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SleepingBear

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I have a question related to the radiant floor system I am installing.

Context: The previous owner of our homes (two located on one property 60 feet from one another other) had ½ PEX oxygenated lines run throughout the entirety each structure. Both homes are Insulated concrete form construction (12 inch exterior walls), slab foundation, and PEX embedded 4 inches in a polished concrete floor. The two homes are both circles. 1 is 50 feet in diameter the other is 30 with a small 150 foot addition. So home 1 is 2000 square feet and home 2 is 900 square feet . Sounds odd, but those are our new tanks of homes.

Unfortunately the radiant floor pipes were never hooked up. The family relied on parlor stoves to heat the two homes. Supposedly they worked so well that they never completed the radiant install. There is a pair of 1 inch PEX leading from the utility room in the big house to the utility space in the small house. The plumbers notes (the homes were constructed 8 years ago) say that each loop is no shorter than 200 and no loner than 300 linear feet though most are closer to 300. There are 11 ½” pairs in the big house and 6 ½” pairs in the small house, with one designated for snowmelt that is currently just a stub out. I pressure tested everything and it all checks out.

The previous owners left a brand new 199,999 BTU Takagi/T-H3-DV-NG to be used to heat both systems. I intend to use it and another tankless water heater for the domestic water supply in each home.

Question:

I am in the process of figuring out the system layout and am unsure of the appropriate pump size and installation locations. If I am to maintain .6 gpm and have a total of 16 300’ loops what pumps are recommended for this sort of configuration? Where would you recommend locating the pump or possibly two separate pumps for this system. Generally, I am concerned that the head from the 60’ run between the homes will force me to use a pump that may be too strong for the rest of the system.

Much appreciated
 

Dana

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Most systems like that would have to be plumbed and pumped primary/secondary, with an appropriate sized pump for each loop. The pumping head of the heat exchanger in the tankless could easily be as-high or higher than the pumping head of the radiant floor plumbing. The tankless can (and should) be operated at high delta-T and low flow, probably lower flow than than what the heating system needs. With unknown loop lengths and no way to calculate the pumping head of the radiation, this is a "design by hackery" project, but with 6-12 loops of half-inch PEX the pumping volumes will higher than you'd want to pound through a tankless water heater on a high duty cycle.

With an infra red camera a small pump, and a big tub of hot water you may be able to trace it all out to get a better handle on it, but it's probably not going to be worth the trouble. A standard fixed speed circulation pump such as a Taco 007 (not the -007e) or Grundfos UP15-42F drawing from a big tub of water driving the supply manifold, and timing how long it takes to fill a 5 gallon bucket at the return manifold will tell you everything you need to know, working backward from the manufacturer's pump curve.

Typical systems (that were actually designed rather than hacked) would be running at about 0.3-0.5 gpm per loop, so with 6 loops you'd be looking at something like 2-3 gpm, and with 12 loops 4-6 gpm. To not wear out flow sensors in a tankless on a regular basis it's best to keep it under 3 gpm. But for optimal efficiency the tankless (unlike a hydronic boiler) usually needs a delta-T of 40F or more, which you're probably not going to get without overheating the place if trying to pump direct. At 40F and 2gpm you're looking at 40,000 BTU/hr which the Takagi can comfortably deliver but 40K is probably more than 1.5x the design heat load of even the bigger house, which would be running higher volumes than that.

If starting from scratch, and meven right now, you'd be better off running Manual-J load calculations on each house before finalizing the radiant design. It's highly likely that the 99% design heat load of a round 900' ICF house is well below even the ~18,500 BTU/hr minimum-fire output of a T-H3-DV-NG, which means it basically never modulates. It's complete and utter overkill. You're probably better off using the T-H3-DV-NGs for the domestic hot water, and finding something more appropriate sized for the radiant slabs. Unfortunately the cheapie/freebie online Manual-J tools don't have ICF options in the pull down menus, so you may be stuck paying a professional engineer with the pro tools and some experience with ICF to run those numbers for you. It's not rocket science to get the load numbers right with ICF, but it's not 7th grade math either. (I wouldn't want to model it with an Xcel spreadsheet, even if it were MY house, when there are already well vetted tools out there.)

Being that it's not oxygen barrier PEX all fittings and pumps would have to be bronze or stainless or plastic, not cheaper iron/steel parts used in most hydronic systems, and the heat source would have to be oxygen tolerant as well.
 

SleepingBear

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Dana,
Thank you for the detailed and thoughtful response. To clarify the pipe that was used is coated with the oxygen barrier. Also the 900 sq foot home is nearly completely open. The bathroom is the only enclosed separated space. The 2000 sq foot home is broken up into 2 bedrooms, 2 baths and a giant kitchen/living room area making up 3/5's of the home.

