First of all you need to find out what the cost of each energy source is and capacity available at your site. As this will be different depending on where in NZ you are. Also is Piped Natural gas available? As it is often around the 6c per kW/hr mark. This means in some cases gas heating will be cheaper to operate than heatpump. And high output gas heaters are most likely cheaper to buy than heatpumps. A Rinnai infinity HD250 instant gas water heater outputs 59.9kW. Retail price for it is $1930+gst. How much does the 60kW output heatpump cost? And does it guarantee a COP of at least 3 to 1 when running at max output?
Presumabily you will also need to install some showers. Which means you will need a big hot water system to supply them. It will probably be more cost effective to have a large hot water cylinder (heated by natural gas) that will supply the showers. And supply "top up" hot water to the spas.
Also consider ventilation to your building. See if you could run the exhaust air through an air to water heatpump. And use the recovered heat to maintain temp in the spas.
You need to calculate how much it will cost to install each system type on the cheap. And calculate running costs for it. Then work out how much extra it will cost to make it more efficient. And how much you will save in running costs. So you can work out "payback periods". As there is no point spending heaps to save on energy. If your savings will just be used for paying money to the bank as interest.
How about this just focussing on the main heating system. Heat recovery will come later...
- 10,000L insulated storage tank containing 36 degree water. - 8x spa baths draw 1200L each from this tank. - Natural gas heaters located on each spa will bring the 36 degree water (which may have lost 10% in piping so now 32.4) up to desired temps(36,40,45 degrees) - The natural gas heaters continue to circulate the bath water locally maintaining the desired temp. - Once 1 hour session is complete then the water is pumped back to the storage tank through a natural gas heater located on the tank to bring the water, after piping heat loss, to 36 degrees. - Within the tank, a solar powered heat pump pool heater maintains the water at 36 degrees. - Ventilation pumps warm air from the spa rooms into the heat pump as well as the storage tank to fill the empty space when water is drawn from it.
Using hot water at those temperatures you need to consider Legionella control because hot water systems (particularly those in public facilities) and spa pools are common sources of Legionnaires disease. You could well have a perfect cocktail of multiple vulnerable systems - hot water storage, spa pools, mechanical ventilation and heat exchange - and the goal temperatures in the optimal range for the bacteria to breed.
bcraig: Anyone know of a 50kw water heater or series of heaters that can operate at 80L/min?
What temp rise do you need? As normally with pool heaters, they have so much flow going through them. That the inlet and outlet temps are not much different. Yet you still getting the full heat input to the water.
Either way definetly consider getting a condensing pool heater as they are more efficient than non condensing ones. And some condensing gas heaters have such a small amount of waste heat going out the flue pipe. They are fitted with plastic flue pipes.
bcraig: How about this just focussing on the main heating system. Heat recovery will come later...
- 10,000L insulated storage tank containing 36 degree water. - 8x spa baths draw 1200L each from this tank. - Natural gas heaters located on each spa will bring the 36 degree water (which may have lost 10% in piping so now 32.4) up to desired temps(36,40,45 degrees) - The natural gas heaters continue to circulate the bath water locally maintaining the desired temp. - Once 1 hour session is complete then the water is pumped back to the storage tank through a natural gas heater located on the tank to bring the water, after piping heat loss, to 36 degrees. - Within the tank, a solar powered heat pump pool heater maintains the water at 36 degrees. - Ventilation pumps warm air from the spa rooms into the heat pump as well as the storage tank to fill the empty space when water is drawn from it.
That sounds incredibly expensive. It would be far cheaper to heat the tank to say 45 degrees and use a mixing valve locally to temper the water down to your desired temp for each zone.
Personally I'd probably look to use a calorifier tank with a heat exchanger built in so you can use a closed system on the heating side and control the flow rate through the hot side to give a controlled tank temp. Alternatively you can use a condensing boiler to do the same thing.
You really should talk to a mechanical consultant, this is what they do for a living.
Try talking to the people that run the hot pools at Lake Tekapo. I believe they use heatpumps to heat their pools. They are a little different as they have a few large pools but they do keep them quite warm and may be able to give you a few ideas.
Also Where in the North island are you? (since you said you have access to Natural Gas you would have to be in the North Island) As I might be interested in doing a full design and build for the plumbing and gas for you.
But simple overview - Use your idea of a 10,000L storage tank. With spa overflows / drains going to tank. Water pumped through continuous flow condensing gas heater to spa. (such as a Rinnai infinity or similar) Heater only used for initial fill and for temp "topups". Each spa has it's own pump just for circulation / filtering. And each spa has a hot water feed and a non heated water feed that comes from main tank but without heating.
But then you will need someone else to design / build the control system. Which will need to control 7 pumps, 12 solenoid valves, 8 water level sensors. And have temperature displays for both users / staff. And controls for more / less temp, spa circulating pumps on / off, spa filling. I can write the "rules" for control. (if user input = A and sensor input = B then control output needs to = X) type of rules. But someone else will need to make a control system to carry out those rules.
And then there is still the water filtration / disinfection system to be designed. Lots of different options, both chemical and non chemical based. And lots of different advantages and dis advantages of each.
Aredwood: Also Where in the North island are you? (since you said you have access to Natural Gas you would have to be in the North Island) As I might be interested in doing a full design and build for the plumbing and gas for you.
But simple overview - Use your idea of a 10,000L storage tank. With spa overflows / drains going to tank. Water pumped through continuous flow condensing gas heater to spa. (such as a Rinnai infinity or similar) Heater only used for initial fill and for temp "topups". Each spa has it's own pump just for circulation / filtering. And each spa has a hot water feed and a non heated water feed that comes from main tank but without heating.
But then you will need someone else to design / build the control system. Which will need to control 7 pumps, 12 solenoid valves, 8 water level sensors. And have temperature displays for both users / staff. And controls for more / less temp, spa circulating pumps on / off, spa filling. I can write the "rules" for control. (if user input = A and sensor input = B then control output needs to = X) type of rules. But someone else will need to make a control system to carry out those rules.
And then there is still the water filtration / disinfection system to be designed. Lots of different options, both chemical and non chemical based. And lots of different advantages and dis advantages of each.
If you use a competent controls contractor you dont write rules like that. You write what you are trying to achieve and then let them write a functional description of how it will be achieved for you to approve.
It's a pretty small job and would suit either a small DDC controller or something a bit simpler like a mini PLC. Looks like around 50 points by the time you are done.
Budget depends on whether you need a front end and graphics or something a bit simpler. Putting a full front end, or using a DDC controller with a webserver on board, in adds significant cost but gives a much easier to use system and let's you manually operate each part of the system.
You wouldn't use solenoid valves, too expensive and useless for anything other than isolation valves. You just need to make sure you use valves that are rated for open systems as most control valves are designed for closed systems.
Fyi Raypak is locally represented, I've seen a few around on jobs. You'd probably use a wall hung condensing boiler or use a non condensing boiler with a heat exchanger.
It's too small for us to do but if you actually get a design done pm me and I can give you a budget cost and some potential people to talk to.
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