New combi boiler setup

kratznick

Hey all, non-plumber here but a contractor and architect who is a big plumbing nerd. I've got a new combi boiler I'm installing at my full gut renovation. Question is can I pressurize the system and get just the DHW side working? That may vary by model I guess -- Bosch Singular 5200 is what I have.

I assumed I could hook up gas, CW in, DHW out with exp tank etc etc, then supply/ return in a short "loop" so I can fill and pressurize the system in order for DHW to work. I'll have isolation valves and unions everywhere so can service and actually finish the radiant side once I have time / money this summer.

Temporary solution to have hot water working while we finish the renovation, since I have backup heat.

Thanks for any help!

GGross

Yes you would build out your primary loop and where your system will connect you can valve off. You will still need all the normal safety devices installed that would be required for a completed install.

Ironman

If you hook up the necessary ancillary components, then yes, it should work.

However, I would highly recommend that you get a heating boiler and an indirect. Combi’s will almost always be greatly over-sized for space heating and short cycle to death. The 3 way valves in them have a very high failure rate and the plate heat exchanger can require descaling frequently depending upon the hardness of your water.

kratznick

So act like I'm going to set the whole system up -- exp tanks, circ pump, dirt mag, check valve etc and then just valve off to make a short loop where it would go out to the rest of the house.

I hear you on the indirect . . . unfortunately I'm really tight on space which is why I went with a combi anyways. I'm aware of the extra maintenance, maybe I'll get creative and try to add an indirect in the attic directly above . . . that poses its own issues.

Thanks for the suggestions.

GGross

Depending on how you pipe it yes. You do not NEED the secondary circ or dirt mag technically for your temporary install, but if it makes sense to install them now then you can install them yes, don't power the secondary circ until the piping is complete.

attached is an example from your Bosch manual, if you had an isolator flange for the secondary circ you could use that to close off the system, and close off the purge valve. Since your heating system will not be operating yet you dont need to loop together your heating side piping. The boiler needs its own loop to function however, and needs to be able to reference the expansion tank, those are the most important bits.

Ironman

A combi isn’t setup for an indirect: it senses flow to switch to domestic.

You may be able to come up with a creative way to make it heat an indirect from the space heating loop while overriding the ODR, but why go that way if you can avoid it?

A combi will probably have about 1/2 to 2/3 the life of a heating only + an indirect.

kratznick

So the loop can "dead end" without going to going back into the return at the boiler, as long as that can be isolated. Then don't power that circuit -- forgive me, like inside the boiler board or with the zone valve?

I am also doing a super simple single zone system but have thermostatic valves at the radiators (buderus panels) -- so that probably helps simplify this a bit. Sounds like there's not a problem just getting everything setup and then strategically placing valves.

kratznick

Oh-- related question. This Bosch unit has a built in circulator pump, and at least according to them it would be sufficient for a single loop system. Is that a bad idea for durability and should I just use a normal external circ pump instead?

GGross

@kratznick

You need to use the internal pump for the boiler loop. Personally I would pipe this boiler primary/secondary, with either a low loss header or closely spaced tees (the diagram shows primary/secondary with close tees) This method of piping requires an external system pump to provide flow for your space heating zones.

If you know the pressure drop and required flow rates of your system, you can add the pressure drop of your boiler into the mix and plot that down onto your boilers internal pump's curve to see if it will be sufficient. It quite possibly is, but without the info I would just pipe primary secondary to avoid issues.

pecmsg

What are the emitters rated for? If above 140 you’ll never see high efficiency!

kratznick

Ironman said:

A combi isn’t setup for an indirect: it senses flow to switch to domestic.

You may be able to come up with a creative way to make it heat an indirect from the space heating loop while overriding the ODR, but why go that way if you can avoid it?

A combi will probably have about 1/2 to 2/3 the life of a heating only + an indirect.

Well I've already purchased, opened, and bolted this one to the wall so I'll revisit when it's time to replace in hopefully 10 years if I service it correctly. Thanks for the advice though.