I like the idea of calculating the length and will probably tackle that in the next couple weeks to confirm those plumbers notes. I hear your concern related to over taxing the Takagi and want to have the system as dialed in as possible to preserve the life of the flow sensors and heat exchangers. To reduce wear and create a simpler system, I am going to refrain from using pumps on individual loops and instead rely on a single pump for each of the two manifolds. Thermostat wire was run along side the 1" loop between the homes so I will be able to conveniently control the two properties separately. I will probably go with Grundfos alpha pumps for both to help modulate temperature and offer a more dialed in control mechanism. From what Ive been told by a local supply house this should help regulate the overuse of the heart exchanger. However that doesn't necessarily solve for the home temperature issue. I'll see if I can track down someone that has access to the pro tools so that I can better evaluate how the ICF structures will respond.

Takagi also offers the T-k4-IN-NG http://www.pexheat.com/T-K4-IN-NG?sc=7&category=206
It offers a minimum BTU of 11,000 and a max still of 190,000, so it should still have more than enough power. Any thought's on the machine?

As you wisely suggested, this would allow me to still use the big boy for the large house domestic water supply.

Also, I am planning on using the attached schematic as as cheat sheet for the layout. Any concerns there?

Thanks again for the insight.

https://www.radiantcompany.com/heatsources/ondemand/primarysecondary-plumbing/

primarysecondary-plumbing
 
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Dana

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it's O2 barrier PEX throughout, stop wasting your time on using a high heat water tube heat exchanger tankless as a boiler- use a fire-tube condensing boiler instead, which 95 times out of 20 can be pumped direct, due to the very low pumping head of the heat exchanger.

An all stainless Westinghouse WBRUNG-080W / HTP UFT-080W (same boiler different sales channel- the tech support line for Westinghouse even rings in HTP's office). At internet pricing they run about $1500 - $1700. There is a lot more design forgiveness than with any tankless, and it's designed to take it, probably outlasting a Takagi by more than 2x. That boiler modulates down to 7600 BTU/hr out in condensing mode, and can handle a very wide range of flows.

You still have to get a handle on the heat load, and you've offered next to zero detail. You don't say what R-value the ICF is, or how much of what type of insulation is on each side. You also don't say how many windows of what U-factor/type/size, how much insulation there is under the slab, how much fluff there is in the attic, etc, or even what your 99% outside design temp is, which makes it difficult to even put an upper bound on it. But let's give it a shot, using an I=B=R type calculation, which for an ICF house is going to overestimate by more than 15%, possibly more than 25%.

Assumptions:

1> The window/floor area is about 15%, and the windows are all U0.32 (single l0w-E argon)

2> Each house has two solid wood exterior doors (about U0.5), four door total.

3> Exterior wall height is 9'

4> The attic has R49 (current code min) or about U0.026.

5> The ICF is 2.25" + 2.25" of Type 2 EPS, R19 total for just the but with interor & exterior finish coatings comes in at U0.045 (steady state performance with no credit for mass effects.

6>The house is outside of Traverse City (99% design temp +2F) with a local design temp of 0F, indoor design temp is 68F, for a delta-T of 68F.

7> The houses are extremely air tight, and use heat recovery ventilation.

The basic calculation is:

U-factor x Area x Delta-T= BTU/hr

With the 50' diameter house there is 1963' of floor area and U0.026 ceiling, for ceiling losses of:

.....U0.026 x 1963' x 68F= 3471 BTU/hr

A 15% window/floor fraction makes for 0.15 x 1963= 294 square feet of U0.32 window for window losses of:

.....U0.32 x 294' x 68F= 6397 BTU/hr

With a pair of exterior 22 square foot U0.5 doors that's 44 square feet, and door losses of:

.....U0.5 x 44' x 68F= 1496 BTU/hr

The perimeter is 157', and 9' tall for 1413' of gross wall area, less 44' of door and 294' of window leaves 1075' of U0.045 wall for losses of:

..... U0.045 x 1075' x 68F= 3290 BTU/hr.

Add it all up and it's 14,654 BTU/hr, which is a whole lot higher than even the smaller Takagi's minimum firing rate, and it's an overstatement of reality in spite of not calculating in slab losses of the radiant through whatever foam you have down there.

Even at a design load of 14,654/1963= 7BTU/hr per square foot of radiant floor, even under the 0F outdoors design condition the floor will only be about 3F warmer than the average room temp. When it's above freezing out it'll be only about 1.5F warmer than room temp- barely noticeable if the slab is insulated (and it had better be!)

The losses of the smaller house will be of course smaller, but not exactly proportional due to having a higher surface area to floor area ratio, and the unknown construction of the 150' addition. But for yuks let's say it's proportional. With a 30' diameter the floor area of the house-proper is 707', plus 150' of addition makes 857'. If proportional the load of that zone is on the order of 14,654 x (857/1963)= 6385 BTU/hr.

