HVAC / radiant /hydronic gurus? help me identify this thermostat

[SanPancho]

its from Danfoss, which seems to be a UK supplier of hvac stuff. its got a special function that goes with our crazy hydronic /radiant system. (old school type-- black goodyear rubber tubing, hot water boiler, operating temp 160F) there appears to be a time delay function built in to the thermostat. you can switch between 8 cycles/4 min or 3 cycles/2min. not sure what the hell that means in actual practice- cycles per hour seems obvious, although the minute part i got no idea...

cant find a damn thing on the Danfoss website, i assume this is "legacy" equipment and no longer supported.

in any case, this little SOB stopped working, gives some error message and without the manual i have no idea if its fixable, but im guessing no. so what i'm hoping is that someone can tell me what kind of thermostat this is so i know what to search for online as replacement. i would assume that "adjustable cycle rate thermostat" would be close, but no luck. i tried a bunch of search terms but only found your regular everyday thermostats, none of them seemed to have adjustable cycle rates. i think that because of the weird design of our system, with the hot freakin water, the on/off cycling is part of the design and kind of important. but have had zero luck trying to find a replacement in the US. (dont want to buy it overseas)

any help appreciated.

 

[VikeMan]

Well, the part number is right on it: 087N725100 RETB

Maybe ask if these guys have one in stock...
https://www.mytub.co.uk/danfoss-ret-b-room-stat-087n725100-product-109728
Or maybe this is it (or replacement)...
https://www.mjtcontrols.co.uk/087n725100-digital-room-thermostat-2

 

[Lowtemp]

That thermostat is discontinued. The time delay is only used if you have it hooked up for cooling. A normal thermostat can be used to replace this.

 

[SanPancho]

Well, the part number is right on it: 087N725100 RETB

So is the voltage.

 

[SanPancho]

That thermostat is discontinued. The time delay is only used if you have it hooked up for cooling. A normal thermostat can be used to replace this.

Well that’s what I have on it now. Found an old one out in the garage with my plumbing stuff. And yes, it works. But the bathroom heats much more slowly than rest of the house.

which leads me to believe that the cycling feature is important. Because it heats so much more slowly than everything else. And the only difference is that this thermostat is constantly running the zone pump, while the other zone pumps all do cycles.
The times I have taken a look at the system in the basement the other zones are showing 150 or 160 water temperatures. But the bathroom zone is down near 100. It would appear that the cycling featur is important for heating, no?

 

[Lowtemp]

Well that’s what I have on it now. Found an old one out in the garage with my plumbing stuff. And yes, it works. But the bathroom heats much more slowly than rest of the house.

which leads me to believe that the cycling feature is important. Because it heats so much more slowly than everything else. And the only difference is that this thermostat is constantly running the zone pump, while the other zone pumps all do cycles.
The times I have taken a look at the system in the basement the other zones are showing 150 or 160 water temperatures. But the bathroom zone is down near 100. It would appear that the cycling featur is important for heating, no?

The cycling feature is shut off on your old thermostat. Look at the second dip switch. Yours is set to "on/off no compressor delay" The delay only works if that switch is set the other way. Hope that helps.

 

[VikeMan]

So is the voltage.

Yes, it is. Note that it's labeled for both UK mains AC (230 V) and US low "control" voltage AC (24V).

 

[SanPancho]

The cycling feature is shut off on your old thermostat. Look at the second dip switch. Yours is set to "on/off no compressor delay" The delay only works if that switch is set the other way. Hope that helps.

sorry, no, dont go by where the switches are set in the photo. my bad. i played with that guy for a while fiddling with it trying to get it working again.

the correct settings as seen on other tstats are
Heat
chrono /comp delay
3 cycles/ 2min
fahrenheit

and the other zones definitely cycle on and off. i think the idea with these super hot systems is to fill the zone with HOT at 150/160, stop the pump and let them radiate the heat into the joist bay. then they kick on again down near 100ish, which is the lowest water temp ive seen. there's no hot water tank/reservoir or anything like that. so i think the constant pump action for that zone is what's bringing down the water temps and slowing the heating of the bathroom somehow. (maybe some sort of cycle delay on the boiler unit?)

i've only been told about this system design by a few old timer plumbing guys, and gleaned a bit off the interwebs, so not an expert here. but from what i can see in front of me, the only pump constantly running is the bathroom, and that's the coolest zone (when i look at zone's water temp) and slowest heating zone. that's why it seems like the cycling is important to this system design.

 

[Deadalus]

What's the differential set to on the boiler? The thermostat in the bathroom will send out a call for heat. That's turning on your bathroom zone pump. But if the temperature in that loop is dropping to 100, sounds like your boiler isn't receiving the signal to fire. Old systems simply use the differential, which for simplicity I will say is set to 20 degrees. If your system normally is set to work at 160 degrees, when the water drops below 140, the aquastat would tell the boiler to fire. The differential is set in the aquastat.

You have a zoned system however, so your t-stats may run to a zone controller of some sort. Plus, you didn't mention what you have in the bathroom, a radiator, baseboard, or maybe you did but I don't know what black rubber tubing you are referring to. Old school is more like radiators and galvanized pipe or black iron pipe. I think the word used might be appliances (rads, baseboard, etc) but the different appliances use different operating temperatures. Also the type of boiler/furnace used is a factor-gas or oil or electric, wood or coal even.

Rads normally use water in the 170 to 180 range. It's not considered super hot.

