Turning on the boiler in an all TRV controlled radiator setup

cliftonhall

I'm converting from a single-zone, primary loop only, massive pipe system with 20 old, uncontrollable radiators (5,000 sq ft house) to a primary/secondary homerun setup, pex-al-pex, new boiler, and so on with the help of this site and Dan Holohan's books. The new radiators will all have TRVs so each room is its own zone.

I've had to do this conversion in stages. I created a primary loop with the new boiler and created 2 secondary loops, one with a manifold for the new homerun setup and another for the old system. Everything is working well, and I'm slowly moving segments of the house to the new secondary, adding TRVs, etc. Eventually, I'll have two or three secondary loops, each handling a manifold and 7 to 10 radiators, all with TRVs.

Based on advice here, I use a autosensing, multispeed circulator on the new secondary so it pumps away when any of the TRVs open up. The old system still rules the heat though since its thermostat turns on the boiler.

The piece of info I haven't found yet is how I control the boiler when I complete my conversion and all my radiators have TRVs. I assume there must be a way to turn on the boiler when any of the TRVs call for heat and start water flowing. The autosensing circulator fires up and can that fire up the boiler?

Note that I have a fixed speed circulator on the primary loop to ensure minimum water flow required for the boiler, so what I'm looking for is a way to turn on the boiler and its circulator when a TRV calls for heat.

Ironman

There are two basic approaches:
1. Put a thermostat in the coldest room of the house and let it turn the system on and off.
2. Get your outdoor reset curve tweaked to where it maintains the proper SWT so that a stat is not needed.

Or, you can do a combination of both. The second is more difficult to achieve if you have different types of emitters requiring different SWTs.

Jamie Hall

If there is no handy way to extract the on signal from the circulator -- and there may not be -- there are any number of flow sensors available which can be used.

EBEBRATT-Ed

What type of boiler do you have ? Mod Con or cast iron?

hot_rod

A hydronic contractor friend in Alaska uses these on his TRV systems. Basically run the wires to the circulator thru the opening and it senses the current draw.

They have been used for years on humidifiers added to furnaces, this company has low current draw versions.

I used one just to start the delta P circulator, from a buffer tank, but it could also send a boiler signal, and pump controls from boiler.

bburd

One very important point:  if you use an indoor thermostat located in the coldest room in the house, the radiator(s) in that room MUST NOT have TRVs. If they do, someone could close the TRV’s and then the thermostat might never be satisfied, overheating the rest of the house.

cliftonhall

Thanks all for the advice. Looks like between flow sensors and current sensors there will be a solution.

EBEBRATT-Ed, I have a Mod Con boiler.

bburd, a good point and actually a problem I've had to work around while I'm in my partially converted state. I've had to move the thermostat around the house to fire the boiler for the old, non-TRV loop so that the TRV loop will get any heat.