Anti freeze system

DyfedWyn

Now that we've had our first cold night of winter (I live on Anglesey, north Wales) the anti freeze system of my air source heat pump and heating system has kicked in. When the outside temperature falls to 5 degrees and below, the water pump switches on and ensures the water in the system is flowing, thus ensuring it doesn't freeze. All well and good, since that's the whole point of it.

But ... the temperature of the water in hot water tank drops significantly at the same time. Over night, e.g., it dropped from around 46 degrees to 35 degrees. No water was used by me during this period.

This problem happened throughout last winter too (my first winter with the new system - installed under Eco 4) and though the installer sent out an engineer to have a look at it, he was rather perplexed as to what the cause of the issue was.

Has anyone else experienced this issue? I don't believe it to be normal. My neighbours with similar systems don't have this problem. How about anyone else on here?

Thanks.

matt_drummer

Presumably your heating isn't on when this happens?

The heat pump needs warm water to prevent freezing so it circulates the water around your hot water tank and that takes heat from the water stored in the tank.

My heating is normally on at this point so it is never normally an issue.

On the rare occasions when it is 5c or less outside and my heating isn't on my heat pump is set up to run the heating at 26c flow temperature.

Maybe yours can be set up to do something similar?

Or perhaps ensure your heating is always on when it is cold.

Reed_Richards

My system has a buffer tank.  There is a lot of debate around buffer tanks and whether they reduce efficiency by allowing some mixing between the flow and return water.  But in my system it's the water in this buffer tank that is used for defrosting, not the water in the DHW cylinder.

matt_drummer

Reed_Richards said:

My system has a buffer tank.  There is a lot of debate around buffer tanks and whether they reduce efficiency by allowing some mixing between the flow and return water.  But in my system it's the water in this buffer tank that is used for defrosting, not the water in the DHW cylinder.

Hi Reed,

This isn't defrosting though, and a buffer tank or volumiser still needs to have warm water in it for this purpose.

This is frost protection.

I don't what heat pump the OP has but my frost protection is configurable, it can even be turned off.

What the OP is experiencing is not defrosting, it is the self protection of the heat pump to prevent the water inside it freezing.

Reed_Richards

Today it's about 2 C outside.  Although my heating is on and my DHW is on (although set to a lower temperature than the tank is likely to be at because I boost the hot water when I have cheap electricity).  Periodically it reports that it is "defrosting", the LWT becomes lower than the RWT but not at the expense of the water in the heating system or in the DHW cylinder.  I have always assumed it was running in reverse and actively cooling the water in the buffer tank to melt the frost that might otherwise block the air flow to the fans.  I agree that that's not the same thing as just circulating water but how do you tell the difference by observation? 

matt_drummer

Hi Reed,

Defrosting is required when the heat pump is running and ice has formed on the evaporator. The ice blocks the fins and eventually the heat pump can no longer pull air across the evaporator in sufficient volume.

If you can see the heat pump you will see the ice and if you can monitor electricity consumption you will see the electricity use increase as the heat pump struggles to operate.

A valve inside the heat pump that controls the flow of refrigerant reverses the direction of flow of the refrigerant so that it is now warmed up by the flow of water through the heat pump from the home.

The warm refrigerant then melts the ice on the evaporator and operation returns to normal.

As the heat pump is running it takes energy from the house, the water flow direction does not change, it is just the refrigerant that changes its direction of flow.

But the refrigerant is now cooling the water in the house rather than heating it. A buffer tank or volumiser ensures sufficient volume of warm water to carry out the defrost.

What the OP is experiencing is not to clear ice built up on the evaporator as there won't be any at 5c when the heat pump is not running. It is just circulating water to ensure nothing freezes and it gets the water by circulating through the dhw tank as this is the only place with warm water.

The anti frost settings should be adjustable.

As I said, my heating turns on at 5c outside with a flow temperature of 26c when it isn't already running.

NedS

Hi @DyfedWyn

I have a Samsung Gen6 ASHP and mine exhibits exactly the same behaviour. Do you have a Samsung ASHP too?

