Constant Circulation Setup - Tekmar 260/Taco SR502 DHW Priority

Lynnwill

Based on responses obtained in a recent related post, I want to establish a constant circulation pattern in my hydronic system. A few details: I have a gas Burnam boiler with output rated (DOE) at 136MBH. There is also an indirect water heater connected to the boiler. There are two zones aside from the water heater. A pump is dedicated to a radiant zone in a kitchen; The remaining house is on a separetly pumped zone. Lastly there is a boiler system pump. All circulators are Taco 007. The system is a converted gravity type with 1050 EDR – a lot of water. Controls are with a Tekmar 260 connected to a Taco SR502 with 2 internal relays.

I plan to setup constant circulation by adding a thermal sensor to the Tekmar. I will the set the room thermostat a few degrees above the Tekmar setpoint with no setback. The outdoor reset characteristic curve will then adjust water temperature to maintain comfort and temperature while lowering fuel cost. With this scheme I will no longer have a mass of water sitting cold waiting for heat and circulation. The Tekmar will be set to 74 degree indoor, the thermostat will be set to 76-78. The Tekmar is also set for 180 degree max, 140 degree min and 160 degree design temps with a “5” radiation setting.

The issue: Upon hot water heater demand the boiler will shoot to 180 degrees. The main zone circulator and radiant zone pumps are connected to the Taco relay. Upon demand by the indirect water heater the zone pumps will continue to operate – with 180 degree boiler water this will completely throw off the reset. How can I establish a priority so the two zones will temporarily shut down on water heater demand? Should I power the relay box through connections #7 & #8 of the Tekmar? Finally, should I just disable DWH priority in the Tekmar?

As an aside I use the 24VAC connection block and internal transformer in the relay box for demand contact power.

GW

Can that 260 take an indoor sensor? You can't have a true constant circ with your Taco SR 502.

Also, are both zones high mass cast iron rads? The answer makes a difference on what comments you'll get

Also, I'm not sure why your space heat pumps are still running on a dhw call (clearly you're idicating you don't want that), it seems like you're mis-wired, presumably the 260 had a priority ability amd you may need to have your 502 get a signal from the 260 ( do you have a ZC term?). I am not looking at wiring diagrams as a type, but you need to knock out the pumps by rewiring, or a field installed double throw relay switch to kill the 502 when Dhw calls.

Also, as discussed in your other posts, your probably gonna need a mixing valve. You have a giant emitter system, so you don't need much temp (which is great, but your boiler will take an early nap due to condensation).

I can't picture your boiler pump, a pic would be cool to see what your talking about. You have primary secondary or a basic/simple bypass pump? If the latter, what tells the pump to operate? Does it run on any call for boiler?

Gary

Lynnwill

In answer to your questions:

Yes - The Tekmar 260 has capability for addition of a temperature sensor. This will provide feedback to the reset curve. There is none now but I plan to add one. Essentially the existing thermostat will only be used, without setback, to create a boiler demand. This will be overridden by the Tekmar - it will maintain the programmed indoor design temp.

One zone is high mass converted gravity wit cast iron radiators. The 2nd zone is in floor radiant with plastic piping embedded
in a slab. This is in only one room - kitchen. This zone has its own mixing valve.

The boiler pump is activated directly by the boiler upon any demand. It is on the supply side. The photos only show near boiler piping 1-1/4". These connect to 2" black pipe mains for heating zone distribution. The radiant floor supply/return is 3/4".

The SR 502 controls the zone pumps based on thermostat demand and receives no signal from the Tekmar when the Tekmar gets a DWH demand. The DWH pump is controlled by the Tekmar. Upon a DWH demand the Tekmar targets 180 degree boiler water.

Note in the photos "rear piping" from left to right are the radiant zone mixing valve and pump, main heating zone pump and indirect water heater pump.

Lynnwill

The boiler eventually gets above condensing temperatures. The problem is when demand stops due to a satisfied thermometer and/or setback. All the now cool water from the large main former gravity/cast Iron system comes crashing in to be reheated. Constant circulation, as described to me earlier, seems to make the most sense to alleviate this condition.

Lynnwill

I think i got it! It appears that I should relocate the main zone pump power source and control from the SR502 to the P1 output (#7&#8) on the Tekmar 260. The Tekmar should then shut down the main zone pump on DWH priority. The main zone thermostat will continue to call for demand - constant circulation. (at least near constant). I have not physically done this. If I am off please comment.

Thanks!!!

GW

I'm not sure that's a good idea, how many amps can the tekmar handle?

Lynnwill

That particular output from the Tekmar is designed to run pumps. Their schematic shows it capable up to 10A.

GW

you're good to go!

Lynnwill

I have successfully set up constant circulation and added an indoor thermostat to my Tekmar 260. The issue now is that the system overshoots temperature.

I have attached a diagram of my current system setup. Photos are shown in an earlier post.

I would LOVE to get some expert comments. Do I need the system pump P1? Each zone has its own pump. Pump P1 runs whenever there is a demand. Is the boiler temperature sensor correctly located? Is my bypass piping enough to protect the boiler or is a system mixing valve still recommended? Any ideas on boiler/indoor temperature maintenance?

GW

I can't see every detail yet it seems like a blend of pri/sec and parrellel piping. The boiler sensor should be moved to the other side of the 1st take off (radiant ), and it's odd to have a low temp circuit before your high temp circuit, if you have the energy I would flip those and I would close space tee the main heating zone/circuit and radiant return into the primary loop. It doesn' tmake good mechanical sense to have your low temp "before" your hi temp.

You're overheating becasue your low limit is hotter than what the system needs. So if you really want to have a well oiled machine you need to go with a mixing valve or an injection pump (way more cost, double the pump for one zone)

The whole point of constant circ as I recall was your rads were quite large and your boiler was running in danger zone temps. So yes you need constant circ.

You certainly don't need an 007 to move water through the boiler and around the loop.

If you don't repipe then there's no reason to move the sensor.

These are just my opinions, I'd repipe and add a mix valve if it were my system. Your dhw of course can not go through the new mix valve. So, the other two zones can (yes, your radiant would pipe through two mixing valves)

icesailor

I saw quite a few older constant circulation/ODR systems without boiler protection. In the Spring and Fall,, they sure did condense because of the cooler water. To the point of boiler failure. You still shut off the pumps in the warm months for "Warm Weather Shut-down".

Its been out there for well over 50 years (my experience). It isn't new. It sounds like a splendid idea. Used in many commercial buildings. I wonder why it never caught on in the residential market. Maybe cost? It cost a lot to install and there was no ROI?

SWEI

I'm going to re-post what I wrote in one of your other threads:

You really would benefit from an ODR-controlled mixing valve as Gary suggested. You need roughly 135ºF water on a design day, cooler than that 97.5% of the time. Your boiler wants to see a minimum return temp around there, so you need to mix down.

As to circulators, you'll need about 7.5 GPM at a 25º ΔT to deliver those BTU's. A 005 might work, but you really should consider an ECM smart circ like the Taco VT2218.