Air Source Heat Pump Buffer Vessel Sizing

[Peter_1]

I neither use inverters, nor do I use a thermal buffer, but more than likely a larger low loss header than most.
I do not have problems with defrost or any issues with controlling comfort levels.
When using reverse cycle defrost (as most manufactures do, as i proved above is a poor method for this application), on systems where zones are controlled, I simple install an overide relay which opens the zone valves during defrost.
As most systems use static heat exchangers and relatively low flow temperatures, sudden but short changes in temperature is going to make little or no effect on household comfort. Neither is short term losses of heat (on and off of a fixed spped heat pump)
Inverters are great technology, but should be used when direct reaction speed of the enviroment is required, basically on forced draft heat exchangers.
Inverters also generally have poor power factor, so not always as good for the enviroment as a fixed speed.
I think most systems are just overkill, use the KISS principle.

MadFridgie -in fact the Mad is wrong chosen in your case - inverters are in split systems (ceiling, wall, ground..) the only setup which will eliminate most complaints about cold drafts. The thermal comfort is much better because the sensible heat is higher once at operating conditions. But that's perhaps what you meant with 'forced draft exchangers'

[MikeHolm]

This forum is probably the closest thing we have to a survey of systems and their problems/attributes. I would like to see if there could be a proper survey of complaints and solutions to see how much of a problem "bufferless" air to liquid systems are. The problem is that happy people don't complain so how would we design such a survey and get a reasonable sample size?

[Peter_1]

In relation to defrost time, 334 Joules per gram is required to change the state of ice to water and 2.03 joules per gram is required to raise the temperature of ice by one degree.

With this in mind, is it safe to calculate, that hypothetically, if you have around 2kg of ice and the ambient temperature is minus 5degreesC, in order to melt the ice, you require 20,300J to raise the temperature to 0, then 668,000J to complete the phase change to water, which equates to a total of 0.191194kWh.
Also, a 50litre vessel of water held at 50 degrees C indoors (20 degree room temperature) should contain 1.75kWh.
Can you then calculate that the vessel will give up 0.20416kW in 7 minutes, enough to melt the 2 kilos of ice?

I'm no physicist, so if any of the calculations aren't correct, i'm more than open to corrections

Haven't read the whole thread, just on post 9. Perhaps someone responded but what you calculated is if you do it in 14 hour. If you want to do this in 7 minutes, you will need +/- 1.7 kW( not calculated it right now) And 2 kg ice on coils is a lot if you ask me.
Then the heat isn't coming all from the indoor: the motor windings are hot when you start your defrost, so a lot of heat will come from the windings. This is how the simple defrosts work on small icemakers, domestics,....

[Peter_1]

...
HOR 4.9kw (heating) and power in 1.3kw (COP3.77), it goes into a reverse cycle defrost, your SST would be high and your SCT would be low, same machine, 7.7Kw cooling, power in 1Kw (COP7.7) so you actual defrost COP ends up being 3.77/7.7= 0.49, you are better using hot gas defrost with an electric boost, defrost COP = 1 less some thermal losses, lets say 0.85 practical defrost COP.

Wow, MF, this needs some further clarification..if SST is high and SCT is low, then your 'defrosting COP' will be very high

[Peter_1]

....

If this is the case, why is everyone not employing inverter compressors. I understand the coeffecient of performance on inverter driven units is less than on fixed speed scroll compressor units, and longevity of the compressor compared to the fixed speed scroll compressor isn't as good,

If you speed down an inverter compressor while heating, still connected to the same evaporator, then SST will rise and this will increase COP.Something which isn't possible with a fixed speed.
If your smallest capacity step matches the smallest needed heating capacity, then you don't need a buffer. We never install one on an inverter one.

[Peter_1]

I
I wholeheartedly agree with you Jon. Electrical consumption meters need to be put in place in order to make air and ground source heat pumps viable. Not only will it give the wider public more confidence in heat pumps, but it will also ensure that customers cannot fall foul of bad installations. Out of interest Jon, which consumption meters are you installing?
...

