Hi

Welcome to the forum.  Please feel free to ask any questions.

In order to help can you give a bit more detail about how it locks up, what is happening at the time, and particularly you say you are heating 'air and water', what exactly do you mean?

Our LG ASHP (split) is supposed to heat both rooms and water.  It does this fine when the outside temp is above 15deg.  However, if the outside temp is low and a lot of hot water is drawn it goes into "Anti Freezing" and gets stuck there.  The 2-way valve is trying to heat the water but the HP is unwilling. An LG employee said this can't happen although he did admit that it should shut down if the return temperature is low (apparently to protect the heat exchanger).

So I have to prevent it from trying to heat water on the coldest days (and heat it using the immersion), or not use the bath.

This is not what I expected.

Posted by: @paul_tanner

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Our LG ASHP (split) is supposed to heat both rooms and water.  It does this fine when the outside temp is above 15deg.  However, if the outside temp is low and a lot of hot water is drawn it goes into "Anti Freezing" and gets stuck there.  The 2-way valve is trying to heat the water but the HP is unwilling. An LG employee said this can't happen although he did admit that it should shut down if the return temperature is low (apparently to protect the heat exchanger).

So I have to prevent it from trying to heat water on the coldest days (and heat it using the immersion), or not use the bath.

This is not what I expected.

It sounds like it has insufficient system volume for the defrost (energy for the defrost comes from the hot water in the system).  This problem is soluble but may need a volumiser or another change to the system.

Can you confirm

"It does this fine when the outside temp is above 15deg" do you mean 15C, it shouldnt start defrosting until about 5C

"The 2-way valve is trying to heat the water but the HP is unwilling."  So does this problem only occur when the heat pump is trying to heat DHW and/or only when heating DHW after you have drawn a bath

"The 2-way valve is trying to heat the water but the HP is unwilling."  Do you mean 2 way or do you mean diverter valve (either space heating or DHW)

What are the settings for your DHW, is it timed (if so when) or on continuous reheat?

Do you have any kind of volumiser or buffer tank, if so what can you tell us about it and in particular where is it in relation to the diverter valve?

What make, model and size is the DHW cylinder?

Yes. I meant 15C. As I mentioned, an LG guy said this is the lowest the return temp should go.

The problem is after drawing a bath. water is priority so diverter favours water.

Hot water is continuous heat.  I don't see any setting on this LG Therma V to time it.

Yes we have a Cordivari buffer tank.  It has 4 pipe connections, top 2 on the flow side of the HP and bottom 2 on return side.

The cylinder is a 210L world heat "Climaclyss"

Posted by: @paul_tanner

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Yes. I meant 15C. As I mentioned, an LG guy said this is the lowest the return temp should go.

Ah that makes sense (I thought you meant an OAT of 15C).  It confirms that the problem is insufficient water available from the system for defrost.

Posted by: @paul_tanner

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The problem is after drawing a bath. water is priority so diverter favours water.

Hot water is continuous heat.  I don't see any setting on this LG Therma V to time it.

That also helps confirm the problem.  I dont know the ThermaV well but Im surprised it doesnt allow timed reheat, although given how bad the problem is that would probably just 'put it off'.

Posted by: @paul_tanner

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Yes we have a Cordivari buffer tank.  It has 4 pipe connections, top 2 on the flow side of the HP and bottom 2 on return side.

The cylinder is a 210L world heat "Climaclyss"

This should definitely be fixable, the key is that there must be sufficient water accessible to the heat pump during defrost to melt the ice.  Evidently there isnt.  There could be various reasons and the buffer tank complicates things.  So a couple more questions:

• What is the relationship between the buffer tank, the diverter valve and the DHW tank (in terms of the plumbing and order of the various elements) - a sketch would be ideal!
• What is the size of the buffer tank?
• Can you confirm that this is the cylinder?
• What is the approx floor area of your house and what are the emitters/how many?
• What model heat pump is it?

Plumbing: pls see https://docs.google.com/drawings/d/1HBV5rq6VGlIHMFC2OIHCt3xAr8k6glABxneqvHnanVQ

Buffer tank: 51L

Cylinder: yes, that one

Floor area: 160 m2

ASHP model: HN1616M NK5 + HU121MA U33

OK, all as I guessed.

So, if the HP is heating DHW when defrost occurs then the defrost energy has to come from the DHW tank, unless it switches away from DHW during the defrost operation (I have no reason to believe it would, but at the same time no reason to know that it doesnt).

If you have just run a very large bath, its entirely likely that a substantial part of the the tank at the bottom is cold, at or near the temperature of the incoming mains.  The top may still be hot but the bottom will be cold, assuming the tank is tolerably stratified. 

The water from the heat pump will enter towards the top and exit towards the bottom, so it will to some extent carry heat from top to bottom but also tend to be cooled just before it exits en route to the heat pump, where it has the job of melting the ice.

Taking all this together its plausible at least that the heat pump cannot extract sufficient energy for the defrost (particularly if the defrost is early on in the reheat cycle), the return temperature falls below 15C, and so the heat pump shuts down.  I guess this is a risk with all heat pumps but yours is the first time I have seen somebody report it here.

You might want to have a bit of a feel around pipework during defrost to check if Im right and make some observations on timings etc.

Even if Im not precisely right I am pretty certain that the underlying problem is that the heat pump cant (or doesnt) extract sufficient energy from the system for defrost, and it it that you need to tackle.  

I can think of a few options, some of which depend on the precise explanation, some of which are a bit more tolerant to the precise detail.

1. Have a smaller bath

2. Ensure that the DHW is heated before you draw the bath

3. Heat the DHW to a higher temperature so less is drawn off during a bath

4. Provided it is located near the bottom, enable the immersion during defrost (the LG may directly support this as a 'backup' mode

5. If your LG has an inline willis heater inside the casing (ie a backup heater), enable this during defrost (I think thats called something like rapid defrost mode on the LG)

6. When its cold outside (5C or below), use the immersion heater instead of the HP to heat DHW

7. Add a volumiser (2 port buffer) in the return between the cylinder and the HP but (I think - subject to change) after the point where the return from the DHW joins the return from the emitters.  Then you should always have at least some water in the system at elevated temperature.

8. Get a bigger DHW tank

There are probably a few others, but essentially you are trying to ensure that a few 10s of l of warm water are available for defrost.

Time, I think, for some experimentation!

Many thanks, this is very helpful.

I have been using "6" i.e. turning off DHW when it's cold and using the immersion instead.  This is expensive but at least the heating is protected.

I will look into the other options.

Regards.

Posted by: @paul_tanner

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Many thanks, this is very helpful.

I have been using "6" i.e. turning off DHW when it's cold and using the immersion instead.  This is expensive but at least the heating is protected.

I will look into the other options.

Regards.

If you have any cheap tariff the cost difference might not be that great.  Its cheaper for me to heat DHW on the immersion at night, than on the HP during the day (of course HP during the night is cheaper still, but as it means I do less heating during the night maybe not cheaper in total!

I have a 16kw LG Therma V. Used for over 4 years without problem.

Are you using weather compensation (AI)? 

There is an excellent LG User Facebook group which includes installers & engineers. Might be worth asking there

Good point. Thanks.

Ideally I should get the HP to not try to heat water at night and instead heat the water with the immersion during the low-tariff period.  The LG controller does not seem to cater for timed hot water but I guess there's a possible solution around an external device blocking the diverter when the DHW temp is low.