I am a homeowner with a Tekmar 256. Its sensor reads the buffer tank temp and controls a geothermal heat pump. I did not install it but I have done my homework and can program the Tekmar.

First, my Tekmar 256 Outdoor Sensor reads too high by 2-5 Degrees. I believe the Tekmar is off because I have another thermometer next to it that is giving a reading consistent with a third outdoor thermometer (and subjective feel).

Being off by 5 degrees throws the boiler target curve off. Since there is not an offset adjustment for the sensor, is there another way to fool it into adjusting the curve correctly. I've tried raising the Indoor Temp by 5 degrees but this doesn't correctly compensate - too many variables.

A second question. I would like to better understand how the auto differential mode works. My "boiler" is a geothermal system and I'd like to know what differential the Tekmar is shooting for in light, medium and heavy loads.

Lastly, I have noticed that the differential setting doesn't always equate to a correct cut in/out (especially when first installed). Are there cases where let's say a 10 degree differential doesn't cut in/out at 5 degrees above / below the Boiler Target?

Thank you for your help.

 

very possible the Tekmar is off. Seen lots of them off by a bit.
Even 2 sensors in the same enclosure.
It's a 10 K NTC, this means it should read 10 K at 25C, and the resistance will drop as the temp goes down. So adding a pinch of resistance in series will brig a sensor that is reporting low up to proper temp. Putting some in parallel would bring it down. But I don't think you would ever get it right enough and I question how accurate the curve would be after that.
I would just increase the set point a bit and live with it.

Depending on the size relationship between the water to water heat pump and the buffer tank determines the needed differential. The smaller the differential the better, but you can run into problems with short cycling and return water temp problems. There are a number of problems with outdoor reset and stored water temps, I still believe in it though.

A 10 degree differential should always be symmetrical on boiler controls, should be.... I do not think the Tekmar control would set it up a asymmetrical differential.

 

What I am noticing is the Differential is off. When Boiler Target is set for 87*F, I watched the Geo come on at 82*F and off at 89*F. The differential is set at 12 degrees. Any ideas what could be happening?

 

can U post a quick wiring diagram ?

 

Are you sure it's not satisfying the heat call and shutting down?

 

My setting say I can't upload attachments. Here are links to my wiring diagrams. The Tekmar is labeled ODR in the first diagram.
Geo-Wiring---Supply-Voltage---Web.jpg

Geo-Wiring---Switching-Circ---Web.jpg

The thernostat is only capable of opening / closing hydronic valves. When the call for heat is satisfied, the valves close.

The geo heating loop is independent of the floor hydronic loop. The Tekmar is supposed to be the only device which can switch off the Geo once system water gets up to temp. But it is shutting off only 2 degrees above Boiler Target.

However as you can see the system is more complicated than that. The following are unique to my system, but neither should be what is causing the problem.

First, once the thermostat is in "Cool" mode, the Tekmar is disabled and an aquastat takes over (in the summer, the Geo only heats the domestic hot water so there is no need for ODR).

Second, there is an additional aquastat in the buffer tank well which recircs water in the system loop when buffer tank temp drops.

Third, there is yet a third aquastat which controls my backup gas boiler when the thermostat calls for aux heat.

 

gonna take a bit to decifer the stratagy behind that...
My first guess is a wiring glitch that is allowing some other control to drop our the boiler (geo in this case) call from the Tekmar.
That is the one issue with a bunch of relays, it can lead to some strange behaviours.

 

I've noticed the "Boiler Demand" has an arrow over it which is always there, even when the thermostat is not calling for heat and the boiler water temp is adequate.

It may be that my other aquastats are giving a demand signal. Can multiple devices give a demand signal to the 256?

If so, I presume only one of them needs to be giving this signal. It seems that the only way for the Tekmar to shut off too early is

1) for all Aquastats & Thermostats wired to give the demand signal to be satisfied
2) for the 24v power source to be cut off - which in this case looks to be provided by the TACO. Is 24v is always present for the Tekmar to work with?

