(Please Help) Low Flow Radiant system with a Tankless water heater

AaronHoop

I have a Takagi T-H3-DV-N 10-Gallon Per Minute, 199,000 BTU tankless system with a 8 loop manifold (only using 7 loops). Each loop is under 250ft pex (some much shorter). for the pump I have a Grundfos 59896155 UP15-42F SuperBrute Recirculating Pump (1/25 HP)

My main issue (i think) is that I am only getting 0.7 GMP going through my water heater - from what I have read on this site it sounds like I might need a bigger pump and/ or need to replace my tankless with a boiler (I think I read somewhere on where that tankless units are not designed for low-pressure systems)

If anyone could help me figure out what I can do to get more heat into my house I would greatly appreciate it.

PS my incoming water temp is 90 and my out going is 140

EBEBRATT-Ed

A boiler is always better.

if you dont have enough flow through the water heater to make it work you could add another pump and pipe the water heater primary secondary to give it the flow it needs

AaronHoop

Ironman

A tankless is not designed, controlled or approved as a boiler. It’s the wrong tool for the job and its heat exchanger has way too much resistance to flow (head) for a hydronic system.

As Ed mentioned, you could pipe it p/s with a large circulator on the boiler, but it will be a short lived solution.

If a tankless could replace a boiler, there’d be no need to make boilers.

AaronHoop

I intend on replacing the Tankless next season with a boiler, but for now I am wondering if there is anything I can do to improve what I have.

"you could pipe it p/s with a large circulator on the boiler, but it will be a short lived solution." could you explain what you mean? what does p/s stand for?

GroundUp

What they said. To be more specific, you'll need to repipe the system with another, larger high head circulator to pump through the tankless only by use of closely spaced tees or a hydraulic separator. Then the existing 15-42 would continue to circulate the floor loops. This is necessary in most boilers too, so in a few years when the Takagi dies it will be an easy swap to a boiler at that point.

AaronHoop

Thank you both for the response
I dont think I am fully understanding what your advice is. What do you mean "add another pump and pipe the water heater primary secondary to give it the flow it needs"

I get adding another pump, but what does "piping the water heater primary secondary" mean?

Ironman

Primary/secondary piping.

Ironman

The boiler and its new, large circulator would pump to and from the bull on the Tees; the system would pump through the run of the Tees.

You need at least a Grundfos ups26-99 (or equivalent) to pump the boiler.

Even though the diagram shows a size reduction in the bull of the Tees, keep that pipe diameter at least as large as the boiler connections.

AaronHoop

Does this look like I am understanding your advice on the P/S loop?

Mosherd1

As others have said, it’s not ideal but will work, I know this because I am currently using a tankless for my radiant system as well.  It was put in as a temporary solution until all of our home additions and renovations got figured out to properly size a boiler.  I am running a Takagi AT-H3M-DV-N. I have mine set up primary/secondary using a buffer tank instead of closely spaced T’s because my house is micro zoned. For my Primary loop pump going through the tankless, I am using a Taco 0015. That gives me 2.5 gpm through the tankless on high, and 2.2 gpm through it on medium.  If you have any questions on my set up, please feel free to ask. 

Ironman

AaronHoop said:

Does this look like I am understanding your advice on the P/S loop?

Yes. It doesn’t matter whether the tankless or the system come off of the bull (side) of the Tees, but it’s either or, not a variation.

mattmia2

Almost, but the boiler connects with tees to the main loop, not the main loop to the boiler with tees.

Page 7 of this:

https://www.caleffi.com/sites/default/files/file/hydraulicseparation-tr07.pdf

hot_rod

Skip the Y strainer

Ironman

AaronHoop

Does this look better @mattmia2?

Mosherd1

Not quite on the above picture.  The Grunfos pump needs to be in the loop of piping with the boiler circuit.  And then no closely spaced T’s between the expansion tank and your manifold.  The boiler is one loop and the rest of the system is another loop. 

Ironman

AaronHoop said:

Does this look better @mattmia2?

No, that would be primary/secondary/tertiary which would require a third pump and would be totally unnecessary.

You can’t make it any simpler or plainer than the hand diagrams that hot_rod and I posted. 

EternalNoob

Hi Aaron,

As others have stated clearly primary-secondary loops and perhaps also changing to a boiler is the way to go according to convention, and I'm not disagreeing with anything previously stated, *however...

I have seen systems built like yours work in milder climates where the over-all heating needs are low. In fact it is a somewhat common DIY radiant install here in California, especially as a back-up to a solar thermal system which is doing the lions share of the work, or for small cottages / tiny houses or well insulated passive houses. It's not ideal, and will probably result in a short life-span for your takagi, but it's cheap and simple and it can work. If you want to re-pipe your system with p/s loops and/or buy a boiler, it may be better to find a radiant professional, as it seems like your understanding of the subject is limited and this might be biting off more than you want to chew. The subtleties are important to get right. However, if you want to make a few adjustments to what you have, you probably can get yourself some decent heat for the remnant of the heating season and buy yourself some time while you figure out your complete re-do.

That you are getting .7 gpm through a 10 gpm tankless means there is obviously a more acute problem. First i would try cleaning the filters at the isolation valves leading to the takagi. There are good youtube vids on how to clean isolation valve filters and flushing the WH itself couldn't hurt. They get contaminated with rust or debris floating around your radiant system quite easily and will dramatically restrict the flow.

Additionally i would turn down the temperature setting on it to 120F or maybe even 110 (if it goes that low?). Lower temp will be healthier for your radiant floors, regardless, but secondly because, as far as I understand it Takagi tankless WHs modulate flow rate in order to provide a given temperature. It's like they can only produce a given quantity of heat, so if you are asking for a temperature higher than what it is capable of producing at 10gpm, it will clinch the flow down in order to provide that temperature. Here are the flow rates listed on the spec sheet listed on supplyhouse:

Flow Rate at 70F Temp Rise (GPM): 5.4
Flow Rate at 60F Temp Rise (GPM): 6.3
Flow Rate at 50F Temp Rise (GPM): 7.5
Flow Rate at 40F Temp Rise (GPM): 9.5

There also may be any number of other issues restricting flow: clogged or bad balancing valves at the manifold? Bad pump? Did you forget to fully open any shut-off-valves?...

pacopancho

Ironman said:

Thanks iron, for this diagram. I understand how my 'stats control the secondary loop circulators when there's a call for heat but I don't understand
what would activate the circulator that's going to the water heater. How does it get instructed to operate? Many thanks! Paco

Ironman

Whatever calls the system circulator on should also call the boiler circulator on.

hot_rod

I like the piping schematic, I’d be tempted to move the expansion on the loop just to the left of the closely spaced tee   Air purger is fine where it is. Both circs would be pumping away👍🏻