Outdoor Reset without TRVs

Allied

As it is currently set up I have partially closed a couple valves. I'm happy with the balancing for now, but may consider TRVs in the future.

Allied

Outdoor Reset without TRVs

Have a one zone gravity coversion, P/S, CI rads, that I would like to add outdoor reset and constant circulation. Looking at tekmarcontrols.com they seem to suggest using TRVs with their 256 and 260 controls.

I am not ready to add TRVs at this time. Would the comfort level be OK?

I would be reseting the system loop not the boiler loop. Planning on wiring the system circulator for constant, putting the thermistor on the system return pipe, and letting the Tekmar control the boiler instead of the thermostat.

Thanks for your time.

Mike T., Swampeast MO

Not sure if you could achieve truly constant circulation in such a situation of even if it would be desirable.

If you have constant circulation and have the system loop temperature continuously operating off the reset curve your curve would have to be perfectly accuracte under all weather conditions and the radiation perfectly balanced amongst the spaces.

If you have the thermostat act as a "kill" for the primary but keep the radiation circulator running there's still LOTS of heat contained in those big gravity pipes and the space will probably overshoot. Any radiation balance issues will probably be aggravated. I think standing iron water systems produce quite good balance even when not particularly well balanced because once the circulation stops the radiators continue to liberate heat--and they do it FASTER in spaces that were below the t-stat setting and SLOWER in spaces that were above the t-stat setting. Keeping the circulator running would upset this "balance by imbalance."

You can probably achieve long periods of circulation if you control the circulator via the t-stat and establish a reset curve BARELY suitable to provide heat to the house. This might tend to exascerbate temperature imbalances as well--I'm not sure. If you regularly set back the t-stat to any substantial degree, such a "starvation" curve would give VERY slow response during recovery.

These are likely the reasons you find TRVs recommended with such a setup.

Allied

Thanks Mike for your comments. I'll have to think on them.

With my single zone, full outdoor reset with indoor temperature feedback (thus the Tekmar 260 vs 256 or AQ475) seems ideal. Constant circulation and continuously modulating the water temp for the heat loss.

My goals. Less temperature swings and eliminate expansion noises.

Mark J Strawcutter

balancing

I agree with Mike's concern about imbalance between rooms.

You could compensate to some degree (no pun intended) by partially closing the valves in too-hot rooms.

Mark

[Deleted User]

Go for it.

The 260 is a friendly control. Easy to dial in what you want. Concerned about overshooting? Strap the I/D sensor to the supply about 10' downstream from the boiler. Potential drawback w/ sensor location? Might get some short cycles in mild weather. You can leave the stat in series as a temp override &/or for setback. Or install a separate timer thru 13 & 14 for setback.

Constant circulation is the Benz of heating converted gravity c/i systems. But, since you will be letting the boiler temp drop to ambient when no heat is required, make sure your near boiler piping will reduce condensation.

The TEKMAR folks are from the land where TRVs are law. They are also paired, over there, w/ variable speed pumps, low volume systems, flow balancers, panel radiators, & stuff many of us have not seen. Fuel prices are 2 1/2 times ours. That is why they push for the optimum. You can always add TRVs, if you need them, later.

Allied

I really don't see any reason to use TRVs.

As to my current balancing, only the upstairs bedrooms are turned down a bit. The only other place I might consider another one is in the kitchen. This all seems to balanced adequately with the current valves.

I also don't know how one would do setback with TRVs in the bedrooms.

Mark J Strawcutter

more than balancing

You can do setback by connecting a setback thermostat to the reset controller instead of jumpering the thermostat contacts. Daytime program it to extremely high temp - reset and TRVs control temp. Nightime set it back to low temp - TRVs will open wide and thermostat controls temp.

In addition to balancing, TRVs give you room-by-room zoning without just riding the reset curve. But the value depends on how you use the spaces.

Since you've got things balanced pretty well, I'd say go for the reset and see how it works. Worse case you disable it or add TRVs.

Mark

Mike T., Swampeast MO

The indoor feedback will probably help but it's still a digital device. Again, to have the circulator running truly constantly your reset curve would have to be absolutely on-the-money for all indoor and outdoor conditions.

I guess you're planning on having the indoor temp sensor modify the pre-set outdoor reset curve. Don't forget that gravity systems have LOTS of volume and thus contain MASSIVE amounts of potential heat. If the indoor sensor takes action on rising temp the space temp will continue to rise due to the heat already in the system; if it takes action on falling temp there will be quite a lag as the boiler has to heat all of the volume of the system.

Am not saying that such won't work--just that it would be difficult to keep the circulator running absolutely constantly without some form of proportional flow control lest you actually increase temperature swings. It won't hurt to try as you can always re-wire to operate in a different fashion. My guess though is that you will have to stop the circulator sometimes--even more than just for warm weather shutdown.