Am I on the right track?

 

Yikes..

Tekmar should remain powered up all the time.
The boiler demand has likely been jumped, thus why you see a constant demand.

There are a few schools of though, first is to let the boiler control maintain the tank temp regardless of a call for heat. The other is to only start to heat the tank once there is a call for heat, then stop the heating process once the call comes off. Both have merits...

Depending on the relationship between your building load and the heat pump sizing is how I would decide. With a multizoned or smaller heat pump with a back up source, I would allow that tank to be at system temp all the time.
If the heat pump is well matched to the load and it's a single load then I would have the heat demand tied to a call for heat.

 

You are correct, it is jumpered. This is to keep the tank warm for the DHW.

The only things I can think of trying are:

1) To adjust the differential and see if the cut off boiler temperature is the same (some other device is cutting off the geo) or if the cut-off temp is higher (then something is not quite right).

2) Switch to auto-differential and see what happens.

 

This is the message Tekmar support left for me regarding the differential not working on the 256.

There is an anomaly in the software that is causing that. The only thing you can do is to set a wider differential. You should be able to get the operation you want by setting a wider diff. There is something in the software that divides or cuts that differential in half again. So it’s like its only operating on 25% of the diff on the high side. On the low side it’s not a linear relationship. That’s the explanation I got from an engineer today. We are aware of it. Most applications use the auto-diff function and we understand the auto-diff function might not be predictable enough for you with a heat pump application. The control is developed for gas boilers or oil boilers not heat pumps. So characteristics are a little different.


On Monday I will be demanding a refund or replacement with a different model that may actually work. I find this outrageous that they know about this but haven't fixed the software. It seems absurd to sell a product that they know doesn't work. I worry they will give me the line again about how the control is developed for gas boilers or oil boilers not heat pumps and tough luck.

Does anyone know of any other options for an ODR that will work with a heat pump?

 

Ohhh, I can tell you stories about Tekmar...
Software revisions, blah blah...

They have pissed me off more than I care to admit.

Good luck getting your money back, if you do look at HBX.
The have a great little control that will even stage your back up heat on heat pumps.

It the ECO 500... may not be fully advertised yet but I have used 2 to date and they seem pretty good.
I am hoping to send some feedback to the company at some point soon.

 

A bit to my surprise, Tekmar was very reasonable. The Tech support rep agreed to refund via the distributor. He said they will fix this with the next revision, perhaps even within a month.

This sounds too good to be true and based on your comment of "Software revisions, blah blah...", I am a bit skeptical.

They indicated their next model up the 260 does not have that problem. It's $300 more... so that isn't an option.

I found a link for the HBX ECO-1000, no links to the ECO-500 on Google that I could find.

I am also looking at the Johnson Controls A350R, but can't find a detailed installation manual to know if it will work.

Thanks for your input. I'll try to remember to update this post if I decide to update with a newer 256 batch.

 

Here is a link to the HBX I was talking about
http://www.hbxcontrols.com/Resources...500_manual.pdf

Give it a looksee, and let me know your thoughts.

 

Indeed Tekmar fixed the differential issues and the new batches work as advertised. I can now recommend it for Geothermal. The only downside:

The odr curve allows an odr target of 10 degrees above boiler design. My geo unit has a max ewt of 120F. With a differential of 12 degrees, this means that the max I can set boiler design to is 104 due to 6 degree for the differential (half of 12) and 10 degrees for the curve as noted above.

My only other gipe is that the auto differential is undocumented in terms of how it works - what differentials it will use at what loads.

All in all I am very satisfied.

 

Autodiff is a learned PID behavior. It will vary according to load and temperature, but not always the same diff for a given outdoor temp.

 

I understand that autodiff is a learned PID behavior based on load and temp. Is there any way to know what the How much will it vary - how low can the diff go? I'm most concerned about short-cycling.