you most likely want to install the tubing in plates. either lightweight or heavy plates, and if lightweight we recommend PAP tubing, not PEX, to reduce expansion noise issues.

I'm not sure I understand what you are doing though. are you replacing the baseboard or just finishing the existing install?

i am planning to put water something (backer board or that kind of material and i cant remember the name but prevents backer board from water leaks)

Schluter -DITRA or -KERDI?

I sure would like to see a picture of your mechanical room. I would not jump to the conclusion that a tankless is a better choice. The system you have may work fine if you had someone out there that knew what they were looking at. What you are describing , there are still systems which go in today utilizing a central boiler and multiple heat exchangers. The key here is pump and heat exchanger efficiencies along with the boiler efficiency.

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It is easy to replace an expansion tank, once you drain the water out of the part of the system the expansion tank is attached to. Hopefully you have drains and valves so you can isolate one section without having to drain the entire system. Once you have drained it, you should be able to unscrew the current tank and screw on the new one. Adjust the expansion tank pressure to your system pressure before you install it. After it is installed, refill, and purge the air out of that section.
-Rosalinda

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The only "zone" that has any significant thermal mass to it is the pool and the DHW (assuming it's a indirect tank? Didn't see on in the pix.). Without a significant sized buffer tank even the smallest low-mass Mighty Therm is going to short-cycle like crazy on zone calls when the pool isn't calling for heat. What are the BTU or MBH ratings on the nameplate? This looks and sounds like a real hack/kludge of a system, or that it's not been fully explained. I didn't see evidence of a heating system buffer in the pictures, nor an indirect, no tanks (other than expansion tanks) whatsoever, which just isn't right for a micro-zoned system.

The pool is very high mass, and it's peak load may be an order of magnitude higher than the whole-house peak load, and could easily be literally 100x the average heat load of the smallest heating zone. Running a micro-zoned heating system with a monsterously oversized (for the space heating load) pool heater would be difficult do efficiently- it's cheaper/better to run a separate boilers for loads so dramatically different in thermal mass & magnitude.

The temp you'd need to run baseboards may or may not be compatible with radiant. It depends on the actual design loads and baseboard length/type in those zones. If they're fin-tube and you're willing to pay a huge premium for the comfort of radiant floor in those zones (or a smaller premium for replacing them with panel radiators) go ahead and scrap them, but do the heat-loss math first (you'll have to do it anyway if you want a system that really works.) If the existing baseboards can deliver design day heat at 140F or less, you might consider keeping them.

Most decently insulated reasonably sized homes in WA have design-day heat loads of under 75MBH, but you need to do the room-by-room and whole house heat loss calc to properly size the heating plant. A tankless (almost surely oversized for the space heating load) or even a right-sized mod-con will also likely short-cycle into low efficiency and early mortality on an unbuffered 8-zone system.

The right thing to do is to have a competent hydronic designer decipher the existing system layout for you, then propose the necessary changes to make it work at optimal efficiency (which will probably involve separating the pool heating function and putting in a smaller boiler and a buffer tank for the rest), then put out the pieces that are not a DIY type project for bid. Putting even 144MBH (the smallest Laars M.T.- it's probably even worse) of low-mass boiler output into low-mass zones with design day loads of 2-20 MBH isn't good (or even bad) design- it's brain-dead, NON-design. This project is way to messy to attempt a "design by webforum" approach too, which is rarely optimal even on simpler systems (but can sometimes at least work.)

