Jump to content

ASHP Frosting Cure


SteamyTea

Recommended Posts

After a recent conversation with @joe90 about ASHPs frosting up, I have had a few minutes to think about this problem.  As usual, data is needed.

Does anyone monitor the input and output air temperature and relative humidity of their ASHP?  This would be a couple of sensors either side of the radiator and in the airflow.  Ideally knowing when the ASHP is on would be useful as well.

 

My thinking of the frosting problem is to monitor the temperature and RH on either side and calculate the dew point temperature.  Initially the ASHP could be turned off, for a set period of time, when it reaches the median dew point.  This could be offset differently with experience and data for the fine tuning

If anyone is at my end of Cornwall I would be willing to set up some logging, but as not many members are down here @bissoejosh being the only one I can think of and have no idea if he has an ASHP, it would be down to others to help me out.

 

So help needed.

Link to comment
Share on other sites

It would be an interesting research project, what is more efficient:

  • ASHP alternating operating/defrost.
  • ASHP reducing power to prevent frosting and direct electric backup to compensate.

Obviously it is heavily environmentally and ASHP design dependant but some general trends that could be applied to making the units more efficient.

Link to comment
Share on other sites

Yes ST and I discussed this as I initially thought the ASHP could turn off just before defrosting occurred, therefore you could get the highest water temp for given weather conditions. I thought the ASHP should know when it’s about to defrost ( then  go into defrost mode) but perhaps it recognises when it’s frosted up, too late?. Does anyone know how an ASHP knows it needs to defrost?

 

ST, you can monitor mine when I get round to wiring it up ( among the other jobs on my most important to finish list ?).

Link to comment
Share on other sites

1 hour ago, le-cerveau said:

It would be an interesting research project,

Yes, and one that we can easily do on here I think.

 

I always like to collect data first as that gives a baseline to work to.

17 minutes ago, joe90 said:

ST, you can monitor mine when I get round to wiring it up ( among the other jobs on my most important to finish list

I will, and have remembered that I work with someone that has an ASHP.  I think the whole estate has them and I know a couple of his neighbours too.

Better make up a simple, reliable monitor that can work outside.

Edited by SteamyTea
Link to comment
Share on other sites

An off-steer, but....

Oversize the ASHP when you know the local climate will put you into defrost cycles? From what info I've gathered here, space heating wouldn't necessarily put you into the realms of 'over-working' and frost, but definitely a subject I will follow with interest. 

Seems matching demand with the HP is what to avoid, and running DHW temps seems to be mostly what's problematic as the HP will be driving harder for longer. 

Multi-faceted as always, as size of HP and load will be a major factor, along with the micro-climate. 

Link to comment
Share on other sites

To a certain extend over sizing is the cure, but then if it does frost up it either takes longer to defrost, or more energy to defrost.

 

The general idea is to sense when defrost may occur, and stop (or slow down) the ASHP.

So taking a day like yesterday that was hot and dry, it could run a higher output temperature, today it could not.

The tricky bit is getting a decent external Temp-RH curve to work with, and knowing how much that curve can be varied for different conditions.

Link to comment
Share on other sites

So here is a time series chart from yesterday, as long as the ASHP turns off (or reduces power output) before the air after the radiator is above 12.35°C (the highest dew point temperature), then there should be no serious frosting (though the radiator fins may be colder).  By finding the sweet spot between the ambient air temperature and the 'exhaust' temperature, it should be possible to run at a higher CoP over a given time period.

Obviously there is a minimum desired temperature for any hot water and the ASHP may not be able to deliver that, but that just leaves less of a top up.

What it really boils down to is reducing the defrost cycles, but without cycling the ASHP too often during for  frosting avoidance.

example Dew Point.jpg

Edited by SteamyTea
Link to comment
Share on other sites

Actually, I think the big lesson from that is the potential value of weather forecasts to improving COP. Most of the hot water will be used in the morning in an average household, and not further required until the evening. Putting an inhibit on the unit until 10am and then working at max power would be a very effective strategy on that day - whereas if it was cold all day then the best strategy would be to run at lowish power rates intermittently throughout the day and leave the unit to defrost naturally in between. The whole Internet Of Things will actually make this quite easy to implement.

