Help/Tips on combi unit install

Jer74

Good Morning

I am located on the west coast of Canada and have a 1200 Sq/ft garage with a 1000 Sq/ft apartment above that has hot water heat. The system was installed approx 20 years ago by the previous owner and uses a Rinnai RL94 tankless DHW and a heat exchanger for three zones (shop floor, apartment above, and a 400 sq/ft separate shop floor which will be deleted) and also provides the DHW. It is a goofy system and does not work very well in the winter so our tenants are using electric heaters as a supplement. The other hitch is that the floor and in wall pipe is poly b, which I am a bit nervous about since that was installed in 1984 with a proper boiler system before they switched to the Rinnai (which is not a combi unit to be clear).

I am hoping to get some advice from you experts as I am thinking of doing this myself and want to be sure I am on the right track. My thoughts are to use a Navian NFC-H series unit as they seem to be the most popular in our area and if its anything like the Rinnai I will need parts eventually.

1- Will this unit do a decent job?

2- Should I replace the poly b to avoid using a heat exchanger (which I have replaced 3 times in 10 years due to corrosion and bad heat transfer) and delete the shop in floor heating in favour of some type of wall mounted air handler system? The apartment is a two bedroom with baseboard style units 5 in total.

3- will the internal pump that comes with the Navian handle the demand described above, or would I also need and external pump?

I am sure there will be more questions along the way, but for now that will get me started. I will also be putting together a parts list so any thing you guys may have to offer in that regard would be much appreciated.

Thanks in advance for your advice and expertise.
Jer

hot_rod

If it is an easy task, get rid of the PB. If not, flush the system after running a cleaner, then add a hydronic conditioner, Rhomar, Fernox, etc. That will keep corrosion at bay. But it needs an occasional check and inhibitor boost. The test kits will tell you when to add more boost chemical.

Most all mod cons benefit from a hydraulic sep, or primary secondary piping. You want good flow through the boiler under any zone condition.
Is it zones with valves or circulators currently? I prefer zone valves with an ECM delta P circulator. Fig 7-4c, shows the concept of variable speed circulation.

Jer74

Thanks Hot Rod

The PB wont be easy as it runs in finished walls and in the concrete floor of the shop. I could delete the in floor heat and go with air handler, and then cut some drywall, but if adding a conditioner works on 40 plus year old PB then that may be best. With the conditioner do I still need a heat exchanger? Currently its a plate style unit that seems to plug with corrosion every 3 years or so.

Currently the zones have valves controlled by the thermostat's in each zone, zone 1 = apartment zone 2 =shop, and zone 3 = rear shop that doesn't really need heat so I may delete to keep things simple. The system actually had a driveway loop also if you can believe it but I deleted that as the line in the driveway cracked, I cant believe the guy expected the Rinnai to handle it all. 3 pumps are running the zones now and the way it works is thermostat sends signal to an electronic switch/relay that turns pump on and the Rinnai (I believe) senses the water flow and turns on.

As far as head loss, the upstairs baseboards are less than 10 feet above the unit, and the shop floor is below. My understanding is that the Navian has an internal pump and ability to run three zones. In my simple brain, I was thinking the 2 thermostats could signal the Navian and zone valve to allow flow eliminating the need for another pump or two. Not against the cost of pumps, just hoping for the simplest design possible as the current system makes my eyes hurt everytime I look at it. If I can figure out how to post a picture I will, and wait for the internet to blow up with comments.....its ugly.

hot_rod

Was the HX to run glycol due to the driveway loops, or just to separate all the PB? Did the boiler or system side sludge up?
Either with or without the HX I would use a hydronic conditioner.

Look up the piping suggestions in the manual, this model only shows the primary secondary piping.

If it is a combi, the onboard pump is mainly for the onboard DHW heat exchanger. A zoned system may not be ideal for direct piping.

Head loss in a hydronic system refers to the piping friction loss, not the elevation of the heat emitters.

Lift head is a term used for sizing open sump pumps, and refers to how high the vertical lift is. Similar term, different meanings.

Big Ed_4

I would use the boiler for heat and a heat exchanger for hot water .

EBEBRATT-Ed

I would stay away from Navien

Jer74

Thanks Big Ed_4

Not 100% familiar with this concept but I will try to gauge my understanding.

Boiler runs heating loops for both heating zones, and then a Plate style heat exchanger for the DHW, is it as simple as it sounds? few questions.

1- I get the basic heating, but what tells the boiler to turn on for the DHW? is there a flow sensor on the inlet to the heat exchanger that triggers the hot side to flow?

2- Are the heat exchangers efficient enough to transfer the heat for DHW? I assume yes since you suggested it, but my plate style seems to loose a lot of heat across it.

3- I was told that the poly B I have does not love hi temperatures above 120f? Is that true, and if so would 120 be enough to run DHW through a HX?

If this is do able I like the idea as it seems like a much more basic system than a Combi..... at least to a lay person such as myself.


Thanks
Jer

Jer74

Thanks Hot Rod

The system actually had 2 HX's, 1 for the glycol driveway loop, and the second for heating which I believe was to keep the poly B from the actual Rinnai unit. I was told the PB is oxidative? and can cause corrosion in the unit? The system side I believe was corroded which was hindering its ability to transfer heat. I cant remember the exact numbers but it was below the suggested heat transfer that i looked up for that style HX by a lot.

Thanks for the lesson on head loss, do I understand correctly that excess use of 90 degree fittings contribute to this calculation, similar to venting?

Appreciate your knowledge
Jer

hot_rod

Yes the PB tube can allow a lot of oxygen into the system. The oxygen oxidizes or corrodes any ferrous metals it comes in contact with.
You might consider a stainless body pump, Amtrol radiant expansion tank and all copper, brass or plastic components on the PS side of the HX. As it will be a reoccurring problem

The two options are all in ferrous components or keep adding oxygen scavenger chemicals.

The non ferrous route is a onetime fix.