Critique my 25 year-old In-Floor Radiant System w/ Indirect DHW Heater

kipper

Some background here: I live at 8300' in the mountains of Colorado. I purchased a house late 2015 that was built in 1992. It has radiant in-floor heating (concrete slab), 865sq/ft lower level, partially subterranean. Upper floor is also 865sq/ft heated by baseboard radiant.

Recently my Superstor 60gal (SSU-60, installed in 2009) indirect dhw storage tank started leaking around the aquastat well. I replaced the well and that didnt solve the issue. I suspect a pressure surge from the town water supply, faulty PRV, and years of improperly configured zones/pumps (overheated water in storage) caused the premature failure.

The tank was recently replaced (same unit, updated design with permanent aquastat well) and I set out to educate myself on the specifics of my setup. I put together this diagram to help me understand the existing system and use this knowledge to update and expand it. My plan is to add a ~700sqft attached garage with in-floor radiant slab, also a ~700sqft studio above the garage with in-floor (staple up) radiant. I intend to have one boiler and one storage tank for the whole structure.

1) Living space ~2400sqft, 4 zones (pex slab lower level, staple up in-floor upper level)
2) 700sqft garage, in-floor radiant, seperate zone kept ~50F in the winter, ambient temp avg 20F
3) One Nest thermostat per zone, total 5
4) Taco ZVC406-EXP-4 OR Tekmar 306V (DHW Priority is important, also Post Purge into DHW tank)
5) Variable power Grundfos Alpha2 pumps (see #6)
6) Tesla Powerwall grid-charged for backup power to the circulator pumps during (frequent!) 1-12hr power outages

Right now, my primary concerns are 1) when DHW is calling, hot water flows through the primary pump and heats the floors, even when the pump is off and in-floor isnt calling. This causes two reasonable-length showers to take 2-3hrs of boiler time to recover. 2) related to 1, in the summer my floors are heated even when the thermostat isnt calling 3)

Any feedback would be much appreciated, including boiler sizing, zone controller (taco vs tekmar), variable power circulators, storage tank sizing, etc etc.

hot_rod

You need a repipe. Did the person replacing the tank offer any suggestions. Do you plan on replacing the boiler?
Is zone one really a 10" slab?

kipper

hot rod said:

You need a repipe. Did the person replacing the tank offer any suggestions. Do you plan on replacing the boiler?
Is zone one really a 10" slab?

Im still working with the plumber that installed the new storage tank. He also installed a new PRV at the main, an expansion tank on the cold side, some shutoff valves on the Superstor, and a new zone valve motor on the dhw return. Yes I will probably have to replace the boiler, and yes zone 1 is a radiant slab. The slab is probably not 10" thick, that was a poor guess. What's your repipe suggestion? I was thinking a zone valve on the system pump to avoid in-floor heating when only dhw is calling. Thanks for the reply!

hot_rod

Is there any information on the original design of the system? Any heat load calculation, tubing layout, equipment sizing data? Any pictures taken during the tube installation?

Without any original design information you may need to start from scratch.

A room by room load calculation would indicate how much tubing and what temperature and flow rates are needed. With those type of heat emitters radiant slab and radiators you will probably need a two temperature system.

With all the rooms totaled you would size the boiler, circulators, etc. This design and load data becomes a "road map" for a successful system design and installation.

The boiler you have would need to have return temperature protection to prevent it from running with low return temperatures for extended periods.

The movement now is towards high efficiency modulating condensing boilers mod cons. That would assure best efficiency, and protection from low operating conditions.

It may be worth hiring an experienced hydronic designer to do this important design step, then you could select components and the best boiler piping. And also get some bids for the work based on one accurate design.

Plenty of top notch hydronic guys across Colorado if you need that assistance.

kipper

hot rod said:

Is there any information on the original design of the system? Any heat load calculation, tubing layout, equipment sizing data? Any pictures taken during the tube installation?

Without any original design information you may need to start from scratch.