So total load you're looking at about 21,000 BTU/hr, but the smaller zone's DESIGN load is barely more than half the minimum output of the smaller Takagi, and only 85% of the min-fire output of a UFT/WBRUNG-080W boiler. With the Takagi that means it's going to cycle on zone calls no matter what. But if under outdoor reset control (not an option with a tankless) the boiler would run very long, nearly continuous highly efficient burns even when it's only in the 40s F outside, even if the IBR calc is overshooting reality by 25% (which it might be.)

Go ahead and run your own IBR load numbers for each house/zone separately, using the actual window area & U-factors, and attic R etc. If you find a competent engineer to run the real load numbers we'll see just how much overshoot there is relative to your IBR calculation (not my WAG based on a bunch of assumptions.)

Either Grundfos Alphas or Taco Viridians would be appropriate for the application, but they aren't infinitely tune-able- you still have to pick the right ones. It's good to calculate the pumping head on the radiation and (if water -tube tankless) figure out what it's going to need for a primary pump. At 2gpm and a 20F temperature rise it already has the combined loads of both houses covered, so something between 1-2 gpm and would do it without over-pumping the hell out of the tankless.
 

SleepingBear

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Thanks for this. Nice guess on the geography, Im about 35 minutes west of Traverse.

Dana you've gone far above and beyond. If I was aware of your interest in the project I would have gladly sent you all the specs you assumed. That said, the tutorial you provide along side the J Manual calcs will put me in a position to make a much more informed decision on layout and hardware. Ill work on putting all the numbers together tomorrow and share what I find.

I have no specific notes on the insulation in the slab, just that insulation was installed. Both homes have an inordinate amount of large windows and glass doors. The small home actually has two pairs of large glass patio doors. The windows in both homes are almost all Anderson 400s, I'll have to look into their U rating tomorrow. Also, you might like to know/get a kick out of the fact that both homes have large light shafts that greatly aid in passive cooling. The small house has one that is 6 foot in diameter extending 10 feet above the 9.5 foot ceiling height. While the large home is outfitted with a square 8x8 shaped light chute extending 15 feet above the 9.5 foot ceilings. The top of each has 8 inoperable small rectangular windows only venting is in the attic space.

I'm in the process of finishing the renovation of the small house now. Ill send along some photos if your interested.

Thanks again
 

SleepingBear

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Dana, I ran the numbers this evening. You were remarkably close. Well done. The math on the large house is below. 10,810.1 BTU is lower than I expected. As you mentioned the small house is about proportional. I didn't attempt to assign any additional value to each homes light shaft - any ideas? I am having a tough time finding that info online.

After the math and some reconsideration I am going to go the boiler route. You listed all the reasons; its a 1000 USD higher investment on the front end that will pay dividends over its longer life. I've installed a number of tankless units for domestic water supplies but this will be my first boiler/radiant heat install. That said, Ive got a couple additional questions.

1. Although this boiler is intended to also be used for domestic water supply, I don't have the room to use it in conjunction with a tank. The utility room is small and Ive got a Fleck 600sxt/salt tank, 11 port manifold, well pump, filters, maines and all the under slab water lines in the space as is. Its going to be tight. Will NOT using a tank pose any issue for the boiler? If so, I I'll sell the new 199,00 BTU Takagi that came with the home... might have to build a loft of sorts to squeeze it all in with appropriate spacing.

2. I downloaded the primary/secondary installation schematic for the UFT/WBRUNG-080W and compiled a fittings list based on their system requirements. Would you mind taking a look at the following to see if I missed anything or if I simply misunderstood something?


