I have an indoor pool (approx 12,000 gals) and want radiant floor heat in the master suite (approx 550 sq ft). We are installing a gas fired combi unit to heat both. My thinking is that the Pool heat exchanger would operate as an indirect water heater on the DHW side of the boiler and the radiant would run off the other side. My mechanic is advising to setup the system as 2 zones on the radiant heat side. This makes no sense to me as the water temperatures required are so different. I am fine with the pool taking priority away from the radiant for the heating cycles required. Any thoughts?

Typically, an indirect DHW water heater is the highest priority heating zone to ensure that you get DHW when you want it. However, you wouldn’t want the pool heating zone to prevent the house from being properly heated. So the pool should be a lower priority heating zone and you also need to properly size the heat source for both the pool and house heat loss (e.g., ACCA Manual J8 or ASHRAE analysis). Personally, I would want a dedicated and independent heat source for the pool.

As I said, I'm fine with the pool heater being the highest priority. Manual J is done and I believe we are sized correctly. The real question is how to pipe the combi boiler. The pool heat cycle would be 3 hours from 12-3pm and 3 hours 12-3am, daily. It would appear that this will be more than enough time for pool heating to maintain at 82 degrees, and the appropriate times for the master suite to lose temperature. It would seem that 2 zones off the radiant side and nothing off the DHW is the wrong way to pipe that combi boiler.

Well, the right way in my opinion, would be to have dedicated and independent heat sources for the DHW, pool and HR system. This way each heat source can be optimally sized for best efficiency and you never have a situation where you are operating a grossly oversized heat source to generate only a small amount of required heat gain. Besides inefficiency, this can also cause heat source short cycling which can significantly reduce the life of the heat source. If the heat source has modulation, that can go a long ways to mitigating this issue. So would a buffer tank, but this is best avoided whenever possible. However, without knowing the details of your systems, I can’t credibly speak to how well what you propose will work. I am wondering why the company that designed your systems can’t resolve this for you?

This project is a renovation. The wing with the pool and master suite were on a dedicated oil fired boiler (that froze and cracked) and a dedicated (way over sized LP gas pool heater) that has since failed. The rest of the house is currently operating with an oil fired boiler heating radiant baseboard and an indirect water heater for the whole house. We are phasing the renovation to complete the wing then transition the rest of the house from oil to gas fired radiant floors and indirect DHW. There would then be 2 separate gas boilers - 1 with 1 zone heat and the pool heater - the other with 4 zones heat and DHW. I believe the systems are sized correctly but the question remains - How to pipe the first combi boiler. The issue comes up because the contractor is overwhelmed with work and has not done a great job of managing his workers in the field. I believe some decisions were left to his mechanic and we now have a situation that would require some redoing of new piping if my point of view is correct. He is not denying the plan at this point but is vaguely defending the mechanics decision. Thanks for your input.