Seeking highest quality millivolt thermostat for precise temperature control

Motorapido

I am seeking a high quality millivolt thermostat with precise temperature adjustment. The one I have now (Empire brand -- but looks identical to others with other brands I see online since they are probably all made by the same Chinese manufacturer) is very imprecise in its adjustment slider. I heat my vacation house in northern Pennsylvania with a gas heater stove that operates with a thermopile gas valve and requires a millivolt thermostat-- very important deep in the woods where winter electricity outages are frequent and prolonged). I strive to keep the temperature at 50 degrees when we are not there for more than a day to prevent plumbing trouble, and the temperature slider is very imprecise on my millivolt thermostat making it hard to achieve the desired 50 degree setting. From the off position as you push the slider toward higher temperatures, you feel resistance from the spring coil and you have to push the slider a good bit past the temperature you're seeking since it springs back when you stop pushing it. Anyhow, I am looking for a more precisely controllable millivolt thermostat, maybe with a dial that clicks into position as you move it through the temperature selection range. After a good bit of online searching, I'm not finding anything. Any recommendations?

Jamie Hall

If you could find a mercury T87 -- kind of hard to come by -- in the low range (even harder to come by these days) if would be perfect. Otherwise... at least some microswitch thermostats have a low enough resistance in the switch to work. Anything digital won't, unless it has a very low resistance relay.

Motorapido

Here's one on eBay. https://www.ebay.com/itm/324937950024?hash=item4ba7cfa348:g:-AEAAOSwWEhhtz76 No bids so far. Not sure what the temperature range is on this one. Does a standard t87 have a low temperature setting of at most 50 degrees F? I didn't realize the t87s weren't 24v powered. So essentially, at t87 operates the same as a millivolt thermostat -- simply makes/breaks the circuit to the gas valve?

ChrisJ

Does the anticipator work on a mv system?

ratio

It would appear that there's a separate model number for a millivolt style T87, @GW has (had?) one here, the TS86A. The manual says it has a preset (non-adjustable) anticipator.

WMno57

Any mercury millivolt stat will work.
If the range does not go as low as 50, just mount it to the wall off level. I have a vacation home that I maintain at 50 with an off level mercury stat. Been working fine for years. The setting scale will be off, I just turn it up till I'm comfortable.

Jamie Hall

ChrisJ said:

Does the anticipator work on a mv system?

No -- but that shouldn't be a problem.

The beauty is that the mercury switch has -- and maintains indefinitely -- a contact resistance in the low miliohm to microhm range.

The standard T87 goes down to 50. There was one once... sigh... which went down to 35.

Chris_L

ratio said:

It would appear that there's a separate model number for a millivolt style T87, @GW has (had?) one here, the TS86A. The manual says it has a preset (non-adjustable) anticipator.

Right. I keep one as a spare in the basement. See photo below for the specs on the box.




I keep it as a spare because I've been using digital programmable thermostats on my two millivolt systems for years. They work fine as long as they are rated for millivolt systems, though the batteries may not last much longer than a year.

The most precise thermostat I've ever had is a White-Rogers digital as it offered the finest swing adjustment. But I can't find a model number on it, and I am sure it is no longer made.

GW

not sure the regular t87 is good with milli

EBEBRATT-Ed

From what I recall the difference between the T86 and T87 is the 86 has no anticipator.

T86 is good for millivolt. T86 used to also be hooked up 2 wire common (R), W and Y and used to drive a 2 position damper motor R-W closed damper R-Y opened damper.

If we needed a replacement and didn't have a T86 we would use an 87 and turn the anticipator all the way up

mattmia2

I think if you use a 24v mechanical t-stat, turning the anticipator to where the resistance is nearly 0 is important.

ChrisJ

mattmia2 said:

I think if you use a 24v mechanical t-stat, turning the anticipator to where the resistance is nearly 0 is important.

So much for the op's precise temperature control request eh?

Kind of the opposite.

Jamie Hall

I think I mumbled about all this in another thread a while back.

If by precise temperature control you mean a thermostat or other device which will operate at the exactly the same sensed temperature every time, a mercury T86 or a T87 with the anticipator disabled is your toy or choice, if you don't want to spend a fortune on it.

However, if you mean you that the space temperature itself will be maintained within a narrow -- perhaps a very narrow -- band, you have a very different problem.

First, it is possible in principle to build a temperature control system which will hold the internal space temperature to a very narrow band; indeed, it is theoretically possible to build one which will hold a closed space at a constant temperature, provided that part of the system includes a means of sensing any variable heat sources within the space instantly, and has control power (heating or cooling) which is continuously variable instantly. The system must also be able to sense changes in external heat load on the controlled space.

For all the nasty remarks I make about Nests and their kin, if they are coupled to a heat source with very fast response, such as a high turnover forced air system powered by a very low mass modulating furnace, they can do this actually rather well with their combination of temperature measurement, occupancy sensors, and outdoor condition sensors.