Then look once to this site www.liveheatpump.be I don't have any connections with this brand but as a technician, I like the way how they prove figures, not being afraid that the figures will turn against them.

[Peter_1]

T....
Defrost is reverse cycle but circulator will run and energy will be taken from UFH or radiator ssytem without buffer however this will not affect room temeprature in the 3 minute defrost cycle, more to the point even with a buffer installed the same amount of energy would be required from the water circuit during defrost you would simply have to reheat the water in the buffer as oppose to the emmitters, in practice makes no difference.

For a standard install buffer cylinders on inverters are not necessary as load can be varied direct from the unit itself, the buffer is a trick reserved for fixed speed to get around their inability to do so. also if you fit a buffer on an inverter driven unit would it not have been better (and cheaper) to buy a fixed speed unit instead as in weather compensated mode response time on say 300 litres of water will negate the advantages of the inverter anyway!

With regards to monitoring Mitsi have installed a number of remote web based monitoring systems on Ecodans which they have been carrying out for nearly 3 years now, none of which systems have buffers installed, this allows a seasonal performance to be measured, there are a large number of these sites accross the UK the majority of which are achieving SCOPs of 3 to 4 including DHW in real houses of all types, they have over 20 years of independantly accredited field trial data available. (see extract attached)
Attachment 6742

Agree for 100% with you, are this web controlled machines consultable on the net? If the figures are that good as they claim, then they should release those public.

[Peter_1]

I think having just read some of your comments on different threads on this forum it is clear you dont fit or have an extensive knowledge of inverter controlled systems. Your knowledge of heat transfer and the impact of temperature difference seems flawed, its laughable that you think t...

It's clear you don't have the ability to follow a discussion in an adult, a mature way without becoming personal to make your point.
IMHO

[mad fridgie]

MadFridgie -in fact the Mad is wrong chosen in your case - inverters are in split systems (ceiling, wall, ground..) the only setup which will eliminate most complaints about cold drafts. The thermal comfort is much better because the sensible heat is higher once at operating conditions. But that's perhaps what you meant with 'forced draft exchangers'

Hi Peter, read again, I am saying inverters are not suitable for static heat exchangers. ( i mean underfloor or radiators) but needed for forced draft great exchangers, like most common split AC systems

[Peter_1]

Sometimes difficult to understand technical issues in another language. The term 'forced draft exchangers' was something which I struggled with.

I agree completely with you that inverters are somehow useless in a static system.
The only advantage could be that the radiant heating is proportionally controlled with the outdoor temperature as the driven parameter (and some fuzzy logically predicting what needs to be preheated when past temperatures are known) So you could make 'colder warm water' at a lower speed with the same coil outside, increasing SST when the water doesn't need to be that hot.
Anyway, we install often DX/water heatpumps and we rarely use a VFD on the compressors for the reason you gave. We installed a year ago a VFD on the DX/water of a friend , very interested in controls and heatpumps and he's following this for us. What he does is reducing the speed manually when outside temperature increases so that the compressors runs longer at an elevated SST. He has an hour meter on it and he can read the kW the VFD is pulling at that moment. A kWh meter should be even better and perhaps we will install this on his system. The VFD costs him nothing because the Government is paying 90% of he costs for a VFD on a compressor.

[chillerman2006]

Sometimes difficult to understand technical issues in another language. The term 'forced draft exchangers' was something which I struggled with.

Hi Peter

Your doing better than me mate I am having trouble following this thread too

Can I presume 'forced draft exchangers' are HX with a fan (standard evaporator) ???

R's chillerman

[MikeHolm]

Hi Peter

Your doing better than me mate I am having trouble following this thread too

Can I presume 'forced draft exchangers' are HX with a fan (standard evaporator) ???

R's chillerman

CM, the "forced draft exchanger" is the indoor unit used with a fan, like any single head ductless split. clear as mud?

[chillerman2006]

CM, the "forced draft exchanger" is the indoor unit used with a fan, like any single head ductless split. clear as mud?

Yes
Thanks Mike