I do not have any documents related to the original heating system. Before I start from scratch I'd like some feedback on what could be done to prevent the zone 1 in-floor heat from heating when only dhw is calling. There's two project phases here 1) make small improvements to what i have now 2) re-engineer the entire system after the garage is built. Phase 2 will be next summer, so I'd like to fix the one problem here before then.

Ive been studying modern designs and Im seeing three circulators for my type of setup. System pump, boiler pump, and zone 1 pump. Right now I only have two. Do modern pumps have internal valves to prevent flow when not calling? Or should a zone valve been installed before/after the Zone 1 pump?

Zman

Because your boiler pump is directly piped to the zone 1 mixing assembly with no zone valve or primary /secondary piping, I am sure it is ghosting considerable flow.
I think your fix would be to replace your mixing valve with one of these https://s3.amazonaws.com/s3.supplyhouse.com/manuals/1348766886771/81453_PROD_FILE.pdf
It would provide you with outdoor reset and boiler condensation protection. I believe that the valve will function as a zone valve, preventing the bipass of water when there is no call for heat. I would verify that with the manufacture and add a zone valve if it does not.

SuperJ

There's a lot going on there. But look into the concept of hydraulic separation. Your primary loop isn't really a primary loop unless you count the indirect tank/radiant baseboard. And with infloor heating you definitely are at risk for condensing.
You need a way for water to around in a circle thru the boiler no matter what the loads are doing.

A couple thoughts:
-The location of the expansion tank seems suspect, probably should be re-piped to the return manifold.
-Consider a need a bridge below the infloor mixing valve connecting the SWT to the RWT. (and make sure the supply and return from the infloor loop are right beside each other).
-Might need a separate pump/repipe on your indirect tank (with checkvalve).
-All these things depend on your design goal, more than one way to solve your problems.

But there is enough going on there, that a single fix isn't going to make it right. Needs to be designed/thought out completely before you start changing things.

Zman

The cast iron boiler should be pretty indifferent to minimum flow rates.
The system has quite a few issues. As a band aid, I think the mixing valve will work fine. Moving forward a solid long range redesign is in order.

kipper

Zman said:

Because your boiler pump is directly piped to the zone 1 mixing assembly with no zone valve or primary /secondary piping, I am sure it is ghosting considerable flow.
I think your fix would be to replace your mixing valve with one of these https://s3.amazonaws.com/s3.supplyhouse.com/manuals/1348766886771/81453_PROD_FILE.pdf
It would provide you with outdoor reset and boiler condensation protection. I believe that the valve will function as a zone valve, preventing the bipass of water when there is no call for heat. I would verify that with the manufacture and add a zone valve if it does not.

I dont see anywhere in the documentation that this mixing valve would function as a zone valve. Will my 25y/o Burnham support Outdoor Reset? Are you suggesting a re-wire to allow the boiler to run at full temp for dhw calls, and lower temp otherwise when it's warm out, overriding the static setting on the current mix valve? What about simply adding a zone valve now before the System Pump to eliminate ghost flow?

I presume there's a reason why this system was designed this way, but for instance this morning it was ~25F outside, the Nest was set to 67F, and after two showers the boiler ran for ~3 hours, heating the house to 72F, until the dhw tank was satisfied.

Zman

I don't suggest you reset the boiler. You would gain some comfort by resetting the temp to the slab.It would also be a good idea to use the set the anti condensate feature to prevent the boiler from condensing when the slab calls, dragging down the temps to the other zones.
The mixing valve may turn to the closed position when there is no call for heat. Tech support could answer that question.

hot_rod

amazing that someone has lived with that condition for so many years?

Mark Eatherton

hot rod said:

amazing that someone has lived with that condition for so many years?

Yeah, they wanted hot floors... He gave it to them.

ME

kipper

hot rod said:

amazing that someone has lived with that condition for so many years?