In general practice, however, we usually are happy if we can maintain the temperature within half a degree of the desired set point. This can be done without external environment sensors, and in fact has been done for a century or so with varying degrees of success. The problem presented, of course, is that there is a lag between the sensed temperature moving off the setpoint and the application of heat to restore it, and a further lag between ending the heat supplied to the heating means -- radiators, for instance -- and when that means stops raising the temperature of the space. This means, simplified, that the system must start ramping up the heat supply before the space temperature drops too far, and likewise must turn it off before the space temperature goes too high.

Considering a radiator system for simplicity, this means that the control device must be able to sense that the temperature is going to get too cold and turn on the heat at some time before it actually gets too cold, and likewise that it must be able to sense that the temperature is going to get too hot and turn off the heat source in advance.

Which is called anticipation. It requires three things. First, some knowledge built in as to how the system responds. Second, the actual temperature error. Third, the rate of change of the temperature error.

From there there are many paths which one can take to arrive at decent or at least acceptable temperature control.

But I think this essay is long enough already...

GW

Jamie you keep forgetting to mention your engineering background

mattmia2

ChrisJ said:

mattmia2 said:

I think if you use a 24v mechanical t-stat, turning the anticipator to where the resistance is nearly 0 is important.

So much for the op's precise temperature control request eh?

Kind of the opposite.

A resistor sized for 24vac isn't going to dissipate any appreciable power at ~ .5 v anyhow. It will only serve to reduce the current enough to keep the valve from opening.

Jamie Hall

GW said:

Jamie you keep forgetting to mention your engineering background

If I had half the hands on practical experience and knowledge a lot of you folks have I'd be very happy.

ChrisJ

mattmia2 said:

ChrisJ said:

mattmia2 said:

I think if you use a 24v mechanical t-stat, turning the anticipator to where the resistance is nearly 0 is important.

So much for the op's precise temperature control request eh?

Kind of the opposite.

A resistor sized for 24vac isn't going to dissipate any appreciable power at ~ .5 v anyhow. It will only serve to reduce the current enough to keep the valve from opening.

I get that,
But the point was they're looking for a fairly precise control.
A T87 with no anticipator is far from precise especially with cast iron radiation.

That's going to give what, a 3-5 degree range between the boiler firing, and everything finally settling?

Yuck.

The relays in a typical VisionPro have too much resistance to work with millivolt systems?

Jamie Hall

Actually a T87 without the anticipator might do very well, as the application -- if I understand correctly -- is a gas fired heating stove, which should have very fast response and little inherent overshoot. The question -- to which I don't know the answer although I could find out in a few minutes if I weren't otherwise busy -- is what is the actual dead band of the T87's mercury switch? That is to say, what is the temperature difference required to shift the bimetal enough, once the bulb is tipped in one direction, back to the other direction?

ChrisJ

Jamie Hall said:

Actually a T87 without the anticipator might do very well, as the application -- if I understand correctly -- is a gas fired heating stove, which should have very fast response and little inherent overshoot. The question -- to which I don't know the answer although I could find out in a few minutes if I weren't otherwise busy -- is what is the actual dead band of the T87's mercury switch? That is to say, what is the temperature difference required to shift the bimetal enough, once the bulb is tipped in one direction, back to the other direction?

If I had to guess, based on all of the digital thermostats that came out after it etc, it's probably about a 1F degree swing. Meaning +- 0.5F.

I'd be surprised if it was far from that.

Motorapido

The relays in a typical VisionPro have too much resistance to work with millivolt systems?

Interesting that you mentioned a VisionPro. I have a VisionPro wired in parallel with the existing Empire millivolt thermostat. I bought a combo fan control relay center at the advice of wise contributors to HeatingHelp, and I power the VisionPro from the 24v transformer on the fan control relay. The VisionPro acts as the primary thermostat. When it makes or breaks, it triggers the relay in the fan control relay, which I have wired to the gas valve on the gas heater stove (as I said, in parallel with the millivolt thermostat). The VisionPro communicates with my Smart Things smart home hub and from anywhere in the world, I can monitor and control the temperature at the cabin from the Honeywell app on my phone. If the electricity goes down, the Honeywell and the fan control relay are dead, but the millivolt thermostat wired in parallel with the VisionPro continues to operate at the low setting I select on the millivolt stat, to prevent freeze-up of water lines in winter.

I'm pretty sure the VisionPro won't play nicely with the thermopile heater unless I use the VisionPro simply to trigger the relay on the fan control. But even if it did, I still need a non-powered millivolt stat to guarantee that if the electricity goes out, the millivolt stat maintains 50 degrees inside so the pipes don't freeze.

ChrisJ

Motorapido said:

The relays in a typical VisionPro have too much resistance to work with millivolt systems?

