Variable Speed Pump - Need help

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hkillam

New Member
Apr 23, 2008
41
Rice Co, MN
I'm heating a single building of about 3500 Sq Ft (will heat a second, smaller building planned for construction in about 5 years) with a Garn WHS2000, with Radiant in floor Pex. This is a vacation property for the moment, and won't be occupied full time for another 5 years. Here is my problem: I have a propane boiler in the building, which functions as a backup heat system. This propane system is providing backup heat about 50 to 80 % of the time, as I can't get the Garn fired as often as I'd like. Given the location of the heat exchanger, the propane boiler ends up back-feeding heat to the garn unit, via the flat plate Hx. If I turn off the circ pump from the Garn (Taco 007F5), I can see the propane boiler output temp jump rather fast, but with Circ on, it can burn for an hour without getting hot enough to hit its setpoint and stop firing. A lot of that heat is being backfed to the Garn. I can also see this by monitoring the temp of the supply and return, just before and just after the Hx. If my supply is at 100F, and the Propane boiler is firing and up to 130F, I'll see the return to the Garn going out at 115F. I need to keep circulating to prevent freezing, of course.

To a certain extent, this is good - it keeps the Garn from getting so cold that it freezes. But, it ends up costing me in Propane, as well as power to run the Taco 24/7. Is there such a thing as a variable speed or multi speed pump that can receive temp inputs and vary its pumping rate? What I'm picturing is a way to compare the supply temp and the return temp, and control pump speed based on the difference.

For example, if the supply and return temp are the same (or within a reasonable degree the same), then pump at low speed as the building isn't using any heat.
If the Supply is colder then the return, pump at a low speed as the propane system is firing and pumping heat out of the heated building and back into the Garn.
If the Supply is below 135F, then pump at low speed, as it is below the temp needed for heating the building.
If the Supply is above 135, and the return temp is 10 or more lower then supply, pump at high speed as the heated building is requiring and using heat from the Garn.

I know there are commercial solutions to this problem, but I'm struggling to understand them. Some just seem too complicated for my situation - I don't think I need to monitor outdoor air temp, or Propane boiler temp, or heated building temp, etc. The solution I descriped above would only require temp sensors located close to the Garn - one on supply and one on return, with no control wiring going to the heated building or outside the Garn shed. Are there existing commercial solutions that I've missed, or simple DIY options that I can implement? I'm comfortable changing pumps and can handle low voltage and residential voltage electrical systems, and think I could manage some modest control programing.

I'm anxious to hear suggestions or answer any follow up questions.
 
Hey Hugh. The answer is a Tekmar 157. Here is my post on this very subject: https://www.hearth.com/econtent/index.php/forums/viewthread/31287/

You want a delta T based pump controller. I am going to install one to control the secondary pump supplying my house HX. Are you sure that is going to do what you want as far as freeze protection for the GARN?

Do you have the electric heater option on your unit? If so, do you have any elements installed? You could run one of them to keep the unit above freezing. Probably cheaper than propane. If you don't have any elements installed, putting them in now would be a rather wet experience :lol:
 
Hugh, Jim,

A Tekmar 356 was recommended to me for my Garn. I'm still working out the details on my system and have bought the controller, but I think the idea is going to be to

keep the Garn water moving at a trickle, even if it's cool, for freeze protection. The 356 will be running a Grundfos 15-58 on my set-up which is a three speed pump.

Jim, I think that's the way Tat has his running, wasn't it?


Hugh,

Tat's thread is here somewhere called "My Garn Barn". You can check it out

Rick
 
I don't have electric elements installed. Sadly, my local electric Coop doesn't allow off peak DHW or Space heating unless you reside at the property year round. So I won't qualify for that for some time... Regarding freeze protection, if I have a pump running, even at very low speeds, and with occasional backfeed heating from the propane boiler, I'm not concerned. This assumes, of course, that I can program the Tekmar 157 to run nearly forever at a 20% rate when storage temps in the Garn are below about 120F. Currently I'm paying about $0.20 a day to run the pump, which isn't a big deal when heating with wood, and is OK as freeze protection, BUT when those cents are also drawing propane generated heat out of the backup boiler, the expense escalates quickly.

The Tekmar 157 looks like a possibility, and if I'm reading the documentation correctly, it appears that it could use my existing Taco 007F5, which is great - I assumed I'd need a new pump.

I may come back with more follow up questions regarding your install experience. This solution seems to have more options then I need, but is quite reasonably priced, so I guess that is OK.
 
Rick/Hugh,

The 157 is a differential setpoint controller. The 356 is a diff setpoint with setback. I think the 356 may be a better option than the 157, I was just answering Hugh's primary question as to how to set up a Delta T speed control.

I think any Cap start wet rotor circ will work with the 157 or 356. The 15-58s are listed as compatible. My pumps are bigger, but have the same cap start and speed controls (slightly larger caps).
 
Well, I've spent a lot of time reading the manual for the Tekmar 157. Looks like a really good product, but I'm still not clear if it would do what I want. It seems to be made for controling a solar water heater or wood fired heating with REMOTE storage. With the Garn built in storage, I don't know if I can get it to do what I want. I just want a way to reduce (not turn off) pump speed if the Garn water temp is below a particular threshold, and to pump at max speed if supply temp is above a threshold, but only when return temp is 10F or more below the supply temp (indicating a heating demand). I'd use just one pump connected to the 157.

Here is what I need to program, using my admittedly poor programing logic;
If supply temp <135, pump at lowest speed (must keep pumping for freeze protection)
If supply temp >135, and return temp <10F below supply temp, pump at lowest speed (no heat demand)
IF supply temp =>135F, AND return temp lower then supply temp by >10F, pump at full speed (provide the max possible BTU to Hx because there is a heat demand and sufficient BTUs available)

The only adjustments I'd make to the above logic is to vary the thresholds (not sure if 135F is the exact temp I'd need to set) and delta T differences, based on the results I see.

Anyone out there used a Tekmar 157 with a Garn for the type of control I'm looking for? Or does anyone know the two products well enough to make a good prediction about meeting my needs?
 
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