The previous owner suggested I open the windows in the master at night to "cool off the house". Well when it's 10F ambient that's not pleasant. I did find the master bedroom loop and just shut that off, bedroom stays a nice and cool 64F nearly all year.

kipper

Zman said:

I don't suggest you reset the boiler. You would gain some comfort by resetting the temp to the slab.It would also be a good idea to use the set the anti condensate feature to prevent the boiler from condensing and dragging down the temps to the other zones.
The mixing valve may turn to the closed position when there is no call for heat. Tech support could answer that question.

OK, i'll call Taco tomorrow and find out.

Thanks for all the suggestions so far, keep them coming!

kipper

SuperJ said:

-The location of the expansion tank seems suspect, probably should be re-piped to the return manifold.
-Might need a separate pump/repipe on your indirect tank (with checkvalve).

Which expansion tank, the 30 or st-12, and why?

What's the reason people use 3 pumps in a system like this? My concern is power consumption in the event of a power outage. Also complexity. If I need 3 pumps to make it right, I'll switch to the Alpha2 as I understand the variable power helps with lower power draw. Thoughts?

Zman

There are quite a few piping errors like that in this system. The goal is to get it working OK for now and redesign/repipe as part of the remodel project.

SuperJ

Agree that mixing valve would prevent the phantom flow during DHW requests.
It would need to be setup to go full recirc whenever the slab pump is off (don't want it mixing phantom flow). EDIT just realized this is what @Zman said.
If DHW capacity is a concern, then you could set the DHW as a priority with relays to disable the infloor pump during DHW heating. If you did that, you would want an elevated temp during DHW requests to ensure fast recovery so space heating could resume.

kipper

SuperJ said:

If DHW capacity is a concern, then you could set the DHW as a priority with relays to disable the infloor pump during DHW heating.

Im very interested in Priority DHW. Im looking at the Taco ZVC406-EXP-4 vs Tekmar 306V. Any recommendations here? Note: right now i do not have a central zone controller, the system is all daisy-chained off the Taco pump (which coincidentally does support priority, but i'd like one central controller).

I want a 6 zone unit so I can isolate the master bedroom on it's own valve, plus add the garage and studio. So my thoughts here are:

Zone 1) DHW (priority)
Zone 2) First Floor Great room
Zone 3) First Floor Master
Zone 4) Second Floor
Zone 5) Garage slab
Zone 6) Studio above garage

kipper

Zman said:

There are quite a few piping errors like that in this system. The goal is to get it working OK for now and redesign/repipe as part of the remodel project.

Thank you, this is correct, I'd like to solve the ghost flow in Zone 1 and also add a controller for priority, and next summer upgrade the boiler (if necessary) and redesign/re-pipe the whole system.

Right now your suggestion seems to make the most sense, upgrade the mixing valve with a unit that does block flow if the in-floor Zone 1 isnt calling. My plumber suggested just adding a zone valve before the System Pump. The taco unit you suggested is $3-400 while a single zone valve is $100. Both require pipe work, so Im thinking the Taco mix valve you suggested would be a better long-term investment for the features you described above.

I really like the idea of keeping the pumps flowing more often at a reduced temperature, instead of the huge temp swings Im seeing now.

SuperJ

Those zone controllers could work in conjunction with the mix valve. You would have to tie the signals from your infloor thermostats in, the zone controller would start the pump, and enable the mix valve. But, realize the zone controllers are doing something different than the mix valve, and won't really solve your phantom flow problem on their own.

I don't think the $100 zone valve on the in floor circuit is a good long term use of money, even if it is cheaper today. Make sure it won't cause your pump to deadhead. @Zman 's mix valve suggestion will serve you whether or not you upgrade to the 6 zone control panel. Be careful on the ordering and setup though, it looks like a very configurable device that could easily be setup wrong. A good system is more than just a bunch of good parts thrown together.

