Combi Boiler w/ Domestic Recirc/Buffer Tank

rocky_mtn

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.

Ironman

Why don't you do and see how it works?

You could install 3 ball valves so that you could bypass the electric tank if it doesn't perform like you want.

A word of caution: you will be making a great legionella breeding machine!

rocky_mtn

Legionella because the storage tank is just for storage and not plugged in? The circ pump I'm using is a Grundfos UP10-16 A PM and it has a weekly disinfection function it runs automatically for 15 minutes regardless of what setting is selected. Would't that eliminate that worry? I really appreciate the response.

Ironman

It won't do a thing to kill legionella. You have to get the water temp above 130* to stop it from breeding and above 140* to kill it.

The original outbreak of legionella that killed the Legionaires was traced to the hot water re-circulation line.

There are several threads on here about legionella. Do a search and you'll learn a lot.

Any tank water heater that I install (gas, indirect, etc.) is set to 140* or above.

rocky_mtn

That's cool, I'll just plug it in, turn the thermostat up and mix it down leaving the storage tank. Like I said, it won't be running nearly as often with the brazed plate doing most the heavy lifting for heating the water. Thanks for the insight ironman!

njtommy

I would just do it. its not going to cost you much for the tank water to be heated from 120 to 140. My Navien combi is the same way slow to get DHW.

njtommy

This drawing shows what you want to do for the most part.

rocky_mtn

Right on. The Navien application drawing shows the circ teeing back into the cold feed. Picture attached. I think I'm going to just pull the trigger on this next weekend. I just hate having that storage tank in the crawlspace with piping just bypassing it, when I know it can cut delivery time drastically. My old recirc system was the Grundfos tee under the furthest fixture, so I have no dedicated return. That will be fixed too, I just need to determine where I should have it return to, the domestic side of the boiler, or just tie it back to the drain on the electric storage tank. Thanks for the insight guys.

LTsgarage

Rocky_mtn. Did you do the to your system? Did it work? I did a system kind of like yours several years back. Because of the Navien combo unit not having the recirc system built into it. I purchased 2 different units. One for radiant heat the other for domestic hot water with the recirc pump & buffer tank built in. The home already had the recirc line run throughout it.
I actually found in the long run. It was cheaper this way.
I removed a 100 gallon gas water tank and a 4 zone boiler system.

So. How did yours work out?
I think this is a year down the road??

rocky_mtn

> @LTsgarage said:
> Rocky_mtn. Did you do the to your system? Did it work? I did a system kind of like yours several years back. Because of the Navien combo unit not having the recirc system built into it. I purchased 2 different units. One for radiant heat the other for domestic hot water with the recirc pump & buffer tank built in. The home already had the recirc line run throughout it.
> I actually found in the long run. It was cheaper this way.
> I removed a 100 gallon gas water tank and a 4 zone boiler system.
>
> So. How did yours work out?
> I think this is a year down the road??

It worked like a champ. Put the bypass in over the storage tank to see if I like the way it performed, and if not, bypass it. Never needed to bypass it and I get hot water at every fixture within 5 seconds max. It's like every tap is always, at all times, 4 feet from the water heater. I did tie in a dedicated return, put the circ pump in that section of new return line, with a check in the pump and returned it to the cold water inlet on the domestic side of the combi. Also put a check upstream on the cold feed to the domestic inlet. One thing I did do, thanks to @Ironman was put a mixing valve after the storage tank. I vented the combi in schedule 80 CPVC to allow higher operating temps on the DHW side. Set the dip switches accordingly to run high temps and the DHW setpoint at 150° running to the storage tank, and mixed it down post storage tank to 130° feeding the houses taps. Works amazing and couldn't be happier. Let me know if you have any questions.

hot_rod

It's possible your fuel bill and electric bill will go up keeping the tank and recirc hot, maybe even more than the water bill increase? But I understand the frustration waiting for DHW. Was the wait due to ramp up of the heater, or the long run to the fixtures?

I see several of the tankless water heater brands now installing small, several gallon, DHW buffer tanks, & recirc from the unit.

A selling feature of combos and tankless is the lack of legionella breeding with no or little stored DHW.

Insulated recirc lines, of course.

rocky_mtn

> @hot rod_7 said:
> It's possible your fuel bill and electric bill will go up keeping the tank and recirc hot, maybe even more than the water bill increase? But I understand the frustration waiting for DHW. Was the wait due to ramp up of the heater, or the long run to the fixtures?
>
> I see several of the tankless water heater brands now installing small, several gallon, DHW buffer tanks, & recirc from the unit.
>
> A selling feature of combos and tankless is the lack of legionella breeding with no or little stored DHW.
>
> Insulated recirc lines, of course.

I did insulate the lines with 1/2" wall split foam insulation to minimize heat loss. Not only the recirc line, but the entire hot loop from the water heater all the way back to the check on the inlet/return. Ahh, the beauty of everything being accessible in a 5 foot crawlspace 😁. The wait for hot water was due to 2 things in my opinion, which you nailed both on the head. The ramp up and purging of the cooled lines, and the design of the original plumbing. Picture an 80' long house ranch style, 40' wide and the water lines circled the entire crawlspace in a "U" shape around the perimeter catching each fixture as it goes down the line. Delivery took a while, up to 65-70 seconds in the master bath (the end of the "U"). This is where I tapped in the dedicated return tee.
The electricity bill has still went down by a substantial amount, but I pay a lot per kw hour where I live in the Rocky Mountains of Colorado. Went from approaching $300 in the coldest winter months to $160 max so far with similarly fridgid temps outside. The water bill decrease was the least of my concerns when the wife was complaining about it taking over 60 seconds to warm water to the master...that took priority. Ha. I'm only sending 150° water to the storage tank 30' away from the boiler and it has a 38 gallon capacity. It's then mixed down to 130° to feed the house safely and returned to the domestic inlet to be reheated and sent back to the system. I find it doesn't circ nearly as often as I thought it might with this setup due to the aquastat in the pump and the temp sensor attached to the copper just before it re-enters the inlet stream. It kicks on a little more than 2 times an hour, but not quite 3. It's about every 25ish minutes with my setup. Which works nicely as my radiant doesn't even noticed the circ, even though it's disabling the space heating once it kicks on. The mass stays nice and warm at all times.

hot_rod

Sounds like you have a good solution.

As a side note, 1" wall insulation is now required in areas that have adopted the Energy Code