Panel radiators with TRVs and Grundfos alpha: how to call for heat?

bendavster72

Hi,

I am working on a hydronic heating system for my house in Pennsylvania.

One of the circuits will be around 12 panel radiators in a home-run setup with a Viega manifold, controlled using TRV. The circular is a Grundfos Alpha (set to proportional pressure, or perhaps constant pressure.)

The boiler is a high resistance mod/con boiler, and at least at the beginning and end of the heating season I do not expect it to be running 24/7. The boiler set point is controlled with outdoor reset.

Without a thermostat on this circuit, what is the best way to call for heat? From reading older threads (pre 2021) it seems that people have used flow sensors ( Sika was floated) or measuring the circulator’s current. Are there any best practices or are these still the recommended options?

thanks!

ben

Kaos

I'm assuming you are looking at primary/secondary loop? Possible without as well but you have to be careful with zoning.

You have some options. Set the boiler to run all the time, zones pull heat from the primary loop as the TRVs open. The only boiler control is outdoor reset.

The other one is to have part of the zones as your reference zone, these don't have a TRV but there is a thermostat. The reference zone should be set up so it receives less heat than the TRV zones. The thermostat can now drive a relay module to control your Alpha pump and also turn on the boiler. The reference zone should be big enough that it won't short cycle your boiler.

bendavster72

Thanks!

Yes, it is p/s piping. The near boiler piping is essentially standard, with closely spaced tees and (very) generously sized headers for hydraulic separation.

I prefer to not have the boiler run 24/7 - it cannot modulate below 20% and is oversized already after the house was insulated. I considered adding a thermostat in an adjacent colder zone, but the only suitable place would be right by the front door. The thermostat also wouldn’t be able to detect if someone cranked up one of the radiators - but my understanding is that the circulator would be able to detect this and turn on.

Ben

hot_rod

yes a . 5 gpm flow switch is an option, or thus RIB relay

a post from a few months back

IMG_9145.png

bendavster72

https://forum.heatinghelp.com/discussion/comment/1825809#Comment_1825809

thanks, Bob! Are you aware of anyone using a flow switch to energize a relay in a hydronic setting over a longer period? I certainly see how it could work, but any recommendations on brands and where to get these? Is long-term drift an issue for these sensors? This certainly seems like it should be a more common problem…

Kaos

I think an adjustable current sensing relay on the pump is a much more reliable option. In constant pressure mode, the power (thus current) of these ECM pumps is proportional to flow rate, you can set the relay to trigger your boiler when enough flow rate is reached so your boiler is not likely to short cycle.

hot_rod

https://forum.heatinghelp.com/discussion/comment/1825821#Comment_1825821

Sitka flow switch is a brand you find in many tankess and combi units. I imagine they see a lot more cycles in a tankless than a hydronic application. Possibly hundreds a cycles a day in a heavily used DHW application like a hotel.

With a properly sized and installed hydronic system running outdoor reset, there should not be a lot of cycles.

Some have claimed to get constant circulation with a dialed in ODR system. The TRVs like constant flow as they respond within seconds with an always circulating loop.

With the relay, set it to trigger when your smallest zone or loop calls. That could be 1/2 gpm or several gpm. Your design should spell out flow rates of all the individual circuits. Or trial and error.