The Grundfos Alpha 2 you want, is a good pump but I think you should hold off until you get your piping sorted. I have one (serving TRV'd panel rads) and it's great but pump selection is really system dependent. If you end up micro zoning your in-floor it could be a good use of a delta P ECM pump (like the Alpha2). Your primary loop might end up being a different animal though.

kipper

Here's another datapoint. Two showers this morning caused the DHW tank to call for recovery. I manually shut off the system pump inlet and zone 1 return valve. Full DHW recovery was ~45min without dumping excess heat into Zone 1. I think a large contributor to this recovery time is my town water supply temp, it's at ~35F right now. I dont have a measurement for summertime but it's always very cold, i'd guess less than 45F.

Based on my Nest history, thermostat set at 67F, the Zone 1 calls for heat for 4-6 hours nightly between midnight and 8AM. When I wake up the floors downstairs are ~82F.

Mark Eatherton

What town are you in? I'm in Denver and do hydronic consulting all around the state. Seven two zero, three seven five, three one zero seven.

ME

kipper

Mark Eatherton said:

What town are you in? I'm in Denver and do hydronic consulting all around the state.

Im in western Boulder County.

HomerJSmith

Expansion tanks--What is the purpose of the ST-12? You have no check valve on the cold water supply to SSU-60. The Watts LF25 aub allows water to flow back to the street main as the water in the SSU-60 is heated and expands. The Watts should be set at 45-55 psi not 70 psi. You would be surprised at how much more water you use in a shower with the pressure at 70 psi rather than 45-50 psi.

You can raise the temperature of the SSU-60 aquastat and use a proportional thermostatic mixing valve on the tank to keep the hot water supply temp to 120 degs which would extend the shower time.

What is the boiler supply temperature? It should be about 180 degs for radiant baseboard and the SSU-60 (typical). Why is the radiant baseboard inactive? When the boiler is running what is the temperature on the boiler tridicator? Does the temp ever reach the boiler aquastat temp setting when everything is operating and cycle thru the aquastat differential (usually 15 degs)? I fear the boiler may be under sized for the load requirements and the boiler return water temp is too low and allows condensation of the flue gases.

The Extrol 30 should be moved to 12" before the inlet to the boiler pump.


You might consider this--abandon the DHW Honeywell zone valve and install a three way diverter valve where the supply water to the DHW tank branches to the mixing valve. The DHW tank controls the diverter valve and gives the tank priority over the mixing valve circuit, shutting off flow to the mixing valve and directing all the flow to the DHW tank, with the exception of the inactive radiant baseboard. You can move the supply to the inactive radiant baseboard to the output of the diverter valve to the mixing valve and that would solve that problem. Doing this would direct all the boiler BTU's to the DHW tank decreasing the tank's recovery time.

How are you controlling flow thru Zone 1?

HomerJSmith

Make sure the new boiler you get can supply the required BTU's to the load and the load can overcome the house envelope BTU loss on the coldest day in the winter.

kipper

Thanks for all the comments here! I've taken them all into consideration and made the following changes. I will update the diagram soon.

1) Added a zone valve upstream of Zone 1 pump
2) Installed a TACO ZVC406 zone controller
3) Installed a Tekmar Setpoint 150 for aquastat
4) Rewire the whole system, removing the old 24v transformer
5) Added one z-wave dry contact to the Tekmar to get updates on when DHW is calling
6) Installed a z-wave water leak detector, probe installed over drain in boiler room floor

Everything is working great now.

The Taco is doing DHW priority, and controlling the new Zone 1 valve, which activates the circulator via end switch, which reports back to the Taco and fires up the boiler.

The floors are no longer heating when DHW is calling (I can't believe this system was designed like this and ran for ~25 years in this configuration). The house is so much more comfortable now. And the DHW with digital Tekmar aquastat is doing a great job of keeping the water hot while not running 3-4 times per day. I have it set to 10deg differential.

Next project is installing a thermostatic mixing valve for DHW and keeping the tank at 140F. Any recommendations on a good mix valve for this? Worth the extra money getting a dynamic unit that senses input temp?

Thanks again everyone!