Combi tank vs. Tankless Combi vs. Boiler + indirect? (DHW + basement radiant, house with forced air)

Pajamas

I’ll be replacing my Bradford White 50 gallon power vent combi tank (C-DW2-TW50T10FBN). I inherited this tank (a rental) with my house. I find it’s a little underpowered on the DHW side with the hot water reservoir sometimes running out. My work around for this has been to set thermostat higher and have the overly hot water mix with cold water at the thermostatic mixing valve, thus both extending how long the hot reservoir water lasts and raising the threshold at which recovery kicks in. This obviously works the unit harder, and perhaps has caused more scale (though, being a rental, I’ve never descaled it during it’s 7 year life). After coming back from a recent vacation during which I had shut off the water service and had turned the tank off, I found myself replacing a malfunctioning temp/pressure relief valve, and this showed a fair amount of scale.

House: Toronto (zone 6). 4.5 baths. Forced air throughout, basement supplemented with in floor radiant heating (1000 SF).

My priorities are durability/longevity and frequency/ease of maintenance (I‘m savvy and do most of the home’s upkeep). Efficiency and set up cost are less important (within reason).

I’m struggling to differentiate my best option. I'm inclined to just replace it with a similar combi tank for simplicity, unless I'm missing any important considerations?

1. Replace with a similar combi tank:
   1. Pros: My configuration is already set up for this, less expensive
   2. Cons: Cumbersome descaling compared with tankless? I assume filling draining the tank with vinegar mix is more labour intensive than flushing heat exchangers though I have never done either; Runs out of hot water (though I could upsize from my current 50 gallon tank)
2. Replace with a tankless combi
   1. Pros: Unlimited hot water
   2. Cons: Will require a new direct vent intake to be installed (unless a flexible polypropylene can be directed through the current insulated flexible duct installed to my mechanical room, terminating beside my powervent combi’s room air intake on the floor; Cold water sandwich and not ideal for low flow demand (example washing hands/doing dishes with lukewarm water) so I was wondering about installing a small electric buffer tank downstream to mitigate these.
3. Replace with a boiler and indirect
   1. This setup seems to be the least common around here for whatever reason, and I have been able to find the least information about it

Related question: My local water hardness is 6-7gpg. Most households do not have water softeners (though most households rent their equipment). With this hardness, and considering I will be owning my new equipment, should I be investing in a water softener? Will I just be trading maintenance on the water softeners (buying/replenishing salt) for maintenance on the water heater (periodic descaling), all while adding to installation expense and system complexity?

Many thanks

EBEBRATT-Ed

#3

GroundUp

What is the heat load of the space that the hydronics would need to overcome? Just that 1000SF basement, maybe 10k BTU at design? If that's the case, why not keep what you have? 7 years is nothing for a BW combi and if it's a rental anyway, why are you touching the mechanicals? Is your LL okay with this? I love a boiler and indirect, but with such a low heat load, I don't see the benefit. Absolutely would not consider a combi, though.

SuperTech

Boiler and indirect water heater is the best option in my opinion. Combis and tankless water heaters are higher maintenance and don't last as long in my experience.

EdTheHeaterMan

I assume that you own the rental property. You are the landlord? Are you living in one of the units? Is the other unit(s) vacant also when you are on vacation so you can turn off the heat and hot water? Or are you the tenant as @GroundUp asked? If so, then why are you doing this job?

EDIT: or this your home and only the equipment being rented? I have heard that is a thing in some places.

Either way… I vote for #3

Cyclist77

I am in the process of changing my coal stoker boiler that powers our radiant heat to a NG boiler and indirect DHW tank. Actually have a chimney fellow coming this morning to talk SS liner . Getting all the facts . Still deciding on the heating plant, ci boiler or condensing boiler.

But the indirect dhw is for sure.

clammy

Since your heating load is so low and low temp take a look at turbomaxx indirects . for myself and the few i ve installed they producing a decent amount of portable hot water and are quite easy to flush the coil if installed to do so . The real plus is that the tank holds boiler water volume and is equipped w supply and return tappings for drawing heat from and w a small heating loads especially radiant this would help keep your boiler from short cycling on it aquastat . There not cheap but they do last as long as installed properly w heating side expansion tank the extra water volume be taken into account . As in all things suggested manufactures yearly maintenance should be followed to ensure proper operation and safety but ya can always fall back on break and react .peace and good luck clammy

hot_rod

that is not very hard water, so the amount of scale we be related to the temperature, and the volume of water used. My vote would be a 75 gallon combicor tank.

I rarely hear of anyone using descaler chemicals in a tank type water heater. Draining them occasionally, maybe.

Maybe install a powered anode rod right from day one. Maybe you get 10 years or more with a better protected tank.

You could try one of the other water treatment methods, TAC or magnetic conditioners. Less maintenance and lp no salt to deal with.

Kaos

4.5 bath means there is probably a recirc already, so you have the expensive pump.

What I would do is a get a regular power vented tank (can get a big one with a 75k burner if you want more water) and install a plate heat exchanger for the space heat.

You can use the recirc to pump the plate HX, just set it to run continually in the winter time. Basically, pulling hot water off the top of the tank, to your fixtures, back and through the plate hx to the tank.

The other side of the plate hx would connect to the existing space heat loop.

Much cheaper to buy, standard bits and connects to existing vent, not much extra to install and simpler maintenance. Unlike a tank coil, with a set of isolation valves, the plate HX can be cleaned if needed. I haven't found that to be needed with Toronto water as it is still relatively soft.

If you don't already have a recirc setup, now would be a good excuse to install one.

hot_rod

with that quality of water, if you size a plate hx properly, the channel velocity does a good job of keeping them from scaling. Bigger isn’t always better with plate hx

Simple enough to install them with service valves

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