Interesting that you mentioned a VisionPro. I have a VisionPro wired in parallel with the existing Empire millivolt thermostat. I bought a combo fan control relay center at the advice of wise contributors to HeatingHelp, and I power the VisionPro from the 24v transformer on the fan control relay. The VisionPro acts as the primary thermostat. When it makes or breaks, it triggers the relay in the fan control relay, which I have wired to the gas valve on the gas heater stove (as I said, in parallel with the millivolt thermostat). The VisionPro communicates with my Smart Things smart home hub and from anywhere in the world, I can monitor and control the temperature at the cabin from the Honeywell app on my phone. If the electricity goes down, the Honeywell and the fan control relay are dead, but the millivolt thermostat wired in parallel with the VisionPro continues to operate at the low setting I select on the millivolt stat, to prevent freeze-up of water lines in winter.

I'm pretty sure the VisionPro won't play nicely with the thermopile heater unless I use the VisionPro simply to trigger the relay on the fan control. But even if it did, I still need a non-powered millivolt stat to guarantee that if the electricity goes out, the millivolt stat maintains 50 degrees inside so the pipes don't freeze.



The older VisonPro's used batteries and didn't need to be powered.

Like a VP8000 series with the green screen.

Matt_67

Honeywell focuspro th5110d1022 is rated for millivolt use. It’s battery powered.

PRR

Yes, any Mercury 'stat with no or low-Ω anticipator should work. You can't buy one new. I saved old ones then finally turned them in for proper disposal.

And yet I have a works-without-power gas fireplace and I added a thermostat. Works terrific. Sets smooth as silk, little drag, no spring-back. Overshoots a degree or two, which is fine for a vacant house.

It is a Robertshaw. I am not sure what part number, here's the pic. It works on a reed-switch and a magnet on a bi-metal. Of course bi-metal is no-volt but will not SNAP on and off; I guess the magnet and tricky stamping makes snappy hysteresis.


I hope you can find one (or a few). I also have a 240V 'stat I bought in 2011 and I just discovered Honeywell has discontinued their whole line of line stats and are re-branding a product I do not like as much. OTOH in this crowd someone may have that Shaw stat in a dusty box waiting for you. A spare might be wise.

PRR

Mine is a Robertshaw 9204, is listed on Amazon, but SupplyHouse says this is a 24V model? There seem to be too many versions of this basic chassis. But the MilliVolt jobs have fixed anticipation (you don't have much leeway on super low power systems) and mine has the "Longer cycles" adjuster. Yet it works fine on what I think is just a thermcouple and mV gas valve.
https://www.amazon.com/Robertshaw-9204H-Thermostat/dp/B0051BUDW8
https://www.supplyhouse.com/Robertshaw-9204H-24V-Horizontal-Mount-Thermostat-1Heat

EDIT: my fireplace thermostat runs on 3V from batteries. That means a 24V stat "may" work depending on valve load. And it was supplied by the gas fireplace store installers, who would know what works.

It is still a great stat. But if it truly works on milliVolt is unclear. All wiring is exposed so a clever person could bypass the anticipator. The raw reed switch should work any "small" voltage.

Motorapido

The Robertshaw website includes a sell sheet for the 9204H thermostat, and they show it as millivolt compatible with a range of 240 to 750 millivolts. I think I have found a good replacement for my Empire millivolt stat, which has terrible spring back as you adjust the temperature and you are also left guessing when attempting to turn the unit down to its lowest setting if it is still on or if you have accidentally turned it off. You can't feel the difference as you move the slider on the Empire stat from the low 50 setting and the fully off setting. I'll give one of these Robert Shaw 9204H stats a try. Many thanks for sharing this info.

PRR

Next winter already.... I got a spare house t-stat and it IS rated for milliVolt work!

Honeywell brand, model RTH2300B; probably others in this range.
https://www.honeywellhome.com/us/en/products/air/thermostats/programmable-thermostats/5-2-day-programmable-thermostat-w-change-reminder-rth2300b1038-e1/

This controls my 24VAC gas burner (and A/C). I like it a lot, most mornings. No funky slider: all digital. (In general I hate buttons but this one is not bad). PRECISE to the half degree (it may read high but very consistent).

-System Type-

(Despite the fact that "Electrical ratings" are all '20-30VAC', which I took to mean it needed dozens of volts to steal power from. Apparently not.)

"Allowable temperature" runs down to 40F.


Keep a look-out for the LOBATT indicator. I had a no-heat morning and, after jumpering the tstat to prove the burner ran, ordered a new one before this chilly idiot thought to change the batteries.


It makes perfect sense that this unit can operate milliVolt systems. It uses almost "no" power except backlight and the instant that it flips the (latching) relay. It could do that on the two AAA batteries for a year or two (I might have got 3 years, which is why I forgot to suspect batteries first...).