How to control primary flow rate of plate exchanger based on DHW demand

[sirocosm]

I am planning the installation of a thermal store in my house. I am thinking to use a plate exchanger for DHW. One problem compared to an internal coil is that for full output the flow on the store side of the exchanger has to be quite high, which disrupts the stratification of the store. I would like to therefore modulate the flow rate on the store side of the plate excahnger based on DHW demand.

I was thinking a reliable way to do this would be to throttle flow on the store side of the exchanger based on the temperature of the water going through it. This could be done with a thermostatically controlled valve, and a pressure modulated pump, such as a Grundfos Alpha 2. It would also eliminate the need for a flow switch on the demand side.

My question is does anyone know what valve could I use for this purpose? It would be like a large TRV but would need to depend on the water temperature rather than the air temperature. Perhaps a normal TMV would work with one leg capped?

 

[echoes]

You can get valves intended for under-floor heating with remote temperature sensors.

This thread is worth a read, if you've not already... https://www.diynot.com/diy/threads/heat-banks.137289/page-5

 

[echoes]

Don't quite see how you would manage without a flow switch though. Also need to avoid the pump speed from fluctuating due to delayed feedback.

 

[sirocosm]

You can get valves intended for under-floor heating with remote temperature sensors.

This thread is worth a read, if you've not already... https://www.diynot.com/diy/threads/heat-banks.137289/page-5

Thanks for that. They talk about the Gledhill doing something very similar, on page 4 ""Gledhill had the best solution IMHO using a variable speed pump and electronic feedback from the DHW output on the plate. "

Instead of electronic feedback with a temperature sensor which requires a special pump, I was thinking to use a simple thermostatic valve that closes when it gets warm. This would cause a pressure modulating Alpha to pump more slowly.

 

[sirocosm]

Don't quite see how you would manage without a flow switch though. Also need to avoid the pump speed from fluctuating due to delayed feedback.

You wouldn't need a flow switch because the pump on the primary side of the DHW exchanger would always be on, and thus also always warm.

 

[echoes]

I understand. That thread does discuss the pros and cons of a pressure modulating pump, particularly in regard to possible instability in the feedback from the temperature sensor on the DHW side of the heat exchanger, the valve and the pump, which may cause the pump speed to 'hunt'.
They solved this (in theory) with a flow regulated shunt across the pump.

I'd be interested in your findings, since I have a very similar system, but I use a crude, open loop, electronic duty-cycle control of the pump together with a flow rate sensor. This works OK, but at high flow rates I do end up with a cylinder of tepid water before too long.

 

[sirocosm]

Thanks again for pointing me to that thread. A user named tsoutwood suggests exactly what I am thinking to do, but they were last seen in 2009. I think also found the valve I am looking for, it is related to the UFH valve. It is called a thermostatic balancing valve, often used to balance a multi zone system. They are adjustable and they close when they get to a set temperature.

I would put the valve on the return of the primary(store) side of the DHW exchanger. If the water in the return is warm, there is no demand and the valve would close.

My current setup does a similar thing with the rads, also mentioned in that thread. TRVs all around, with an Alpha 2 modulating pump and no wall stat. I can tell how many rads are on by looking at the wattage that the pump is using.

 

[echoes]

I see. Rather than use the actual DHW temperature as a control, you intend to use the 'cold' end of the primary.
I'd run through some scenarios, since depending on the temperature of the water in the cylinder, you might get some undesirable effects.

 

[sirocosm]

I am not completely sure how the DHW would drop off as the tank cools, as the primary would flow faster and cause more mixing.

I was thinking that if the DHW side goes from 10 or 15C in to 5OC out, if the tank normally runs at 60C, I could set the target return temperature for 40C or or maybe even less. I am not sure if I could use the same target temperature in winter vs summer, it seems it would be dependent on the mains temperature.

Indeed it would be nice to know if anyone has tried this. If it is a total bust, I can always go back to a flow switch.

 

[echoes]

I've not tried it, but was considering having DHW temperature feedback to either throttle the primary flow (diverting through a shunt) using a remote sending valve, or control the pump duty cycle via a DHW temp sensor electronically. But I would have a flow switch in either case so the pump would only run on HW demand.

 

[sirocosm]

The reason I thought to skip the switch is because the setup works so well with the Alpha and TRVs on all the rads. When all the TRVs are closed the pump power consumption drops to 4W. Plus it would keep the exchanger warm, meaning quicker hot water. If you wanted to be really posh, you could put the exchanges near the point of use, for almost instant hot water.

I was also worried that if you want a slow trickle of lukewarm water at the kitchen sink, the flow might not be high enough to trigger the switch.

 

[hard-work]

He posted the pictures of what he built.
https://www.diynot.com/diy/media/albums/thermal-store.1671/

 

[hard-work]

What you propose is:

1. A 2-port thermostatic valve in the plate primary side
2. A smart pump on the primary side.
3. The thermostatic valve has a remote sensor on the DHW outlet right after the plate.

Correct?

The smart pump is always running at a trickle to keep the plate warm when no there is no DHW demand - when the thermostatic valve is closed the pump slowly turns. The thermostatic valve will occasionally open slightly as the temperature drops slightly.
When a DHW draw off the plate temp drops opening the thermostatic valve which in turn has the smart pump increase its speed, pumping through the plate more hot water from the store.​

The aim (or hope) is that the smart pump will only revolve at the speed matched to provide the necessary DHW to prevent disturbance of the stores stratification.

The smart pump would need some sort of adjustment is it operates independently of the mechanical thermostat. It may work. Inline restrictors may be need for adjustment. All a bit hit and miss and experimental. But it may work. It would be interesting to see the results.

If you want something that clearly works, use the pcb, temperature sensors and modulating pump Gledhill used on their Systemate. The pcb is an off the shelf item made in the Czech Republic or Austria. The parts should be easy to source. YOu cam still have a full DHW and CH thermal store using this pcb.

Another good thread:
https://www.diynot.com/diy/threads/dps-thermal-store-dhw-performance.275808/page-4

The Gledhill pcb:
https://www.keeptheheaton.com/produ...yJhN2d2JaKrxVwBa_6FwD1XZwE8wvx8kaAphoEALw_wcB

Some pcbs are reconned and cheaper.

I like these threads.

 

[sirocosm]

Thanks for the links.

What I have in mind is almost as you have written, except for #3. The thermal valve would sense the the return from the primary, no need for a remote sensor. Demand on the DHW side would cool the primary side and that is all you need to detect (I think).

It would be very simple. The 2-port thermal flow valve is analog and requires no power, and the only electronics are inside the pump, which is a relatively cheap commodity item nowadays. It is essentially the same as fitting a smartpump to a radiator with a TRV. The only difference is the TRV senses air temp so it could be on either side of the rad. Here the thermal flow valve would sense the water temperature, so it would need to be on the return so it could sense the cool side of the primary. If it drops, it opens up to increase the flow.

 

[hard-work]

"The thermal valve would sense the the return from the primary". What do you mean, "the return"? The the thermo valve is after the plate heat exchanger? (where the pump will be). You need DHW temperature control. What you propose, having the DHW temperature control only on the primary side will not work.

Having 1 to 3 as I described above, will probably not work, as two separate control systems (the internals of the pump and the thermo valves) may be fighting against each other. The DHW temperature thermo valve in the DHW pipe, will open and close to the DHW setpoint. This raises the flow and pressure of the water running through the smart pump, which will raise and lower its speed out of sync' with the DHW setpoint.

A direct control link has to be between the DHW setpoint sensing location and the modulating pump.