As you say, when the outside temp drops, the system enters anti-freeze mode and will circulate the main pump to ensure water is flowing and cannot freeze - mine does the for around 5 mins, and is temperature triggered, I think at somewhere below 10C as measured by the LWT sensor in the outdoor unit.

As you say, annoyingly this is circulating through the DHW cylinder, which sucks heat from the DHW. It wouldn't be so bad if it were circulating from the radiators, but I haven't found a way to change this or to disable it. I guess the assumption is that when it's that cold, the heat pump will be running permanently so you would never see this behaviour other than maybe if it were switched off whilst you were away.

We also heat DHW on a schedule, whereas if it were set to reheat automatically upon a given temperature drop, the heat pump would kick in and reheat it whenever it drops and again you'd never notice it.

DyfedWyn

NedS said:

Hi @DyfedWyn

I have a Samsung Gen6 ASHP and mine exhibits exactly the same behaviour. Do you have a Samsung ASHP too?

As you say, when the outside temp drops, the system enters anti-freeze mode and will circulate the main pump to ensure water is flowing and cannot freeze - mine does the for around 5 mins, and is temperature triggered, I think at somewhere below 10C as measured by the LWT sensor in the outdoor unit.

As you say, annoyingly this is circulating through the DHW cylinder, which sucks heat from the DHW. It wouldn't be so bad if it were circulating from the radiators, but I haven't found a way to change this or to disable it. I guess the assumption is that when it's that cold, the heat pump will be running permanently so you would never see this behaviour other than maybe if it were switched off whilst you were away.

We also heat DHW on a schedule, whereas if it were set to reheat automatically upon a given temperature drop, the heat pump would kick in and reheat it whenever it drops and again you'd never notice it.

Thanks for the response. Mine is a Mitsubishi system but probably works in much the same way. Since my original post I've had a chat with an engineer from the company which installed the system for me. As has been commented upon above, he said that the anti-freeze system takes water from either the tank or the rads to keep the heat pump from freezing. I've had the heating off over night since I've had it (can't stand being too hot at night) but he suggested that with having it off there was only warm water from the tank for the system to use. Makes sense. So for last night I set the temp to 17 degrees over night. To be honest, I've no idea if it got cold enough for the heating to kick in over night, but the temp of the tank had dropped again. Even during the day - when the heating is on, as I work from home - the tank temp fell significantly for those periods when anti-freeze kicked in. So unless the heating is permanently on (mine is regulated with a thermostat, as is most people's probably) I can't see that the water in the rads is warm enough for the anti-freeze system.

Luckily the temp here doesn't drop very low too often during the winters, so it's not going to be an issue for me on a regular basis. 

NedS

@DyfedWyn After responding yesterday I googled the issue some more, and did come across one thread which suggested this "feature" can be disabled on a Mitsubishi (no such luck on my Samsung unfortunately).

https://renewableheatinghub.co.uk/forums/renewable-heating-air-source-heap-pumps-ashps/help-needed-please/paged/3#post-15705

It would appear that you can deactivate the 'freeze stat' function as shown below.

Freeze stat function
Menu subtitle Function/ Description
Freeze stat function *1 An operational function to prevent the water circuit from freezing when outdoor ambient temperature drops.
Flow t. The target outlet water temperature at water circuit when operating in Freeze stat function. *2
Outdoor ambient temp. Minimum outdoor ambient temperature which freeze stat function will begin to operate,
(3 - 20ºC) or choose**. If asterisk (**) is chosen freeze stat function is deactivated. (i.e. primary water freeze risk)"

*1. When the system is turned off, freeze stat function is not enabled.
*2. Flow t. is fixed to 20°C and unchangeable.

I found this on page 45 of the FTC5 manual.

Do you have Glycol in the system? If not, then you must leave this enabled otherwise the system may freeze, but if you do have glycol and are confident your circulating fluid will not freeze at the ambient temps you experience, then you could disable the freeze stat function as described above.