Have I spotted a worrying failure mode on our ASHP system or missed something?

[MikeSharp01]

Spent the day working through Failure Modes and Effects Analysis (FMEA) of our ASHP system. I have hit on one which has me wondering if I have missed something.

 

When the ASHP goes to heat the Domestic Hot Water (DHW) it provides a voltage to the changeover valve in the house that switches the ASHP flow from the Under Floor Heating (UFH) circuit to the DHW heating circuit. All well and good except that there is no feedback to the ASHP, that I can spot, that confirms the valve has actually moved and the the circulation is now running round the DHW circuit. In the situation where it fails to change over we will get the much hotter water, normally reserved for heating the DHW, flowing into our UFH which as a strict upper limit on the floor surface because of the floor coverings. There is a temperature probe feeding the DHW cylinder temperature back to the ASHP but the lag on that could be significant even supposing the ASHP has a mechanism for detecting that there is no rise in temperature of the DHW despite it apparently delivering 55^oC water to the cylinder. This seems like an oversight to me, have I missed something or am I expected to provide an external solution - any thoughts or observations?

[DamonHD]

I agree that that fault mode is plausible.

[JohnMo]

15 minutes ago, MikeSharp01 said:

Spent the day working through Failure Modes and Effects Analysis (FMEA) of our ASHP system. I have hit on one which has me wondering if I have missed something.

 

When the ASHP goes to heat the Domestic Hot Water (DHW) it provides a voltage to the changeover valve in the house that switches the ASHP flow from the Under Floor Heating (UFH) circuit to the DHW heating circuit. All well and good except that there is no feedback to the ASHP, that I can spot, that confirms the valve has actually moved and the the circulation is now running round the DHW circuit. In the situation where it fails to change over we will get the much hotter water, normally reserved for heating the DHW, flowing into our UFH which as a strict upper limit on the floor surface because of the floor coverings. There is a temperature probe feeding the DHW cylinder temperature back to the ASHP but the lag on that could be significant even supposing the ASHP has a mechanism for detecting that there is no rise in temperature of the DHW despite it apparently delivering 55^oC water to the cylinder. This seems like an oversight to me, have I missed something or am I expected to provide an external solution - any thoughts or observations?

An ASHP feeding UFH will only add heat slowly as flow temp rise is capped by dT between flow and return.  If I set mine to batch charge - 35 Deg target point, it will take several hours of full load run to get anywhere that target. So if you set timed windows of one hour for DHW, you will have zero issue, if the 3 port diverter failed. Only cold water coming out the tap.

[dpmiller]

one benefit of a TMV on the manifold...

[MikeSharp01]

27 minutes ago, dpmiller said:

one benefit of a TMV on the manifold...

How so?

[dpmiller]

9 minutes ago, MikeSharp01 said:

How so?

stops high temps from getting to the floor

[MikeSharp01]

13 minutes ago, dpmiller said:

Stops high temps from getting to the floor

I guess I will look into that but I suspect it will be simpler just to put a stat on the manifold in the ASHP enable line to shut it down if the water temp at the manifold gets too high.

[JohnMo]

17 minutes ago, MikeSharp01 said:

I guess I will look into that but I suspect it will be simpler just to put a stat on the manifold in the ASHP enable line to shut it down if the water temp at the manifold gets too high.

That is a normal approach, cheap pipe stat.