I have a customer who needs to redo his roof and he absolutely hates the look of his solar thermal system on the roof so he wants to get rid of it and will not install propane or electric resistance for the pool. We are already putting in a 4 ton geo system so we are looking at adding a 6 ton water to water heat pump so he can heat the pool may-october here in Portland Oregon.

The problem I am having is I can't find any sizing data to confirm that I am not over/under sized. The pool is 38,000 gallons and its on a hill so part of it is underground and part of it is above ground with a deck built around it. The WaterFurnace pool sizing calculator won't calculate for May and October because it only calculates months where the average temp is over 60 degrees outside, it also calculates at a water temp of 87 degrees and this person only wants 78-80 degree water temperature. I just don't want this to be undersized since there will be no backup and I have only done one other pool system and one spa, plus the waterfurnace program showed needing 83K BTU's and the output oft he NSW075 will only be about 65K BTU's based on the EWT that we see.

Pool heating can be tricky.The surface area of the pool needs to be factored in due to wind chill factors. The surface area becomes the greatest source for heat loss. Solar blankets definately will reduce overnight heat losses.The formula for determining btus for a pool is a complex equation. Most pool heaters are oversized to accomodate the constant changing weather conditions.The first thing I would determine is whether or not the existing solar was able to heat the pool to the homeowner's satisfaction.Then you could reverse engineer to find out just how many more btus are needed.

why not use the pool as the ground source?

a small cooling tower might be needed for the summer time but you can get the 30% tax credit on the pool itself.

Use the pool as the ground source? You can't do that, I need to HEAT the pool in May when its 54 degrees outside on average. Second a pool has very corrosive water that requires heat exchangers to sepearate it from the water source heat pump. Third, this house uses almost no cooling and if they did use lots of cooling it still would not be enough to heat the pool, they have a heat gain of about 46K BTU's and a loss of 42K BTU's when the pool sizing is recomending 83K BTU's. You can not use the pool as the ground source in many situations and this is certainly not one where it would even come close to working.

We use air sourced heat pumps like Aquacal since there is more heat in the air then there is in the ground in the pool season, specifically when your loop is cold coming off the heating season.
It performs with a much higher efficiency (again, the air is much warmer than the ground) and is much cheaper to install than geo.

Doc is right in nearly all circumstances - realistically almost no one swims outdoors with air temps much below 70. Florida energy code requires air source heat pump pool heaters operate at COP of 4.0 or higher at an ambient temperature of 50. It is tough to get geo to perform much better than that, and pool heaters are optimized for the task with large titanium (Chlorine resistant) heat exchangers allowing high flow - 5+ GPM per ton.

AHRI rates heat pump pool heaters and AHRI standard 1160 specifies rating parameters. Beware claims of very high COP such as 7+. These are based on unrealistically high ambient air temperatures. Applying the low temperature standard shows a COP range of 3.2 to 4.3, with the vast majority coming in a 4.0 so as to meet Florida minimum.

Sizing pool heaters is interesting - much depends on surface area and wind. A quick browse returned requirements ranging from 5-10 Btu / hr / SF / *F. For example, holding a modest 16x25 pool at 80 in 60 degree ambient air takes anywhere from 40-80 kBtuh. Gallonage is less important than surface area and wind.

An exception might be heating an indoor pool during winter - air source is not viable, but heating load should be much less since wind and evaporation are virtually zero.

Aqua Cal is a leading manufacturer, and they have water source units. I would lean toward that unless WF has products designed for pool heating (WF site shows just one air source offering, and I didn't see that model in AHRI)

WaterFurnace mentions pool heating in their NSW brochure, but there is a limit of 0.5 PPM Chlorine for the heat exchanger and most pools are maintained well above that, introducing risk of early failure or requirement for an intermediate heat exchanger - doesn't seem a good fit.

We would be using an intermediate either titanium or stainless steel heat exchanger if we do a geothermal pool heater. The pool is 20X40 and is always covered when not in use. WaterFurnace does make an air source heat pump water heater that is rated up to 150,000 BTU's has anybody ever used these? I have no experience with them and neither does my TM.

I'm sure WF makes good product, but I'd feel better about this one if they joined the other 161 AHRI-rated models.

Posted By engineer on 12 Nov 2012 08:43 PM
I'm sure WF makes good product, but I'd feel better about this one if they joined the other 161 AHRI-rated models.

I will have to do more research into this product because I assume they just rebrand it like they do with their humidifiers, EAC's and HRV systems. So I bet it is AHRI rated just not under their name. Yes I could buy it from another supplier but this is a WaterFurnace lead so I am not going to take work from them after they sent it to me, plus the customer really wants all WaterFurnace products.

From a heating cost, the previous posts about the air source heat pump system makes sense. I would add that if it can run on a timer during the afternoon when the ambient temp is highest that would be best.

Also, since the customer hates the look of solar thermal on the roof (as do most), consider some of the vertical evacuated tubes, like alongside the deck.

WF COP is about 4.2 if rated at AHRI conditions, Aquacal is about 6. Plus Wf does not make 150 KBTUs under AHRI conditions, and Aquacal runs amazingly quite, the pool circulation pump is louder!

Aquacal is no higher than 4.3 at AHRI "low temp" condition, where the work really gets done.

What is WF COP at AHRI low?

Correction: WF at Ahri "high" condition (80F EWT, 63% relative humidity, 80F ambient air) is 5.0, versus 6.1 for Aquacal. "Low" condition is 50F ambient air, no data published for "Low" WE