How to control circulator pumps in radiator system with TRVs?

EN

Hello,  I am trying to design a my hot water radiator heating system. Initially, I was going to use thermostats, sentry valves, and a multiple zone controller, hooked up to a circulation pump on my closed side, and a circulation pump on my open side (instant hot water heater) that runs through a buffer tank and is connected using a heat exchanger.



However, I'm starting to look closer at just having each radiator have a TRV on it, and forgetting all the cost and complexity of sentry valves, manifolds and zones.  Also, with a manifold and sentry valves, I have to worry about balancing the loops, which could be a pain.



An alternative, and much easier solution would be: like before, have my instant hot water heater with a pump, going to buffer tank and heat exchanger.  But on the closed-loop side of the exchanger, just have a hot water (140 degree) supply loop that runs to each of my 4 radiators, 3 baseboard heaters, and 1 towel rack (total loop will be < 110 feet), and then a reverse return loop.  No sentry valves, no manifolds, just a simple loop and a pump.



This scenario would be much simpler and cheaper, and I don't have a problem adjusting the TRVs directly on the radiators, rather than using wall-mounted digital devices.



However, since I am now lacking electronic control, how do I tell my pumps when heating is needed?  I don't want to run my circulation pumps for no reason, and certainly don't want my my instant hot water heater going for no reason.



How can I get around this?  What is the normal scenario for telling the pumps "when to run"?  I understand that a delta-pressure pump can determine when a TRV has been opened, and ramp up, but even this wouldn't help with the "open" side of the system going to the heat exchanger, as its pressure does not change due to influences on the closed side.



Thanks a million!



-Ian

Eastman

Flow switch

You could use a flow switch.



Where are you located?  How many btus does the system need to deliver?  Instant hot water heaters are usually a poor choice for a heat source.



**Ignore my answer.  I was not thinking about the buffer tank.**

Tom_133

If I understand correctly

Ian,



The issue I see in your setup is no call from anywhere to turn on pump?



If this was my scenario I would put a Alpha pump on, TRV's on each heating unit and a 24 volt three way zone valve that would open and close based off a room thermostat in the biggest room. If you are trying to zone each based off the temperature in each room you may need to use a TRV with an end switch.

Ironman

Buffer Tank

Use an ETC and put its sensor in the buffer tank well. Let it control the circ on the open loop and use a Grundfos Alpha on the closed loop side. Power the Alpha continuously.

Eastman

Trying to figure out what you've described.

I don't quite follow your description.  The heat exchanger requires both pumps running to function.  If the pump on the open loop only operates while the instant hot water is recharging the buffer, how is heat exchanged continuously into the closed loop system?



Oh wait, you're saying the heat exchanger is inside the tank?

Ironman

Indirect Tank

Exactly. I'm thinking of using an indirect tank and letting the control bring on the charging pump, which would cause the tankless to fire, similar to how it would be with a boiler and indirect.



I don't think the tankless is a good idea, though. I would just go with a combi boiler if looking for a low end setup. It would cost no more, maybe less, than a tankless and buffer or indirect.