Other ways to provide return temperature protection

[Bob Rohr]

The intent of a return temperature protection "devise" is to assure the return water temperature to the boiler stays hot enough to prevent condensation. Typically 140 F return temperature will assure this.

Ideally you want a device with "intelligence" One that can monitor and respond to temperature. Many methods are out there. Simple bypass piping, or a bypass circ can be used, but again it is best to have some brains.

3 way thermostatic valves are a common, simple way. They use a wax cartridge to respond to temperature and open and close the ports. They are a fairly high pressure drop "parasitic" device as there is a lot going on inside that brass body.

Another common and very simple way is to use a temperature sensitive control like a strap on aquastat, snap switch, setpoint control, etc. The sensor monitors the temperature at the return piping, or sometimes in a well in the boiler, or even strapped to the boiler itself. It merely drops power to the circ until the boiler temperature is sufficient.

Viessmann and many other boilers, gas fired, use this simple method.

My EKO came equipped with a simple snap style mounted along side the temperature control probe. It does cycle a few times until the boiler gets up to speed but works very well.

It also drops out the circ when the fire dies so as to not circulate any heat back out of my buffer tank.

Just another option to consider if you would rather not purchase, install and deal with 3 way thermostatic valves.

hr

 

[mpilihp]

In my setup I will have an aqustat controlling the circ pump for the woodboiler but after it HEATs the load (oil boiler belly/heat zone directly) the return water temp could be below 140 and I think thats why they are saying to have a 3-way valve to maintain that 140 deg temp on the return line correct??

~ Phil

 

[Nofossil]

Unless I'm missing something, there's an important difference between maintaining outlet temperature vs. inlet temperature.

Modulating circulator speed is a great way to maintain a target outlet temperature. The EKO approach is crude but effective. Variable speed circs are a more sophisticated way to accomplish the same thing.

However, that can't change the inlet temperature. If you have 120 degree water coming back from your zones, nothing you can do with the circulator will change that. Seems like you have to have some sort of recirculation loop that bypasses the heat loads. It would be really nice to have the flow through that bypass modulated so that the inlet is as low as possible without dropping into condensation range.

 

[steam man]

I think keeping the outlet temp up kind of infers the boiler must be hot and can keep up with the load. I think the boiler rating, heat load, water volume all play into how well it would work. I just wonder if the pump would have a limit to cycling on and off.

 

[jebatty]

I think keeping the outlet temp up kind of infers the boiler must be hot and can keep up with the load.

Seems logical, and true based on certain operating parameters, but not all. I may have 140 top of storage temp and 100 bottom of storage. Boiler output (say 160) goes to top of tank but return is 100 -- not good. Only one example, but many possibilities here.

The Termovar/circ/aquastat setup on my boiler returns 100% of boiler output to return until return temp hits 150, then aquastat turns on circ. "Cold" water then enters the system, and depending on "cold" water input, the Termovar regulates return to 130 minimum, which rises depending on "cold" input temp.

 

[steam man]

"Seems logical, and true based on certain operating parameters, but not all. I may have 140 top of storage temp and 100 bottom of storage. Boiler output (say 160) goes to top of tank but return is 100—not good. Only one example, but many possibilities here'.

I am thinking that a typical boiler probably wouldn't be able to give a 60 deg rise in one pass through the boiler. I would think it would be more plausible to cycle on at say 180 and off at 160. I can't see the harm to the boiler though the process seems somewhat inefficient.

 

[jebatty]

I have no info as to what is typical as to temp rise through a boiler. But keep in mind that this is one of the purposes to have a Termovar type valve on the boiler. It mixes cold return water with hot boiler supply water to insure that return to the boiler is not cold. That also means that "unneeded" hot supply water goes to the system to be heated. The consequence is that a 60 degree temp rise indeed can and does occur in my boiler operation, albeit the full output is not going into the system, as part is being returned to the boiler and being mixed with other cold return. Since full output is not going into the system, full system return also is not going through the boiler.

As temp of system rises sufficiently (my guess is about to 130-140), the Termovar closes completely and full boiler output goes into the system. The Termovar no longer is diverting hot supply to return.

 

[jebatty]

I would think it would be more plausible to cycle on at say 180 and off at 160. I can’t see the harm to the boiler though the process seems somewhat inefficient.

Do you mean "cycle on at 160 and off at 180"? In this scenario the Termovar is not diverting any hot supply water to return and you are correct, a Termovar is not needed, but in a wood boiler the system is not operating efficiently due to the cycling.

