatures... but I don't know anybody who has one installed for pool heating.

The problem is that sooner or later theory has to meet the cold hard light of day :)

-- Craig
 
Water is the hardest form to heat. That being said a air to water heat pump such as the Pentair Ultratemp will not exchange heat below 50 degrees. I have to many units out that shut down when temps get down there. A ground source heat pump is a different animal. I had a customer investigate this option and the cost of drilling enough wells to heat the pool he would be able to buy propane for thirty years. I am not an expert by any means just speaking from my own personal experience.
 
You won't be happy with a heat pump. It's not going to do what you want during the cold times of the year. Even here in FL heat pumps aren't that good and we don't get as cold as you folks up there.
 
!

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
 
Heat pumps has limitations. When it gets cooler out, btu output will decrease, and can form ice on the evaporator coil. Heat and Cool units will go into a reverse freon flow/defrost mode to melt the ice buildup off the evaporator coil, then switch back to heat mode (therefore can operate down into the 30's but at lower btus). Heat Pumps are also slow to heat, so spot heating a pool with a heat pump is probably not going to be satisfactory.

You cannot compare 100,000 BTUs from a gas/oil heater to 100,000 BTUs from a heat pump when it gets cold, because the output from the heat pump will be more affected than the gas/oil burner and you will lose more BTUs. Remember, you're burning oil to create the heat, whereas the heat pump is relying on the nature of the freon to generate the heat, so there are limits.

However, heat pumps do work within the manufacturers recommended temperature range just fine. 95 degrees would be considered much higher than a normal pool would be maintained during winter months, but 90 may not be out of the question. You may need two heat pumps to do the job though.

The use of a solar blanket (with any heating source) is always recommended.

With a large property, rather than drilling a well, and since you have 6 acres, you can use a ground coil system with a geothermal heatpump. However, this has to be designed and buried deep enough below the frost line, so as to get to a stable ground temperature not prone to freezing.... or a deep well source. In MD, an air source will struggle in the second half of Nov through the first half of April. The remainder of the year, it should be able to get you into the mid 80's. With higher BTU's, as temps drop, btus will drop also, but will continue to provide some heat. It just depends on your expectations. Heck, AquaCal makes a 500,000 BTU air source heat pump that may work for you, if you want to go that route!

Many customers have used a combination of the heat pump to heat a majority of the year, then use a gas/oil heater to supplement the colder months.

By the way, Heat Pumps work very well in Florida, except when there are extended cold fronts into the 30s, in which case you're most likely not going to go swimming anyway.
 
It really appears that you have done the research and explored the options extensively from reading your post.

If it were me, I'd work with the gas company and a contractor to get a sufficient gas line up to the house. Sometimes they will work with you if you sign a contract for a certain minimum usage for a certain time period (i.e. 3-5 years). Even if you spend $10K getting the line up there, you will eventually recover your investment and ultimately be way ahead. You currently know what you are using in oil and have a history, so you can prove to the gas company that you are a serious user. If the local district manager is any good, he or she will get an engineer from the gas company with you right away and survey your property.

Regarding the heat pump with resistive heat backup, that can certainly be done using two heaters. The heat pump will serve to pre-heat the water and the heater will speed up the rate and get the temperature up to the required comfort level. I don't see the controls for such a system being a big deal. The biggest problem here will be cost. Electric rates on the East cost are not cheap, although I don't know what your specific power company charges. You can be sure the rates will continue to climb substantially since all power companies have ongoing "green" initiatives mandated by the EPA. Many (if not most) power generation is being converted to natural gas, so think about it..... you might be able to get the same energy from your gas company without going through all the conversion to electric (through steam generators) and without all the delivery infrastructure (power transmission lines, transformers, switching stations, line protection, etc.). Of course, we know the power company gets their gas cheaper than you can, but the overhead to get electricity to you is multiple times their cost for the same energy.

One other thing you need to consider. You may need an upgrade to your electric service entrance and panel box..... plus the wiring to your heatpump and resistive heat furnace may be pretty extensive.

Just food for thought here.... :)

EDIT: I should probably add here that the ongoing maintenance a few years down the road will be much higher for the heat pump (and controllers) than it will be from a gas fired furnace.
 

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"EDIT: I should probably add here that the ongoing maintenance a few years down the road will be much higher for the heat pump (and controllers) than it will be from a gas fired furnace."

May I ask what this is based on? Other than a fan and compressor, there's very little to wear out in a heat pump. Titanium is used for the heat exchanger, which is much less prone to damage from water chemistry degradation that is copper or cupric-nickel gas heat exchangers.
 
Poolsean said:
"EDIT: I should probably add here that the ongoing maintenance a few years down the road will be much higher for the heat pump (and controllers) than it will be from a gas fired furnace."

May I ask what this is based on? Other than a fan and compressor, there's very little to wear out in a heat pump. Titanium is used for the heat exchanger, which is much less prone to damage from water chemistry degradation that is copper or cupric-nickel gas heat exchangers.
Click to expand...

Although they have improved a lot in recent years, much of the problems were control board and sensor issues. With the onset of on-board diagnostics, things have improved, but I still see a lot of technician labor and occasional shotgunning of replacement controls where it is questionable if they were ever bad. It is true that pool water heater heat pumps are a bit less complicated than home heating/cooling system heat pumps, but they still have issues. A straight forward gas or electric resistive heater is about as simple as it gets and if they ever do need repairing, it is pretty easy.
 
cfeied,

I think you have plenty of "qualified" folks here, whether they are in the industry like myself or thru their own person experiences, and they have provided you with real life knowledge. So now it's up to you to decide what you want to do. As the saying goes "You can lead a horse to water but you can't make him drink." No amount of talking here seems to be going to convince you out of a heat pump. I'm sorry but this sounds too much like the wranglings in Washington.
 
Poolsean said:
Other than a fan and compressor, there's very little to wear out in a heat pump. Titanium is used for the heat exchanger, which is much less prone to damage from water chemistry degradation that is copper or cupric-nickel gas heat exchangers.
Click to expand...

If it's a moving part, it will break. If you purchase a gas pool heater that doesn't have moving parts, it's less to even think about breaking down the road.

I also don't buy into the titanium theory either. I've personally seen plenty of these units destroyed by bad chemistry. They are NOT bullet proof, it just takes longer.
 
Poolsean and Hoosierrun -- your replies are super-helpful to me.

I'm taking the advice to push harder with the gas company and contractor -- great suggestions as to how to communicate with them.

And if I do end up with all electric, at least I have some confirmation that a big heat pump plus supplemental resistive system can get me most of the way back to what I have now. As it happens, I had my electric service massively upgraded when I bought the place, and the main panels are close to the pool area, so that at least is a plus.

ps303, it sounds as if you were having a bad day when you wrote one of your posts -- hope that's all better now! :) Seriously, your advice is much appreciated.

Thanks so much to everybody!!

-- Craig
 
A large supplemental solar hot water heating system could help take the work load off of the oil heater so it has to run less and use less fuel. It could be added to the roof of a building next to the pool to collect the large amounts of waste heat that hit the roof tops around the pool.
 
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