So I'm not getting much response in the Domestic Hot Water thread, so I apologize if its frowned upon posting this in 2 sections. I was just kind of hoping to tackle this soon if it's attainable. Here is my original posting in the Domestic Hot Water thread :
So I will start by stating that I work at a plumbing and heating wholesaler. April 2016 I was approached by our Navien sales rep about the possibility of installing a NCB-240E in my house that I bought back in 2014. The house was built in 1987 and had a long horizontal furnace from the 1987 era in the full size crawl space, and the gas fired 40 gal water heater had been abandoned for an electric 240v unit a year or so before I bought the house. This sounded like a good option as I'm the hydronic salesman at my employer, and Navien was willing to give me the unit at half off wholesale cost. By the time the tubing rep gave me 8 rolls of oxygen barrier tubing, all I really had to buy was the uponor heat transfer plates and the near boiler piping, pump, expansion tank, air seperator...etc. I got a rebate from the gas company and was able to also claim some deductions on my federal tax return essentially paying for the boiler. The heat transfer plates, even at wholesale cost, was the most expensive component. Navien makes a darn decent product, but it would not have been my combi of choice, if the deal wasn't so enticing. My electric bill dropped 65% once I unplugged the electric dual element 38 gal water heater which was awesome, I think I'm on my way to paying those transfer plates off. What I'd like to accomplish is not having to wait forever for my hot water delivery as I did have a Grundfos smart recirc pump on the electric water heater which worked nicely for close to instant tap delivery. What I'm considering, since my water bill has now gone up waiting a full 2 plus minutes for hot water if it has't been in use recently, is re-installing the Grundfos pump. I mounted the combi in my garage, with a hydronic Modine unit heater and the electric 38 gallon is still in the crawler with piping running right by it now bypassing the tank. I've asked numerous plumbers and heating guys around my shop, reps etc...and they all say don't waste my time or effort on doing this. I was thinking of piping the domestic hot out of the combi down to the electric water heater (unplugged if possible) as a buffer tank and installing the circ pump and running a dedicated return back to the combi cold inlet. I would then insulate all the tubing/copper (hybrid system) with split seal insulation. Does anyone care to chime in on this? If I need to plug the electric water heater back in I can, I'd just prefer not to. It wouldn't be working nearly as hard as it was previously as the brazed plate in the Navien would be doing all the heavy lifting with regards to heating. The 38 gallon electric like I said is already there in the crawler, with piping passing right by it, and it just seems every time I ask someone if I should consider using it for storage or buffer, everyone just looks at me cross-eyed saying it defeats the purpose of the tankless...which I do understand their argument, but I've also seen tankless manufacturers "spec" application drawings with a storage tank plumbed inline previous to their unit to "pre-heat" the water therefore lowering the temperature rise. Anyhow, I'd appreciate the opinions of anyone who would care to chime in.