Frozen Heat Pump

the_lunatic_is_in_my_head

NedS said:

There was an interesting video posted on youtube a couple days ago, stressing the importance of system volume, which is particularly relevant to defrost cycles:

Samsung recommend a minimum system volume of 50L for your 12kW gen6 heat pump. Realistically, something around 180-200L would be recommended and should ensure there is no issues performing system defrosts once temps drop down to around 3C or below.

The video explains how you can calculate your system volume, made up from water in the radiators, pipework and any volumiser tank.

Thanks @NedS very useful.  

We have 12 double panel rads + 1 single panel, similar mix of sizes to his chart just without the volumiser so certainly well over 50l. 

He does mention zoning, our house had a weird layout with zone 1 being smaller than 2 plus on a much lower temperature, possible this is the issue and zone 1 needs the temp cranking up to match 2 for the defrost to work more efficiently.

I'll try that as a starting point next time the outside temp drops and see if there is any improvement.  

matelodave

It wouldn't surprise me if the zoning was part of the problem (assuming that there is a problem).

Likewise TRV's, by closing a zone or with TRV's shutting down radiators you are significantly reducing the system volume when it could be trying to defrost.

Does your heatpump circulating pump shut off when the room stats aren't calling for heat, if so, it really doesn't matter how much water is in the system if its not being circulated. AS you said earlier if you shut the system down then it cant defrost as the hot water stopped circulating.

Like any system you need to work out what is happening and when, understand what your controls are doing and how they are set.
Record that info and then try adjusting one parameter at a time, giving the system sufficient time to settle and take effect (at least 24 hours, but preferably more if there are major weather changes). Record what you did and what happened and then perhaps try changing something else but only one thing at a time and make sure you record what you did so you can revert back.

Just tweaking stuff without giving the system a chance to properly react and settle or randomly adjusting things without a bit of a plan wont help to set the system properly.

the_lunatic_is_in_my_head

matelodave said:

Does your heatpump circulating pump shut off when the room stats aren't calling for heat, if so, it really doesn't matter how much water is in the system if its not being circulated.

I would assume so as I was under the impression that was how the system was intended to work, i.e you set your thermostat at x and leave it alone so if that was set at 18 the house sits at 18 all the time and the heat pump (+ by extension water circulation) is only occurring at the point required to ensure the temp doesn't drop below 18? 

matelodave said:

Likewise TRV's, by closing a zone or with TRV's shutting down radiators you are significantly reducing the system volume when it could be trying to defrost.

I don't think the one rad being closed is affecting much but it's not really a problem to leave that open, the other 12 rads are all open on full all the time. 

matelodave said:

It wouldn't surprise me if the zoning was part of the problem (assuming that there is a problem).

This is possibly it, I don't know if the defrost pulls water from zone 1, 2, both or which ever is on/has a suitable flow temp?

michaels

It is definitely worth considering, I have heard of heat pumps drawing from the hot water cylinder to facilitate defrost, not sure if this is design behaviour or if it just depends what heating mode they are in when they decide a defrost is required?

As mentioned above when we were having the very heavy hoar frosts recently my heat pump was tending to ice up to the point it restricted air flow through the fins.  I have probably hobbled the design defrost ability by disabling the auxiliary heater in the pump and have a system volume of only about 125l which it seems may be inadequate.  I as able to force an effective defrost by running the unit in cooling mode for 20 mins.  Luckily this did not seem to lead to an appreciable drop in house temperature.

the_lunatic_is_in_my_head

michaels said:

It is definitely worth considering, I have heard of heat pumps drawing from the hot water cylinder to facilitate defrost, not sure if this is design behaviour or if it just depends what heating mode they are in when they decide a defrost is required?

I may be wrong but I was under the impression the hot water was a closed system so the HW isn't able to be pushed through the heat pump to defrost?

michaels

the_lunatic_is_in_my_head said:

michaels said:

It is definitely worth considering, I have heard of heat pumps drawing from the hot water cylinder to facilitate defrost, not sure if this is design behaviour or if it just depends what heating mode they are in when they decide a defrost is required?

I may be wrong but I was under the impression the hot water was a closed system so the HW isn't able to be pushed through the heat pump to defrost?

The coil in the hot water tank normally has hotter water from the heat pump warming the water in the tank.  In a defrost cycle the heat pump circulates colder water through the hot water tank coil, drawing heat from the stored hot water that is then used to defrost the fins.

the_lunatic_is_in_my_head

michaels said:

the_lunatic_is_in_my_head said:

michaels said:

It is definitely worth considering, I have heard of heat pumps drawing from the hot water cylinder to facilitate defrost, not sure if this is design behaviour or if it just depends what heating mode they are in when they decide a defrost is required?

I may be wrong but I was under the impression the hot water was a closed system so the HW isn't able to be pushed through the heat pump to defrost?

The coil in the hot water tank normally has hotter water from the heat pump warming the water in the tank.  In a defrost cycle the heat pump circulates colder water through the hot water tank coil, drawing heat from the stored hot water that is then used to defrost the fins.

Thank you I see what you mean.  

matelodave

Wen mine goes into defrost mode the circulating pump keeps running (but that's because it runs most of the time anyway - we don't rely on the room stat to control the temp unless the place overheats due to solar gain) and the circulating water temperature plummets to very cold for a few minutes during the cycle.

As we have an underfloor system (overlay rather than in slab) it has sufficient thermal inertia for it to go unnoticed although if you feel the flow pipes you can feel that it gets cold almost instantly when defrost kicks in. It also recover pretty quickly when defrost finishes.