In the meantime, buy a cheapie heat-loss calculator like HVAC Calc (http://www.hvaccomputer.com/) or download an OK freebie like the Slant Fin tool (http://www.pvsullivan.com/Downloads.html ) and enter in the real dimensions, construction & window type & size, outdoor design temp, etc. to get a handle on the room by room, zone by zone heat loads. It'll give you a much better insight on whether a pro's design constraints or heat load is anything like reality. Most of these overestimate reality by 20% or more (the SlantFin tool overestimates by 30%+, in my limited experience), so if somebody is selling you on a boiler or heat-load number that's ANY bigger than what your DIY heat load calc came up with, press them to explain exactly why. It's not uncommon for contractors to do a sloppy rush job of it entering best-guesstimates with some padding "just to be sure", and end up 2x or more over what a careful calc would have come with, and 3x over the real heat load. Anybody you PAY to come up with a design should have good numbers and be able to back them up, but heating & plumbing guys submitting bids may not have the time to take all of the necessary measurements & care, and err to the high side rather than risk the 3AM call from a cold & irate customer, but that's to your detriment.

Nicely put Dana,

Now that we all know how to replace a expansion tank, I wonder how big it is? What the design temperature of the system is? What the volume of the system is? Why did it fail?

When faced with existing equipment that is too big for the job at hand (this happens to the Novice and the Professional for lack of specific training, experience and sometimes the use of "free" on line heat load programs) I will use a buffer tank. If the appliance heats domestic hot water, as appears to be the case here) I will draw heat through a plate HX and drive my radiation with outdoor reset.

If you don't have an accurate heat load for every room, you don't know what your doing.

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I'm not sure what the attraction is for a tankless combi here. In a microzoned it's a LOUSY way to go, since it too will be way oversized for any single zone and also short-cycle itself to death on zone calls if you don't buffer it. (And for the record, this is coming from someone who is currently using a tankless as a boiler, and reasonably happy with it! Call it a hobby... ;-) ) Either use a tank-type combi (which is inherently self-buffering) or system architecture that uses a buffer tank as a high-mass hydraulic separator between the boiler loop and the zone flows.

If it's all inside of conditioned space, there's no upside to adding vapor barriers, only downsides. (Where do you think the water vapor is coming from or going to?) In cool climate residential construction vapor barrier's primary function is to keep humid conditioned space air from allowing moisture to diffuse through interior walls/ceilings and condense on susceptible materials in a much colder part of the assembly (like OSB exterior sheathing & studs in a studwall), or under basement slabs to keep ground moisture from permeating into conditioned space uninvited. They have no relevance to heating systems. Randomly placing vapor barrier materials inside of assemblies increases the risk of moisture traps & rot. Construction assemblies are best designed with defined drying paths to make them resiliant to incidental moisture intrusions, while limiting (but not necessarily absolutely blocking) vapor diffusion in directions that may be seasonally problematic from a condensation point of view.

The radiant system supplies more heat at lower temps if you put the tubing as close to the finish floor as you can. Any backerboard you put between the tubing & heat distribution plates adds R-value between the room you want to heat and the source of the heat. I suppose if your backerboard was something as refractory as aluminum or copper plate it wouldn't much matter though. :-)

Seriously- take the class, do the math (or hire somebody who did/will.) Design by web-forum isn't going to cut it, nor is "design" by hacking on the plumbing until it "works, kinda".

It's cheaper to pay somebody to design it right before you've wandered so far wrong road that you end up paying extra rip a lot of stuff up and start over. It's possible, you're already headed down that road if you haven't done the math on your radiant tubing layout.

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Tell me; can you tell when someone is speaking a language, not their native tongue?

Your outside design temperature based on personal memory?

As Dana says; Seriously.

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You missed my point and are missing the lesson. The language is hydronics and you can't get the answers you need without knowing the question.

First you start with a proper heat load analysis, performed by a qualified designer, experienced in radiant floor heating. If your job is incompatible for radiant floor he will be able to direct you elsewhere.

If you start with a DIY heat load you are lost from the start. Multiple zones of disparate loads are the most challenging, as they will affect comfort, efficiency and reliability.

I would start by finding a contractor that "volunteers" a heat load on receipt of signed contract and can show you a sample of his own computer generated heat load analysis. All the big manufacturers have one and the smart guys use one of them.