Danfoss Thermic Valve

I have an Attack DP35 with a Danfoss VTC511 thermic valve at the boiler return, with a 140F element. The circulator is located downstream (closer to the return) of the valve - just like their installation diagram illustrates. When the boiler output reaches circulator launch temperature of 144F, the return quickly rises to 140F, regardless of the heat load - number of active zones. However, where I'm having a problem is when the heat load increases. Let's say I have my DHW calling for heat, then a zone kicks in. The boiler return does not get back up to 140F until the return manifold heats to that temperature. Is this normal for the Danfoss? If so, it's worthless in my application. If not, any suggestions?

Trex83 said:

Hi George,
please send a quick picture or diagram of your loop. We need to see if you have the VTC piped correctly. PLease reference which page and version of the PDF document from Danfoss and which diagram you are referencing.
The valve can be used for two different application.
For boiler protection, you are correct, it shouldnt let water below 135F or so.
We are using one and quite satisfied.
Cheers,
Trex

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In the attached document, it is the piping orientation for return mounted, other location for circulator. The valve is mounted with the thermostat access down, as in the picture, with the square marking on the valve toward the boiler return. When at or above 140F return at the outlet side of the circulator and a zone opens with the return manifold cold (room temperature), I have seen the boiler return drop as low as 110 and never recover past that until the return manifold rises above that temperature, then it tracks up with the return manifold temperature. When this happens, boiler outlet temperature remains at or above 160F. I have temperature sensors at various locations and have verified temps with an IR thermometer as well. I am using a nofossil control system which datalogs temperature once a minute if you would like to see more detail. I can send that in a .csv, .xls or .xlsx format.

maple1 said:

Can you post a pic of it? Certainly doesn't sound right.

Also, not likely has anything to do with it, but I likely wouldn't use a launch temp below 160. Mine usually doesn't launch from a cold start until it's about 190.

EDIT: This might be a real silly question - but are you sure there's a thermostat in the body? Not sure where you got it, but for example, I think if buying from Pex Supply, you have to buy each seperately.

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My controller won't allow a launch temperature above 160 and the factory default is 144, which is part of the reason it's set there. I removed the thermostat yesterday and tested its opening temperature and that seemed a little low, since it was fully open at 140. According to the data sheet it should not be fully open until it reaches 158. I did verify the part number on the thermostat body and it does indicate it is a 140F. I think once it starts to open, it doesn't close fully until it cools well below 140. I'll post a pic, but it looks just like the diagram in the Danfoss brochure.

maple1 said:

Maybe a defective element?

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Kind of what I was thinking, but I have little experience with this valve. I installed my boiler last year and it did seem to work better at first, but I had other issues, like air in the system, etc. Hard to tell if it always behaved this way, so I really needed to hear what other people have experienced. So, tell me. If you come up to temp on your boiler return, then have a cold zone open like I've described, does the Danfoss recover the return temperature quickly? I'd expect a quick glitch for a minute or two as things stabilize, but this is just, drop like a rock and stay there until the return manifold temp goes up. I'm considering a higher temp thermostat, but really shouldn't need anything larger than the 140.

700renegade said:

Mine is a 140* valve, it does take close to 160* before it diverts the whole flow to the system.

If I turn on a huge load and pull the boiler temp back towards 140* it reacts within a minute or so and goes to full bypass as it should. my boiler never drops below 140*. As a matter of fact I can never get it below 145* or so unless it's out of wood, as below 158 it starts clamping off the flow out of the primary loop.

My bet is on a defective element.

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Thanks to everyone who has posted responses to this thread. It has been very helpful. I'm going to go ahead and get a replacement 140 rather than the 149 I was considering.