Plumbing list for UFT/WBRUNG-080W install
Updated September 1 2019

1 - 3/4 Swing check valve https://www.supplyhouse.com/Bluefin-SCT075-3-4-Threaded-Swing-Check-Valve-Lead-Free
1 - 3/4 Pressure reducing valve https://www.supplyhouse.com/Pressure-Reducing-Valves-17082000
1- 3/4 Back flow preventer https://www.supplyhouse.com/Watts-0063192-BBFP-3-4-IPS-Backflow-Preventer
1 - 3/4 Y strainer https://www.supplyhouse.com/Everflow-100T034-NL-3-4-Bronze-Wye-Strainer-Lead-Free-Threaded
1 - 3/4 Differential bypass valve https://www.supplyhouse.com/Bell-Gossett-113247-3-4-Differential-Bypass-Valve
1 - RX-60 expansion tank https://www.supplyhouse.com/Amtrol-...ol-Expansion-Tank-103-Gallon-Volume-7364000-p
1 - 3/4 air separator https://www.supplyhouse.com/Webstone-74003-3-4-Air-Separator-Threaded
1 - 2 zone pump control
https://www.supplyhouse.com/Azel-Te...ller-Switching-Relay-with-Priority-Protection
4 - 3/4 boiler drain -gate https://www.supplyhouse.com/Bluefin-BDM075-3-4-MPT-Boiler-Drain-Male-Threaded-Lead-Free
6 - 3/4 ball valve https://www.supplyhouse.com/Bluefin-BVT075-3-4-Full-Port-Threaded-Ball-Valve-Lead-Free
1 - Swing check valve https://www.supplyhouse.com/Bluefin-SCT075-3-4-Threaded-Swing-Check-Valve-Lead-Free
1 - 3/4 spirogent air eliminator https://www.supplyhouse.com/Spirotherm-VJR075-3-4-Spirovent-Jr-Air-Eliminator-Threaded-3559000-p
1 - 1/4 Temp and pressure guage https://www.supplyhouse.com/Honeywe...-8-Face-Temperature-Pressure-Gauge-Tridicator
2 - Alpha2 15-55F cast iron https://www.supplyhouse.com/Grundfo...55-F-Cast-Iron-Circulator-Pump-w-Terminal-Box
1 - UP15 58 FC 3 Speed cast iron https://www.supplyhouse.com/Grundfo...ed-Circulator-Pump-1-25-HP-115-volt-4701000-p
1 - flex console 1 https://www.supplyhouse.com/Flamco-97975-Flexconsole-1-2-x-1-2-NPT-Steel-Red




Big house math

1> The window/floor area is about 14%, and the windows are all U0.26

2> Large house has three exterior doors. Back and side are 80 percent glass. Front is a sliding glass double door. Small house has three doors as well. Solid side door and two sliding glass patio doors. Those mostly glass doors were included in the window ratio count. So I’ve got one door in the small house at U.05 .

3> All interior/exterior wall heights in large and small house are 9' 10” Light shaft is noted above in earlier communication.

4> The attics are both at R40or about U0.025.

5> The ICF is 3" + 3" of Nudura https://www.nudura.com/divisions/nudura-products/icf-series

R24 before interior and exterior wall covering. 5/8 drywall on the interior and EPDM (last three feet before grade) and 1 inch or so of stucco on the exterior give us an extra R1. So total comes in at U0.04 (steady state performance with no credit for mass effects -> R50 as advertised.

6>The house is outside of Traverse City (99% design temp +2F) with a local design temp of 0F, indoor design temp is 68F, for a delta-T of 68F.

7> The homes are extremely air tight, and use heat recovery ventilation. – Yes from what I can tell.

The basic calculation is:

U-factor x Area x Delta-T= BTU/hr

With the 50' diameter house there is 1963' of floor area and U0.025 ceiling, for ceiling losses of:

.....U0.025 x 1963' x 68F= 3,337.1 BTU/hr

A 15% window/floor fraction makes for 0.15 x 1963= 219 square feet of U0.26 window for window losses of:

.....U0.26 x 219' x 68F= 3872 BTU/hr

With no exterior doors counted on the big house that's 0 square feet, and door losses of:

.....U0.5 x 0 x 68F= 0 BTU/hr

The perimeter is 157', and 9' 10 tall for 1543' of gross wall area, less 0 of door and 219' of window leaves 1075' of U0.04 wall for losses of:

..... U0.04 x 1324' x 68F= 3601 BTU/hr.

Thanks man
 

Dana

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It's perfectly fine to use the -080W without an indirect water heater. The plumbing and control features on that boiler make it dead-easy to use it with in indirect, but the presence or absence of the indirect has no bearing on how it works in space heating mode.

I'm not convinced the -080W needs to be plumbed primary/secondary on your system, but I'd be surprised if it does. Most systems don't- this boiler can handle a wide range of flows, and has fairly low pumping head. Try to get a better handle on your radiation flows and pumping head before making that decision. I'm not a hydronic system designer, and while I'm familiar enough to point out basic flaws and toward a general direction most likely to succeed, I'm reluctant do walk through a component list and swear that if properly assembled it would work at maximal efficiency & lowest electricity use. Design-by-web-forum has many ways to go astray.

There are a few competent hydronic designers who at least used to do design and design-reviews remotely via internet as a service, one of whom used to post here regularly here. The other has changed jobs in the past handful of years, but may still do that (either independently or through his current employer's books.)
 

Fitter30

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Both secondary circuits use factory made manifolds with flow meters.
Let see what your loads are for both houses. 260' of pex per loop 2990' @ 32 btu per ft.
1 gpm per 10k btu's for flow at 20° td

photostudio_1674938327582.png
 
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