If your other zones have the cycling t-stats, maybe you could create a call for heat and time the zone pump turning on and off to see if you can get a better understanding of what the cycling refers to. Also, you could measure the zones water temperature to get a lower limit on when the boiler fires. You might have some kind of controller that overrides the boiler aquastat. You would likely have a thermowell somewhere away from the boiler to measure the temp and feed the information to the controller. That's how my outdoor reset functions, which is a controller that adjusts for outside temperatures as well as varying the differential. It's "smart" to a degree.

 

[Deadalus]

Actually I think Vikeman did provide a helpful link. The thermostat he linked mentions "cycle rate adjustment". I found this information on cycle rate when I searched it. It's not anything I recall seeing when working on my electric furnace with radiators system. But I did see both Emerson and Honeywell discussing it so I'd look for one of their t-stats that match the settings on your other t-stats or whatever was actually set on the one that was in the bathroom.

 

[Deadalus]

So I am not sure about your boiler but I have an idea what the issue is. Your boiler is potentially gas and it fires so hard that it doesn't need to stay on too long before it heats the water up to it's high limit. Or even is just designed to come on for a set period of time. I think the cycling function must include a reset. But the old replacement you added doesn't have a cycle rate adjustment so it just tells the boiler to come on but not to cycle. It keeps telling the boiler to turn on without sending a reset maybe and then the aquastat's high limit is or a timer is telling the boiler to turn off and the boiler needs a fresh call maybe. You could test that by turning the t-stat up, letting it run, and waiting to see if the boiler refires. And if doesn't, you could turn the t-stat down and up and see if that makes the boiler refire.

 

[bruce_the_loon]

In the first link that Vikeman posted, there was a link to the installation manual for both the direct and RF versions. There is a pairing mechanism to link the thermostat to the base station which may need to be done if this is the RF model.

https://www.mytub.co.uk/product_images/danf25693v09.pdf

 

[Deadalus]

This video does a decent job of explaining cycle rate. It even discusses how there was a similar way to set it in old mercury t-stats (heat anticipator). He doesn't quit detail a cycle though, only making suggestions for cycle rates for various systems.

Is the Chrono set to on for the the others t-stats? The manual @bruce_the_loon links says

CHRONO - energy saving feature which fi res the boiler at regular intervals to maintain a set temperature, achieving a constant ambient environment for the user.

If your differential is really wide and 100's are still above the lower limit, then the old t-stat could be telling the furnace to fire but the furnace doesn't because the aquastat is set lower than that (through the differential, depends on the aquastat model on how exactly). Potentially you have some kind of zone controller along the way to split the t-stat call of heat between the circulation pump and then on to the furnace. The other t-stats cycle off through the Chrono feature and then reinitiate the cycle, hence turning on the furnace regularly and preventing the water in the system from dropping too low.

Depending on possible other system components, the circulator pumps may or may not stay on when the furnace is off. My outdoor reset for example continues to recirculate while a call for heat exists but can tell the electric furnace to turn off. Similar in thought to your idea about dissipating the heat in a zone while the furnace is off. The OR is also smart enough to vary the differential as needed. I have a central pump though with zone valves operated by the t-stats so any call for heat needs to turn on the pump.

 

[SanPancho]

I’ve already theorized on the boiler potentially having a delay/cycle timer situation, and that, combined with a “constant run” thermostat, to be causing the poor bathroom heating. fairly easy diagnosis.

but the only actual hard facts on hand are that the zone with the replacement tstat is the only zone acting funny. Occam’s razor. It’s the tstat. (the replacement one)

its not really worth it to get into the weeds here about the system. based on the two points above, its easy to say the "cycling" functions are required by the system design. so it needs to be replaced with a unit that has similar functionality.

that's why in post #1 i'm asking folks in the HVAC or radiant/hydronic spheres if they can put a name on this feature. "cycling" doesnt work on its own. not "timed cycling", etc. etc. HVAC and plumbing and every other industry has their own nomenclature and jargon. was hoping if it i could search using the commonly-used feature name that would help. but no luck.

luckily i received today a contact back east to get a hold of and see if they can help. (thanks pete! still happy to send you some nice fresh hops!!) and was just yesterday offered a unit that will work, but its a commercial controller unit from a hospital, as big as a carry-on bag and fairly heavy. not worth the hassle. the system is horribly inefficient vs modern hydronic design and getting ripped out next year during expansion.

at the end of the day its just the bathroom, its not the end of the world.

worst case i'll slave it to one of the adjoining bedrooms.

 

[Deadalus]

The issue you are having involves the cycle rate. Vikeman gave you the link where you could have found that but obviously you didn't captain. I even pointed that out to you in post #10 that it's called cycle rate adjustment and provided a tutorial on that in post #13 (How a thermostat works part 4 what is cycle rate and why should I care?). And I'm not an HVAC person. I've only replumbed and zoned my system including a zone controller and an outdoor reset. You mention cycle rate but have switched back to calling it "cycling". But in case you missed what it's called, it's called the cycle rate.

You would need to provide your full system information to a qualified HVAC person to get the right product so everything plays together nicely. There is more information you need to provide to get the correct t-stat, including whether there is heating and cooling involved. It's even important as to how many wires are in your wall as far as wiring the t-stat. A full recommendation by anybody is going to need more information than you have provided before identifying a t-stat with a cycle rate=3. I've already seen a couple that might work for you but you haven't provided enough information. Further some t-stats settings ask for the type of furnace which I can't say with certainty but I suspect makes picking a cycle rate mostly redundant. Some t-stats also use slow, medium, and fast for the cycle rate instead of numbers.

I'd recommend you go to an HVAC site if you absolutely want a pro as I've seen many where the pros help out but I'd recommend a little humility if you do. They might ask you questions that you might think are unimportant but since you have phrased your question exactly like you think you need to that probably won't happen.

Good luck with your cycle rate problem.