I have 1000 gal pressurized storage, and the goal of my operation is for the Tarm to burn full out through the load of wood without cycling at all. I may not quite achieve this at the high end (if system return is above 140-150 or so), but often I will. With storage my heat needs are often met with top of tank at 110-120, bottom of tank much less. So the next time I fire the Tarm (next day, sometimes 3-4 days or more later) I'm getting really cold return water, and the Termovar is very important to insure the boiler is operating efficiently in the burn and without condensation. I'm only looking for sufficient top of tank temp to meet my needs. There is no benefit to heating the entire tank to 160 or more in my application.

 

[steam man]

I would think it would be more plausible to cycle on at say 180 and off at 160. I can’t see the harm to the boiler though the process seems somewhat inefficient.

Do you mean "cycle on at 160 and off at 180"? In this scenario the Termovar is not diverting any hot supply water to return and you are correct, a Termovar is not needed, but in a wood boiler the system is not operating efficiently due to the cycling.

I have 1000 gal pressurized storage, and the goal of my operation is for the Tarm to burn full out through the load of wood without cycling at all. I may not quite achieve this at the high end (if system return is above 140-150 or so), but often I will. With storage my heat needs are often met with top of tank at 110-120, bottom of tank much less. So the next time I fire the Tarm (next day, sometimes 3-4 days or more later) I'm getting really cold return water, and the Termovar is very important to insure the boiler is operating efficiently in the burn and without condensation. I'm only looking for sufficient top of tank temp to meet my needs. There is no benefit to heating the entire tank to 160 or more in my application.

I think we have some confusion here. I am thinking how the on/off pump scheme would work for a way to "avoid, purchasing, and installing, a 3 way valve". I am totally for the Termovar and other valves and would probably not bother with the on/off control. In fact I have a variable speed pump with controls set up ready to install (and an electric actuated 3 way mixing valve). I could most likely just use the variable speed pump to obtain some measure of boiler return protection similiar to the on/off control. My thought was that the on/off pump scheme would turn on at 180 and as the cool water returns the boiler temperature sinks to a point where you still have a "hot" boiler and off at say 160 to minize shock. This action would seem to do basically what the Termovar does at start up. The size of the boiler, amount of storage, heat load, and initial return temp are issues that dictate how well this scheme would work. The Termovar obviously makes it a set it and forget it process. I am not sure what a typical temperature rise is for a boiler that is "sized" properly for a system without storage. Obviously with storage you can have a larger boiler. It would seem that a larger boiler would cycle less than a lower rated one. There probably is no perfect scheme since they all have some cost, whether it is parasitic cost from a 3 way valve or the inefficiency of constant cycling trying to get a system stabilized. I am sure the Euros mentioned above have some standard style systems in mind when they promote this scheme. I just noticed that master of sparks reports that his on/off scheme works very well. Simplicity at its finest..............

Mike

 

[Bob Rohr]

yes, variable speed circ are the ideal way in my opinion. Most all radiant controls include a "boiler return" protection feature. basically a sensor mounts at the return piping and it varies the speed of the boiler circ based on the preset return temperature. this assures exactly the correct return, avoids on/ off pulsing as I mentioned above, and does not use pump head like a 3 way does. 200 bucks plus typically for a control to do this.

You will notice this V/S concept on my stickey above for primary secondary.

Like all hydronic concepts, there are plenty of way to skin a cat.

Hydronics is a fairly forgiving technology. You can be off a bit on pipe, pump, emitter, storage, etc sizing and they still work!

What I have learned over the years is it is fairly easy to calculate and design exactly to the task. Plenty of calculators and formulas are out there to help. really no reason to guesstimate, or experiment. Unless that is a hobby you enjoy.

By far the hardest part of these jobs id defining what you want to happen, when, and how. it is a lot like a business plan. write it down, massage it, ask questions before you break out the told.

The basic laws of thermodynamics haven't changed, and they can't be fooled

hr

 

[pybyr]

when I look at the concept that HR just mentioned (using a variable speed circulator instead of a 3 way valve in order to keep the boiler return temperature at the proper point to avoid creosote), it makes total sense to me

and the same thing is referenced on page 13 of Siegenthaler's presentation "Integrating Solar & Wood into Conventional Hydronic Systems" at:
www.cmxshow.com/docs/CMX_CIPHEX-wood_solar.pdf

but as someone who's no stranger to either plumbing or electronic controls- though I am new to this combination of the two in the form of hydronics- I have not been able to wrap my head around exactly what sensors, controls, and pumps I would use to do this? Can someone please point me to some illustrative device examples & literature so that I can get a better sense of how to go from theory to implementation?

Thanks very much, as always, for the know-how and willingness to share it