Need help with radiant heat set up

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Richard Bradford

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I know half of you will say why are you doing this. Simply put, I am only able to spend so much per check to a new heating system. And I am fairly handy, So its worth the try.
I have a old house that froze last year (not because of me). 18 huge radiators cracked including the ancient boiler.
I have 2 fireplace that heated the house for 170 years and could do it today too but we are all less brutal then our forefathers. So I have installed 6 300ft loops of 1/2 tubing under the floor in the basement. all to a very nice manifold coming off a primary line. I have a Rannai 98 LSi tankless and a Taco bumble bee circulator. I also purchased a Taco SR503-4 switching relay. It will be a closed unit and I am satisfied if it takes the chill out of the floor and I still use the fireplaces until a future time when I can add to it and afford better ways to heat the home.
I have only 1 circulator that will feed the 6 loops. my question is when wiring the relay do I have to wire the Tankless to the relay and if so whats the best way. Or will my thermostat tell the circulator to move the water and the tankless automatically kick on? And do I need to have the temp sensors that came with the circulator installed on the feed and return lines of the manifold?
There are so many conflicting ways online, and I live in a small town that is hard to get good quality help. I appreciate any help you all can spare.
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Dana

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(Wading in with trepidation...)

The best thing you could have done is to install stainless liners in the fireplace flues and put in some pretty-good wood stoves or air-tight wood-burning inserts in the antique fireplaces. A pretty-good EPA rated wood stove would use 1/10th the amount of wood as an antique fireplace, and having the flues plugged with rock-wool insulation around the liners would reduce the heat load by a good fraction too, since those flues (even when unused) are sucking air 24/365, pulling cold air into your house.

There is so much un-kosher with the hydronic floor installation I'm a bit at a loss. If you're at the point where you are asking, "my question is when wiring the relay do I have to wire the Tankless to the relay and if so whats the best way. Or will my thermostat tell the circulator to move the water and the tankless automatically kick on?", it seems pretty unlikely that you have done even the napkin-math version of the design, independently of the plumbing-sculpture implementation. Hydronic heating is more than a plumbing exercise.

While it's possible to successfully use tankless water heaters as hydronic boilers, they aren't particularly well adapted to the application. But without doing the math on the whole thing and using a solid system architecture the likelihood of success is low.

A tankless is agnostic of all other aspects of the plumbing. All it "understands" is flow. To get it to fire up takes a minimum flow (something around 1 gpm typ.), but overpumping it will wear out the flow detectors very quickly. Plumbing the system primary/secondary with the primary (tankless) loop running something like 1.5-2.5 gpm would fire reliably without over-pumping it. A tankless heat exchanger can tolerate very high delta-Ts, so you can play around with the temperature a bit to get the firing rate reasonably dialed to the load. By separating the radiation flows from the tankless flow, the flows can be at very different rates. Pumping direct through the tankless with a fancy with a programmable pump, the flow rates will vary all over the place, and it's unlikely that it will start reliably and not over-pump, destroying the flow sensor & eroding the tankless heat exchanger from the inside out.

On low-mass burners like a tankless, always pump toward the tankless, making the cold-feed side of the tankless the highest pressure point on the system. Hang the expansion tank on the other side of the pump from the tankless to reduce cavitation issues and improve pump longevity.

To keep system like this from short-cycling it's probably better to use an electric HW tank (not wired up) as the point of hydraulic separation.

A simple control setup using an electric hot water heater as a buffer tank is to set up the zone relay with one zone to start the secondary (radiant) pump, and set up an aquastat in the electric tank as the "thermostat" on another zone to turn the primary (tankless) pump on & off. You can probably use the Taco Bumblebee as the secondary pump (even if on a single zoned system it's something of a waste.) You can either take a WAG on the amount of pumping head the tankless represents and spec a pump, or maybe Taco 0010-F3 3-speed can be used for the primary loop, setting it up at the lowest speed initially, as long as it's pumping enough to start the tankless (either way you may want to install a ball-valve to adjust the flow to a non-destructive range.) You will have to run the tankless 20-70 F hotter than you set the aquastat on the tank, so there are limitations to how high the radiation loop temps will be. If the floor PEX doesn't have heat exchanger plates (or at the very least, bolt on finned convectors such as UltraFin) the thing is pretty much doomed anyway.

Before you buy any more hardware, do the math. Start with a room-by-room heat load calculation based on realistic indoor & outdoor design temperatures. Then, when you know how many BTU per square foot it would take to cover even half your design heat load with whatever plate system (or suspended tube) you are going, we'll have a clue as to what your water temperatures need to be. My guess is that you probably have way too sophisticated a pump, and not nearly enough radiation to be useful for much. But as-implemented it looks like a complete rip-up and start over (you can probably leave the radiation loops hooked up to the manifold, the rest has to come out) to end up with something even remotely likely to work.
 
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