Efficient Boiler for Small Heat Load and Multiple Zone Types

[Alan R]

Gentlemen,

I have been reading this form for a number of months and have always been impressed by the depth of knowledge and wide perspective. I am a general contractor installing hot water heat in my own home. I have installed five hot water systems over the years. However, my current project has some particulars that I would appreciate your suggestions on how to best resolve.

The house is under construction, 2800 sf, highly insulated, with a max heat loss at Seattle design temperature (22*F) of only 25,000 BTU/hr. I was planning on using three different heat emitters. A 450 sf slab in the daylight basement with 1/2" pex at 12"c; 400 sf of pex under plywood floor 8" oc with light gauge stamped aluminum diffusers and tile above; and two zones of HW baseboard main floor 32 lineal feet and bedrooms 32 lineal feet.

My question is what boiler and DHW heat source would you recommend?

I have considered combi boilers with built in heat exchangers however, I need to have capacity operate two showers simultaneously. That would bump me up to a Rinnai E110CN, Bosch Greenstar Combi 151, Triangle Tube Challenger Combi 125, or Navien CH 180. All of these are way over capacity for my heating needs. Perhaps a smaller combi with an electric hot water heater as holding tank for reserve capacity?

The alternative would be to get an indirect DHW tank zoned off a smaller boiler. I would like to get the high efficientcy of a modulating and condensing boiler. However, I have been told my hot water baseboard will not work well at 140*F and I will need a higher temperatures to induce convection. That would leave me with a 150*F and higher boiler output based on an out door temperature reset and a mixing valve for stepped down supply to the in floor heat. However, I would loose some of the efficientcy of condensing boiler with the higher output. What might be ideal would be a boiler that could alter supply temperatures depending on the zone calling for heat. Weil McLain's Ultra 80 has two temperature output.

[jadnashua]

To reach maximum heat output, the baseboard needs to be fairly hot, but during most days, it could take the chill off at lower temperatures, especially when the house is well insulated and has no drafts. The in-floor radiant bits would likely need different temps, too, so you may need three different ones. The in-slab stuff would likely use the lowest, followed by the floor radiant, then the baseboard. Each zone, individually, is likely less than the lowest modulated output of the smallest boiler. I would not consider a combi, as you'd be keeping the thing hot all year, not a great idea. I don't have any personal experience with the Ultra, but when I was looking several years ago, the installer said he would not install one, since he'd had too many problems with them. This is one person's opinion, and I cannot say if they've made any changes, or it was valid in the first place. I did end up with a Buderus unit, and have been quite happy with it.

On most boilers, where the indirect is set up as a priority zone, it alters the boiler output temp for the task. You can also achieve different temperatures with a mixing valve, as long as the output is at least as high as the highest value needed. These modular boilers have all sorts of options, controlled by plug-in boards to do nearly anything you might want. It can get expensive, but it's hopefully a one time expense. One Viessman unit I looked at had an external mixing tank that could have the temperature automatically set to whatever was needed. So, depending on how complicated you want to get, you can exactly match your needs.

A condensing boiler's return water temp is a big part of how efficient it is, if you can match the outlet needs, and the return water's temp is cool enough, it can reach its max efficiency.

Someone with a lot more experience than I should be able to help with some specifics.

[Dana]

You can run with fin tube baseboard reliably down to ~120F- whoever told you 140F wouldn't work is just plain wrong. Below 120F the density of the dust-kittens affects convective output, and it's hard to design for. With fin tube you can count on ~200-225BTU/foot @ 120F AWT, and ~300-350BTU/foot @ 140F.

But an any house I intended to LIVE in low-temp convective panel radiators or radiant-baseboard such as the Runtal, or even Burnham Base-Ray/Weil-McLain-Snug cast-iron baseboard) would provide higher comfort and reliable output even at temps below 100F. While convection may be iffy at low temp, they still radiate just fine- it's not a fully linear response, but it's still pretty good- far more predicatable than 100F into fin-tube convectors.

With a room-by-room heat loss any heating contractor worth their salt could spec the room-by-room heat emitters needed to deliver the heat with 120F water, and you could make it a single-temp micro-zoned (or bigger zoned) system for both the baseboard and low-mass radiant floor running off a condensing HW heater such as the Polaris, Vertex, or Vertex-100.The Vertex are pre-engineered with side ports for combi-applications, but don't have internal heat exchangers. The slab's temp requirement would almost certainly be at least 20F colder, but a thermostatic mixing valve could handle that. If you're willing to pay for the extra comfort of outdoor reset/continuous flow, there are outdoor reset controlled mixer valves out there, but I'm not sure it's really called for.

By using a HW heater as the heat source it's inherently buffered, and can't short-cycle no matter how small the zone/heat-load, and there are NO modulating burners (mod con or combi) out there that will modulate below even the design-condition heat load of the smallest zone, let alone the zone's average heat load. With the slab that's not a problem- the slab has sufficient mass, but the low-mass radiant and baseboard (if fin-tube) zones could be iffy. If it's coming in at 25K for whole-house load at 22F, the smallest zone has to be under 11K (nothing goes that low), and even the whole house load is under 11K at 45F outdoors.

How big is the biggest bathtub you'd need to fill? (Thats the only reason to consider a big-burner tankless type combi here.)