@cathoderay unfortunately that's not how pressure and resistance in pipework is. Your pipework from the ASHP will be very large bore and, I would assume, a very short run all in all - maybe 20m there and back.

The pipework in your central heating will be significantly larger in length and as you said much smaller in diameter. This smaller pipework means the pressure in the system will be far higher as the speed to meet the flow rate will increase.

As an example using an online calculator we can play with the numbers. a 28mm copper pipe, pushing 8.5l/min (your flow rate) along 20m has a pressure drop of 0.46kPa. If we lower the size of the pipe to 15mm we get 9.56kPa (20x more pressure). There will be significantly more resistance around your heating circuit than to the exchanger. Especially as its full over various size expansions and openings as well as angles.

Hi @cathoderay

Either increase the setting of your weather compensation or manually set the LWT to say 55C. Then monitor the system and see if you can achieve the desired indoor temperatures.

@batalto - point taken, it's also the linear/area thing, half the diameter = a quarter the area. You are also spot on with the primary circuit length, and it even has some gentle curves instead of 90 degree bends as it is plastic pipework. Easy go, easy flow.

@derek-m - I have turned off weather comp and set the temp manually to 55 degrees on the Midea controller, and will see what happens. Long term, won't this (a) increase my bills because I am not using weather comp and (b) meaning I am in effect using the ASHP more like an on/off fossil fuel boiler, which is not the best way to run them?

Hi @cathoderay

I am not suggesting this as a permanent solution, but to try to identify where any design weaknesses may exist. Is it the heat pump that is undersized or the PHE that is undersized or the secondary water pump that is inadequate or the balancing of the system?

I’m surprised that a 3degC-plus loss in a main heat exchanger is acceptable. It seems crazy to have that when there is so little spare capacity in most HP systems and the efficiency is so dependent on attaining the lowest flow temperature possible.

Posted by: @cycleneil

I’m surprised that a 3degC-plus loss in a main heat exchanger is acceptable. It seems crazy to have that when there is so little spare capacity in most HP systems and the efficiency is so dependent on attaining the lowest flow temperature possible.

If you look at the data from PHE manufacturers a 3C temperature drop would be deemed excellent, they are often quoting a 5C or even 10C differential.

I fail to understand why a system designer would even contemplate using a PHE within a heat pump based system, but I suppose that they don't have to get the system working and pay the running costs.

What does one of these PHE's look like and why would you chose one over and buffer tank?

I believe my Midea system has a direct buffer tank with the glycol pumped around the whole system when heating, would a PHE save the need for so much glycol?

@mookyfoo mine doesn't have a buffer, it has a low loss header

Posted by: @mookyfoo

What does one of these PHE's look like and why would you chose one over and buffer tank?

I believe my Midea system has a direct buffer tank with the glycol pumped around the whole system when heating, would a PHE save the need for so much glycol?

 

If you Google 'Brazed Plate Heat Exchanger' you will see what one looks like. The idea is to allow the transfer of heat energy from one side to the other, whilst maintaining isolation of the often liquids in each side.

Because of the importance, from the efficiency point of view, to keep the operating temperature as low as possible within an ASHP based heating system, I cannot understand why any equipment, be it a PHE or buffer tank, would be included that actually reduces operating efficiency.

Posted by: @derek-m

I am not suggesting this as a permanent solution, but to try to identify where any design weaknesses may exist. Is it the heat pump that is undersized or the PHE that is undersized or the secondary water pump that is inadequate or the balancing of the system?

Ah, OK, thanks. So far, running at 55 degree LWT has the house feeling warmer, room temps up, but it's 'only' 8 degrees outside, so demand is not at its highest. I'll take some readings later this evening and again tomorrow morning and post them here.

@cycleneil, @mookyfoo, @batalto - the low loss header or PHE gets into Midea systems because Freedom, who supply Midea units in the UK says they need to be fitted. Until recently, the Freedom Midea installation manual said you have to fit them, otherwise you void the guarantee.

From a heat efficiency point of view they make no sense at all. The are also an added complication/expense (which the consumer pays for). I think I've read somewhere, possibly even on the forum here, suppliers and installers like them because they protect the primary circuit from crud, so less callouts and repairs under warranty etc.

I can see pro and cons - there is some sense in protecting the primary circuit from heating circuit crud, and it means less circulating glycol. But they need to be efficient, otherwise they add to running costs, and the need for a second circulating pump also increases installation and running costs. Worth bearing in mind that 3 degrees loss may not sound like a lot, but at a low LWT say 30 to 35 degrees it is ~10% of the output, which I suppose might increase running costs by 10% per annum, which at today's prices is going to hundred(s) of £££.

As you know, I have a PHE. The low loss headers look like voodoo to me, can't make sense of how they work. Buffers are I think normally used as thermal stores, to store heat, perhaps to reduce short cycling?

@cathoderay I think your right about the buffer limiting short cycling. Also our 2nd ASHP for the UFH is also plumbed into the buffer (yet to be commissioned) so that may be why Freedom have supplied a buffer. The main disadvantage for me is all this glycol in the system meaning if we want to drain down we need to some how save it. 

Posted by: @mookyfoo

@cathoderay I think your right about the buffer limiting short cycling. Also our 2nd ASHP for the UFH is also plumbed into the buffer (yet to be commissioned) so that may be why Freedom have supplied a buffer. The main disadvantage for me is all this glycol in the system meaning if we want to drain down we need to some how save it. 

From what I gather from various sources, having a buffer tank or low loss header can have a detrimental effect on system operation and efficiency. If the system needs additional capacity, then a simple capacity vessel would seem a better option.

With regard to short cycling, this invariably happens during milder weather conditions, when the heat pumps pulls back to its minimum operating capacity and cannot go any lower, so periodically needs to stop for a period of time. A possible solution, which should be quite easy to incorporate, would be to have timing functions built into the controller, which would prevent the heat pump from restarting until a suitable period of time has elapsed, but then also keep it running for a minimum period once it has started. Whilst operating in such a manner may cause the LWT to vary outside optimal limits, but I doubt that it would have that much effect upon actual indoor temperatures.

One way to eliminate the need for glycol would be to install a split system, so that the water circuit is totally within the heated envelope of the building.

Another method would be to install trace heating on all the external pipework, such that it is only operated if the temperature of the pipework were to fall below say +5C. To safeguard against this happening during a power failure, it would be necessary to also install an automatic drain valve.