I've seen a couple of posts on this forum regarding tankless boilers and water heaters, but haven't found anyone that actually has the combo system themselves. We're considering a tankless boiler/tankless water heater combo setup to replace our original boiler and tank water heater. Specifically under consideration is a Rinnai 94 tankless water heater installed with a Navien "Heating Box" - closed loop, model KD-HBC1000. I'm most interested to hear about experiences (hot water satisfaction, maintenance, etc) unrelated to the price point. I'm aware that this setup can more expensive than two separate units.
I am also going thru the throes of investigating a similar system. With credits available, seems like a great time to incorporate.
Have recd a quote for a Takagi TK-3 that would supply the hot water for house needs as well as the radiant system. Would appreciate guidance or pitfalls with such a system.
I encourage both of you to use the search function of this forum for this subject. It's been discussed many times, which you'll find useful. Both Takagi and Rinnai are excellent brands, though I own neither.
I have an architect friend in Oakland who has been advising me on this matter. I have seen the system he retrofitted into his home. It is sweet. I have posted one of his emails to me here:
"The Munchkin can provide your potable hot water as well as the radiant heat by using and indirect water heater. It uses a heat exchanger to heat the potable water. We have that system and the hot water tank comes with a life time guarantee since it has not flame or moving parts and is made of stainless steel. For your house, I would go with the smallest Munchkin, it would be more than enough."
The company he used was Hydronic Specialties Company. Their website is: http://www.2hsc.com.
There is one issue that I can not find good guidance on. Pump sizing
My Bell & Gossett pump shows 1/6HP, 1725RPM, Single phase, 3.3A on the nameplate.
I understand that my manifold valve positions will effect the needed design as well as the overall nest/floorspace of the copper tubing.
What flow rate and pressure does a pump need to sustain to operate the radiant heating?
Is there a nominal value or design that works better with the tankless design?
Thanks for any guidance. Dave G.
The pump just moves the hot water around in the pipes. I don't know what pump size is needed but my understanding is you can use the old one that is used with the current system if it is still good (mine was just replace a few years ago.) I would ask the radiant heater installer. I do know that the pressure for an all copper system should be around 10-25psi and that is done with a pressure regulator connected to the outside water. You should have a meter on your system to check the pressure.
The Takagi TK-3 can be use for radiant heat as well as hot water but you need one for each. Takagi does sell a combo unit but that is a different model.
Anyways that is all I know and you should ask a radiant heater installer(s) that install tankless systems. The ones listed in the service section can help. I've had good work done by LaChance Heating.