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I am contemplating installing a heat bank type thermal store, with an external plate exchanger for DHW. The pump on the primary (tank) side of the plate exchanger is usually controlled by a flow switch on the DHW side. I am thinking to use a modulating pump (Grunfos apha2) and a thermal balancing valve on the return of the primary side instead of the flow switch. The idea would be that when there is call for hot water, it will cool the primary return, and open the valve and increase the flow. When there is no call the exchanger would heat up and the valve would close and the pump would modulate down.

Has anyone tried this?
 
Presumably the store is fed from a heat pump and you want it to be a constant flow system? It would work, but, I think you would find that the reaction times for could be quite slow. A flow switch will trigger as soon as hot water is drawn and the flow rate exceeds the set point.

To give this further thought (for me), which manufacturers valve are you proposing to use and what are the signal outputs it delivers for integration into the control system?
 
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Presumably the store is fed from a heat pump and you want it to be a constant flow system? It would work, but, I think you would find that the reaction times for could be quite slow. A flow switch will trigger as soon as hot water is drawn and the flow rate exceeds the set point.

To give this further thought (for me), which manufacturers valve are you proposing to use and what are the signal outputs it delivers for integration into the control system?

My thermal store will be heated by a gas boiler. I would go with an internal coil for DHW, but they are typically sized too small if you want run the store a bit cooler, say 60-65C. So yes, it would be a similar situation as if the store was heated by a heat pump.

The modulating pump and flow valve should keep the exhanger warm all the time, so there should be no lag when the water is first turned on. As the cold water cools the primary, the balancing valve would open to maintain the return temperature. This would cause the pump to increase the flow.

I would use a Grunfos Alpha2 modulating pump and a valve like this:
JG Speedfit ¾ - https://www.NoLinkingToThis/p/jg-speedfit-themostatic-balancing-valve/276HT

You can get the valves without the 70C bypass function, which I would not need. The pump would be on all the time. I am not sure how far that valve closes. Ideally you would want one that closed right up like a TRV, but based on the water temperature, rather than air temperature. I also wondered if a 3 port TMV would work, with one port blocked, as they are easier to find.

The nice thing is that there would be no wiring or switches, just power to the pump, and it would work even if the hot water flow was very small, which can sometimes play havoc with a flow switch. It might also help to maintain stratification, especially in the case of low flow, as only enough water to keep the exchanger warm would move. I was thinking to set it to keep the return around 45-50.
 
So the control loop for the “call for heat” will be completely separate?

Yes. The boiler will heat the store water directly, and will be controlled by a cylinder stat.

For the central heating, I will again use a modulating pump with TRVs on all the rads. There will be no need for a room stat.
 
For the central heating, I will again use a modulating pump with TRVs on all the rads. There will be no need for a room stat.
How does this meet the Building Regs requirement to have a boiler interlock?

A boiler interlock is defined as follows:

System controls should be wired so that when there is no demand for space heating or hot water, the boiler and pump are switched off.
 
How does this meet the Building Regs requirement to have a boiler interlock?

A boiler interlock is defined as follows:

System controls should be wired so that when there is no demand for space heating or hot water, the boiler and pump are switched off.

If there is no demand, then the cylinder stat will shut the boiler off, so I believe it should qualify as an interlock.

Otherwise, a plain unvented would be illegal, as you could have the boiler running even if there was no demand for heating or hot water at that moment.
 
That will make an interesting conversation with the Building Control inspector.

What's a "plain unvented"?

I meant if you have a standard setup with an unvented cylinder, there will be periods where there is no heat demand, and no hot water demand, but yet the boiler is still on to recharge the cylinder. That also appears to violate the rule as worded, no?

I believe the rule is intended to prevent people from using the boiler max flow water temperature as the only means to control the boiler because it is inefficient, as when the house warms up the boiler will constantly cycle.
 
I meant if you have a standard setup with an unvented cylinder, there will be periods where there is no heat demand, and no hot water demand, but yet the boiler is still on to recharge the cylinder. That also appears to violate the rule as worded, no?
If that was the case then only a combi boiler would be acceptable for hot water Any system which used stored hot water would fail the test. Recharging the cylinder is a legitimate reason for having the boiler on. The regulations also specify how well the cylinder needs to be insulated, so reducing the heat loss from the cylinder.
 
If that was the case then only a combi boiler would be acceptable for hot water Any system which used stored hot water would fail the test. Recharging the cylinder is a legitimate reason for having the boiler on. The regulations also specify how well the cylinder needs to be insulated, so reducing the heat loss from the cylinder.

Exactly, so a cylinder stat is a legitimate boiler interlock as well.

In the system I am talking about, recharging the cylinder is the only reason to have the boiler on.
 
Exactly, so a cylinder stat is a legitimate boiler interlock as well.

In the system I am talking about, recharging the cylinder is the only reason to have the boiler on.
In your system the boiler is turned off when the cylinder is up to temperature. But the regs say the boiler has to turn off when there is no demand for space heating or hot water, which is not the same thing. The boiler could continue heating the cylinder for some time after the ch/hw demand had ended.
 
The boiler could continue heating the cylinder for some time after the ch/hw demand had ended.

This happens in all normal unvented cylinder installations.
 

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