Repco Boiler Tankless Coil

[NickB84]

Hello all, this is my first post, so my apologies ahead of time....

I have a Repco GMT110 gas fired boiler for baseboard heating. I currently have a pressure issue with the boiler hovering around 30 psi due to relief valve holding it there. I believe I have narrowed down to a leak in the tankless coil, thanks to other threads here and elsewhere (ruled out expansion tank, don't have auto fill valve, aquastat is working and no high temp).

I would like to replace it, however, I have no manual, and I think Repco is out of business so long, I can't find anything online. I am considering switching to an indirect setup, but can't afford to switch right now financially or with the current weather, so I'm just looking to get back up and running quick and cheap.

The coil is a 6 bolt hole, 7 1/4 or 7 1/2 diameter (will take another measurement to confirm) round coil with the well for the aquastat as well as a port for the T/P gauge. I have seen both sizes online, but without the manual, I don't know the GPM of the current coil, which I'm guessing would drive the length and whether or not it would fit in my boiler. I also have not seen one that has both the well and the gauge port.

Any help is very much appreciated. Don't really want to keep running the boiler in this state, but don't want to pay a fortune to have someone replace a coil I can replace if I can find one.

Thanks,
Nick


Another quick thought, also considering abandoning the tankless coil altogether and installing a hot water heater instead. Since its leaking I would have to cap it I think. Considering adding a relief valve there too, just in case the leak became obstructed.

Think it would be nice to secure the boiler in the summer months, however, I've read that boilers may leak if they've never been cold.

Appreciate any thoughts on that as well.

 

[Dana]

If you intend to keep heating your house with a boiler (not necessarily THIS boiler, which is probably well past it's useful lifecycle, probably ridiculously oversized for the space heating load, and operating nowhere near it's original nameplate efficiency) installing an indirect fired water heater as a separate heating zone would be the better solution. Even if the boiler gets swapped out for something fresher (and more right-sized) the water heater wouldn't have to change.

Some cast iron boilers just never die, but that doesn't mean they should be kept in service. (The morbidity vs. mortality problem.) After 25 years of service erosion on both the fire side and water side of the heat exchanger plates have taken a toll on the achievable combustion efficiency, and after 50 years it's usually a very significant toll. A boiler that is 3x+ oversized for the primary load (= space heating) in order to deliver reasonable domestic hot water with a tankless coil never came anywhere near it's AFUE either, due to the higher standby losses from the higher minimum temperature needed for the tankless coil to function, and the extremely low duty cycle of the burner serving the space heating load. If the steady state combustion efficiency was 80% when it was shiny and new, at 3x oversizing and a standby temperature of 160F, it's as-used AFUE will be under 70% (and that's when it was NEW!). See Table 3 in this document.

Cutting and capping the connections to the tankless coil and running them to an indirect is the right approach. But whether it's a standalone water heater or an indirect, you can now safely turn down the low-limit on the boiler to 130-135F to reduce the standby & distribution losses. Even if the aquastat controls have a limited temperature differential between high/low limit, most houses have sufficient radiation to heat the place with 140F boiler output. If it turns out that running it that low doesn't keep up, or causes the boiler to short-cycle badly you can always bump it up 5-10F at a time until it works.

The lower the operating temperature, the more efficient the system will be (unless short-cycling due to insufficient radiation). As long as the entering water at the boiler is averaging 130F or more during a call for heat it's still pretty safe for both the boiler and the flue. If the return water is 125F or colder for extended burns the potential for corrosive condensation inside the boiler or in the flue goes up, so take a few measurements when monkeying with the settings. Measure the average burn times and the return water temperature during the middle of the burn to get a handle on any potential problems. (5 minute burns are fine, 2 minute burns are not.)

Then, start tracking wintertime fuel use against heating degree days to establish a firm upper bound for the actual heat load using the boiler as the measuring instrument. Even though it's old and the as-used efficiency isn't what it says anymore, use the nameplate efficiency. For a primer on how to do this type of quick & easy load calculation see this bit o' bloggery.

With the fuel use load number you can also estimate just how oversized the boiler is, but at the very least it will establish the right output range of any replacement boiler should be. If it's 3x oversized and 30+ years old, even a 1.2x oversizing factor from the calculated number would be sufficient to cover your heating load even through Polar Vortex type cold snaps.