Condensing boiler with an indirect tank

[Justin Keat]

I'd like to get a condensing boiler with and indirect tank. My contractor is suggesting the Laars Mascot LT 199 Combi boiler, saying he has all the parts in his van at all times, which is a pretty good reason to go that way.

This doesn't really make sense to me. The combi boiler has a minature 1-gallon indirect tank already. I called Laars, and the guy there agreed that you didn't want to use the mini indirect take as the input for a big indirect tank. I'm not sure what would be so bad about it, but he was pretty clear about it. He noted that that the calls to the hot water would cause a problem, as it would shut off the heat, but I'm not sure if that was the worst problem.

His suggestion was to use the heat-only version. I'm not sure how to make this work, though. The indirect tank will be supplied at maybe 170, while the radiant heat will be 135, probably modulating lower when it's warmer. How do I combine them? Do I have a shared primary loop that the internal pump runs? When the hot water call goes out, do I then rely on thermostatic valves to limit the other loops to 135?

Does anyone have thoughts about the right way to do this?

 

[Jadnashua]

There are at least several ways to do what you want. One would be to have the heating branch to be tapped off via a tempering valve. On most boilers, you can set up a primary zone, which would be for the indirect. WHen it calls for heat, the boiler would run hotter and shut off the other branch(s) until the indirect is up to temp.

Do not size the boiler larger than needed just to account for the indirect.

Also, do not just match the old boiler size as it's likely at least 2 and maybe 3-4x larger than needed and will short cycle, wear itself out faster, and be much less efficient. Long cycles are good. Numerous short ones are not. Ideally, on your coldest day of the year, the thing would run continuously, just meeting the needs of the house to keep it warm. That's not possible from a practical viewpoint, but your goal should be to optimize it.

IOW, don't match the current size. Do run a heat load analysis. Almost all installers will oversize what you need, costing more to buy and run while being less comfortable. You're most comfortable when the thing doesn't need to cycle much...just runs, and that won't happen when oversized b a lot.

Search a bit on the forum and you'll find a method to get a good idea of the sizing needed, or pay someone that will stake their reputation on the accuracy of the analysis. That will take into account local weather, the size, orientation, number of windows, amount of insulation, and air tightness of the building. A typical installer just wants to sell you something...not necessarily what you really need. Someone that suggests installing a system that doesn't follow the manufacturer's instructions doesn't sound like someone I'd want working on my home.

 

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[Justin Keat]

There are at least several ways to do what you want. One would be to have the heating branch to be tapped off via a tempering valve. On most boilers, you can set up a primary zone, which would be for the indirect. WHen it calls for heat, the boiler would run hotter and shut off the other branch(s) until the indirect is up to temp.

Do not size the boiler larger than needed just to account for the indirect.

Also, do not just match the old boiler size as it's likely at least 2 and maybe 3-4x larger than needed and will short cycle, wear itself out faster, and be much less efficient. Long cycles are good. Numerous short ones are not. Ideally, on your coldest day of the year, the thing would run continuously, just meeting the needs of the house to keep it warm. That's not possible from a practical viewpoint, but your goal should be to optimize it.

IOW, don't match the current size. Do run a heat load analysis. Almost all installers will oversize what you need, costing more to buy and run while being less comfortable. You're most comfortable when the thing doesn't need to cycle much...just runs, and that won't happen when oversized b a lot.

Search a bit on the forum and you'll find a method to get a good idea of the sizing needed, or pay someone that will stake their reputation on the accuracy of the analysis. That will take into account local weather, the size, orientation, number of windows, amount of insulation, and air tightness of the building. A typical installer just wants to sell you something...not necessarily what you really need. Someone that suggests installing a system that doesn't follow the manufacturer's instructions doesn't sound like someone I'd want working on my home.

The sizing question is an interesting one.

In the coldest month last year, I averaged a boiler output of 36k Btus. So the 199 is bigger than necessary for heat. I do want at a bare minimum of 100k, if not 140k, for hot water, though.

Condensing boilers are most efficient when operating at a fraction of their max output, so having extra power available is not a problem, and possibly an advantage. The cost savings for going smaller is pretty small, too.

So the question is whether the minimum power is right.

The Laars 199 will modulate down to 18k BTUs. The main house circuit will consume way more than that, even at, say 100F, so that's not a problem. The circuit that heat the kitchen extension is problematic. It will take 20k BTUs when it starts up and the temp differential is 55 degrees, but that load will drop. Also, that assumes the system temperature is at 135, and I'd like to modulate that down for the main loop. If it's on continuously, at 180 sq ft it's never going to radiate more than 6k Btus, so matching a boiler to that condition is hopeless. I'm not sure if this is a problem.

Unfortunately Laars doesn't make 10:1 turndown boilers smaller than 199. I could maybe go for an HTP UFT 140, which will go down to 13k BTUs. I'm not sure if even that will solve the kitchen problem.