  • Thanks 1
Link to comment
Share on other sites

If there was a local, and more importantly, dependable, web based weather service, that had up to the minute data for your location, I would agree.

Trouble is there isn't.  I have a Met station close to me, but cannot access the live data.

I also think the IoT's potential is overstated and very little benefit to be had.  And the security is going to be truly dreadful.

Link to comment
Share on other sites

11 hours ago, SteamyTea said:

If there was a local, and more importantly, dependable, web based weather service, that had up to the minute data for your location, I would agree.

Trouble is there isn't.  I have a Met station close to me, but cannot access the live data.

I also think the IoT's potential is overstated and very little benefit to be had.  And the security is going to be truly dreadful.

This isn't going to work as a homebrew system - it would have to come from someone like Nest who could afford to pay for met data (incidentally, XCWeather.co.uk has got all the most recent met data plus forcasts on it so the data must be out there somewhere). I don't think accuracy is all that critical though - the heat pump would continue to provide heating and hot water no matter how good the data is, what it does is improve the COP significantly by reducing defrost time and the dT it has to pump heat across.

 

IoT is one of those things that is actually really useful, but nobody really knows how best to use it yet - 3D printing is another example where we're probably only seeing 5-10% of the ultimate capability being exploited. Security for this sort of thing is also probably a lot easier than for other applications - unless you're doing demand response type applications, you set it up as essentially a broadcast system that the heat pump uses as an advisory to work out the most efficient profile for the day.

Link to comment
Share on other sites

Ah, Nest. Amazon has already stopped selling it.

 

I can't see any real advantage in knowing the weather hours in advance for an ASHP.  You might be able to 'top up a bit more' the day before, but I think they will be small gains in the scheme of things (tomorrows weather will be within 80% of today's.  Just that the 80% is not defined too well :D).

That is why I think a stand alone device is probably the better solution, it should really be built into an ASHP as standard.

One of the problems we have on this forum is that we have a higher level of knowledge and realise that there are problems, and we can usually work around them.  This is not true of the general public who think that electricity comes from unicorn farts and a hot tap always delivers hot water.

Edited by SteamyTea
Link to comment
Share on other sites

Perhaps I am wrong but are we barking up the wrong tree?. Rather than predicting weather is it not better to monitor how the ASHP is reacting to the weather conditions, I.e. “ am I about to defrost?” 

18 hours ago, joe90 said:

Does anyone know how an ASHP knows it needs to defrost?

 

Just an idea!

Link to comment
Share on other sites

1 hour ago, joe90 said:

Just an idea!

That is the idea.

49 minutes ago, dpmiller said:

the ASHP will have ambient temperature sensing for defrost control; how many people set it up for their own locality?

I don't know individual models well enough, but by the time it has frosted up, it is too late.

35 minutes ago, Dreadnaught said:

Beginner's question: as a local sensor, why would it benefit from being tuned?

The Temperature/RH curve is tuned, not the sensor.

The idea is to predict, or just sense, when the ambient conditions could cause frosting and then either reduce the ASHP output (lighter load) or switch it off for a period of time.  Both would prevent frosting and the associated time or energy to defrost.

It may be possible to do this with just one sensor, but without data it is hard to tell.  One sensor would, in effect, just be monitoring just the ambient conditions and have, other than the theoretical frosting point (based on dew point), no idea what is happening after that air has passed though the radiator and cooled.  The state of the radiator will be dependant on the temperature of the transfer fluid and the flow rate of the coolant.

  • Thanks 1
Link to comment
Share on other sites

  • 4 weeks later...

@JSHarris

The idea is to monitor the temp/RH before and after the cooling fins with DHT22s and the fin temperature with a DS18B20.

Then, once the data is in, crunch some numbers and see if it is possible to detect the condition that frosting may occur, before it happens.

Edited by SteamyTea
  • Thanks 1
Link to comment
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
×
×
  • Create New...