New Pool - Using Boiler + Heat Exchanger. Sizing Suggestions?

[tuffcalc]

Hi all,

Building a new house with a pool. The pool is fairly small, 10 x 20 feet, approximately 6,500 gallons, inground. I would like the pool to be around 85f. Live in Toronto Canada.

Originally specified a Pentair ETi400 (400k BTU - way oversized but would heat up fast) pool heater, but it looks like the mechanical room doesn't have enough physical space to include it.

We've now decided to use the same boiler that will be used for the home infloor heating to save space. It's a Lochinvar Knight WHN086 85,000BTU boiler at 95% efficiency (so approximately 80,000 BTU output). http://lochinvar.com/_linefiles/WHN-05.pdf . The boiler will feed a 400,000 BTU Bowman pool heat exchanger (it's a CN400Ti - http://www.poolweb.net/Brochures/CN-TI_Brochure.pdf .

I'm hesitant to upsize the boiler as some of the infloor heating zones in the home are very small, and on mild days I'm worried it will cycle too much. I've lived with uncomfortable oversized heating equipment and I never want to do it again.

I was wondering if the 85k BTU boiler might be undersized for this application. I've tried a bunch of online pool calculations and it either seems to work or is marginal. Would love some real world experience.

Thanks for everyone's help!

 

[Swampwoman]

I can't help much, but maybe a little

Your pool is about 1/4 of my 24,000 gallons. Mine, using a natural gas dedicated pool heater, takes 266 btu to get one degree an hour at about 80% efficiency (so requires 200,000 btu an hour to get one degree.)

Yours will require about 50,000 btu to increase one degree an hour, which will take approx 60,000 btu to generate at 80-ish percent efficiency.

So in theory, notwithstanding your in floor requirements, 85k btu would have you covered.

However, the heat transfer of the pool exchanger will drop the temp of the circulating water (eg about a 50 percent differential between the two as I believe radiant boilers run at 180 degree water). This may create a constant demand that is artificial -- which im guessing would be handled by zoning on your boiler.

I guess the question to ask the hvac guy is what kind of effect he'd expect if you're heating the pool on a chilly autumn day while heating the house while taking a hot shower or doing laundry -- would you have enough overhead in terms of btu? My completely uninformed hunch is that you might be a bit shy on overhead in this type of demand situation, but I truly do not know what kind of loss you'll have in the transfer.

You may have a trade off in pool heat speed or home heating efficiency, but on the other hand it ay make perfect sense. Hopefully someone will come along with direct hvac and transfer setup experience

 

[CJadamec]

With your condensing boiler and small sized loops do not increase the size of your boiler. You will have short cycle issues.

You would have your pool zone setup the same way as you would a indirect domestic hot water heater. Use of a priority zone setup would insure your house got heat over the pool.

Honestly though if you have the space on the wall for a second boiler dedicated to the pool that would be the best way to go rather than overwork your home heating boiler trying to heat up a pool.

 

[tuffcalc]

Thank you both. It sounds like it will work.

There is already a redundant forced air HVAC system being put in the home (need ductwork for AC). On those edge scenarios where the boiler is overworked - the furnace can keep up. We are heating water separately with two Rinnai tankless units. Hence my hesitation to get a separate dedicated boiler or pool heater.

We are setting up the pool as "Domestic Hot Water Priority". The way the Lochnivar Knight boiler works - if there is a heat call from the pool the heating loops in the house shut off for 30 minutes (or whatever time we program up to 55 minutes), then switch to the radiant loops for 15 minutes, then back ... and so forth.

So my main concern is whether or not the boiler, on its own, can bring the pool up to temp and keep it there. Sounds like it will work especially with Swampwoman's real world example.

 

[Swampwoman]

If you have an aux heat source for the house and you're not feeding the hot water from the boiler, then it should work from the standpoint of adequate btu for a degree an hour providing same is not lost in transfer, which is the part I'm unsure about.

Be sure to plan on a good pool cover to maintain your temp and achieve set heat.
Depending where you are in Canada, it would not be uncommon to loose 6 degrees a day overnight which means six hours of run time to get it back

 

[ps0303]

Is this setup standard for Canada?

The heater, unless because of some code up there or personal preference, doesn't need to be inside. You could pipe it to the outside.

 

[Swampwoman]

Paul, i don't think its exclusively a Canadian thing but there does tend to be a healthy number of boiler installers familiar with indirect pool heat north of 40 As an ex-pat, I've not yet found a similar plethora of expertise in my current snowy state for some reason, but that might just be because back home my friend teaches boiler work at a local college so I'd hear about such things more

I have seen a couple of posters discussing indirect exchangers who I think may have been from Canada, but I can't recall their handles to share with our poster.

Tuffcalc, have you tried a search in TFP with "indirect pool heat" or "titanium heat exchangers"?

 

[tuffcalc]

Is this setup standard for Canada?

The heater, unless because of some code up there or personal preference, doesn't need to be inside. You could pipe it to the outside.

I prefer it to be inside so the back yard is quiet. Since the boiler is only used in the winter for radiant heat - makes sense for us to repurpose it as a pool heater in the summer.

 

[ps0303]

I prefer it to be inside so the back yard is quiet. Since the boiler is only used in the winter for radiant heat - makes sense for us to repurpose it as a pool heater in the summer.

OK... Just some back info, if you get a Raypak/Rheem gas pool heater, they are very quiet as compared to other brands. Raypak/Rheem do not have a blower motor and are just a natural draft unit.

Good luck with the boiler option. We don't have those here in this area.