Balanced coverage?

[nowty]

Have we all got one of them? During a confined space control course I was on the N Irishmen called it "lumpy water!"
A

No just showing what can be done if you put your mind to it, with little cost and a bit of elbow grease. Could as well be an ASHP or an air to air split HP, which might be my next project.

[Ken]

The problem is that most people do not have rads big enough to transfer heat without increasing the HP flow temp. They need a huge emitter area eg UFH so that the flow temp can be kept low but they also need a good flow rate so that as the water passes through the HP there is only a small difference in temp between input and output temps. This translates into long run times and in "beast from the east" moments perhaps permantly on ,in complete contrast to a boiler, The long run time does not increase the cost because the modern ASHP are inverter driven and will modulate down to maintain the set output temp so reducing leccy consumption and improving the COP.

The big difference between Scandanvia and UK is the humidity levels. In the UK we have very high humidity levels and the HP makes a lot of condesate water but at near to 0C (0C to 4C) the cooling effect from the HP can cause the HP to freeze up and has to be defrosted before continuing. Under these conditions the COP drops dramitically but is helped if you keep the Delta t on the inlet/outlet temps low.

Not sure how many installers set HPs up correctly in the first place or offer the correct sizing of HP or the advice to double the size of all the rads. By offereing the correct set up that installer may not get the business so they offer a smaller/cheaper HP and "the rads should be OK". The MCS guidelines
if carried out correctly are very good but ....

PS if you have a HP set up 1/2 the size of the proper job it will cater OK for 80% of the time so after that you need a plan B or just pay up for the extra running costs which can be significant.

[nowty]

When I started my refurbishment 10+ years ago (still haven't finished !), one of the first jobs was to replace the gas boiler. Along with that I replaced all the radiators and oversized them. This was to maximise the condensing efficiency of the new gas boiler, nothing to do with a heat pump. But it just turned out to help with the addition of the heatpump. All I needed to do was add a few more radiators which was a fairly simple job to increase the total volume which alleviated the need for a buffer tank. Although in the milder conditions I sometimes run the hot water and heating together which effectively adds the hot water tank as a buffer tank to reduce the heatpump cycling.

One of my ex work colleagues keeps contacting me because he's seen a heatpump on ebay and just assumes he can plumb it in his existing heating system like I did, but he does not understand a lot of thought and calculations had to be done in order for it to work reasonably.

Interestingly Kensa (who only do GSHP) refuse do make a modulating heatpump, they believe it reduces the reliability and longevity of them.

[Joeboy]

Excellent stuff, so looking for a low slow flow over a greater area of emitter to allow the water to be heated by ashp and radiated into home over a longer timline at a lower temp. What sort of water temperature can be achieved on an ashp?

Is it really the case that these can run at 300% efficiency compared to the per kWh consumed in operation in good conditions? In bad that drops right off?

We've got 15 rads in the house, most are doubles and one is a triple.

Slowly slowly catchy monkey.....

[Countrypaul]

A domestic ASHP can often achieve 60C (Mitsubishi Ecodan for example), however what you should be looking at is how low a temperature can I usefully get away with to heat the house. The higher the temperature you require from the ASHP the lower its efficiency or COP (coefficient of performance), likewise the lower the air temperature it is obtaining heat from the lower the COP. A set of examples of the COPs you possibly obtain at various ambient air temperatures and various water output temperatures for various size ASHPs is given here: https://library.mitsubishielectric.co.u ... AA#page-60 - sorry its the Ecodan again, that's just because that is what I have. It is NOT a recommendation or advertisement for them.

You will see that a COP of 8 is achieveable when its warm outside and you only require slightly warmer water!

Something that is often overlooked, beware of the piping that your CH system uses, if it uses microbore then simply swapping to an ASHP is likely to result in extremely disappointing performance. An ASHP moves large amounts of warm water - generally I believe you should be looking at 22mm pipework, you may get way with 15mm if that is a short distance to the radiator, but not if that is used to supply many radiators. If you require a lot of heat you may be better off with 28mm. Just for reference we have 28mm plastic from ASHP to Thermal Store and 28mm from TS to ground floor UFH manifold. We have 22mm plastic from TS to first floor UFH manifold - but that only suplies heat for 3 bathrooms one 4m2, one 6m2, one 8m2 (room sizes not heated floor area!). We have the mixing valves close to the TS rather than close to the manifold.

Two other aspets that are ofen not discussed, one is that ASHPs often hae small heaters present to maintain the fluid temperature inside at a good working temperature, this can often amount to more than 1kWh per day even when the ASHP is not used. There is also an overhead in starting up the ASHP and warming all the pipework, so if only used for a short period this overhead can severely dent the COP.

With radiators it is not just whether it is a double or tripple but what area they have, though I believe you can get fams for some that increase the effective heat output significantly. others on here should be able to add more detail.

[nowty]

Excellent stuff, so looking for a low slow flow over a greater area of emitter to allow the water to be heated by ashp and radiated into home over a longer timline at a lower temp. What sort of water temperature can be achieved on an ashp?

Is it really the case that these can run at 300% efficiency compared to the per kWh consumed in operation in good conditions? In bad that drops right off?

We've got 15 rads in the house, most are doubles and one is a triple.

Slowly slowly catchy monkey.....

There are some higher heat output models but most generally go up to 60 degrees, the data sheet might say 65 or 70 degrees but you have to realise in the real world the heat exchanger is always going to run a few degrees hotter than the actual output into your water tank or radiators. From memory my heatpump cut out when the output was at 65 degrees from over pressure but and the internal gas temp was at 70 degrees. But your hammering the compressor because of the higher gas pressures so reducing the life expectancy of them. Also the efficiency drops the higher you go, thats why underflow heating is best where you can keep the temp to 25 to 35 degrees. My radiators typically run between 35 and 45 degrees. For the hot water I run it up to 50 degrees with the heatpump, then top it up with the immersions to 60 degrees.

Other problem is excessive cycling, think of a lightbulb (especially the old ones), the spec might say 30,000 hrs but thats only if its left switched on 24/7. Switch it on and off regularly and that drops dramatically.

Below is my calculated real world efficiencies of my heatpump verses temp, I calculated it by heating up a 35 litre builders bucket of water.

[Joeboy]

You guys are flowing so much good info here that it should be a sticky. We are 22mm pipe ftom current gas boiler water circuit and drop to 15mm at the branch lines to rads. Excellent point there as are all the tables. It's funny, I was sitting having breakfast with swmbo a moment ago and it hit me how I am still changing. A year ago the seesaw was more towards finance in my head. Now I'm more at the money is not so important, the carbon count is. In saying that I read earlier today that it's March 22 for the rhi incentives ending.. WE will see how it goes with Ripple and the electric heating backed up by woodstove in the early part of Winter.

I could get onside with a FF free system that does most of the heavy lifting within a reasonable cop window (say 50 deg at 2.5) backed up by immersion coil in HWT and storage heaters/woodstove when it's too cold for the heatpump to be efficient.... hmm, even swmbo isn't ranting as much at the idea of change. Well, she is. It's just less....a bit.

[nowty]

Another thing to add, I can remember if its been mentioned or not.

ASHP is more efficient than a GSHP in summer and vice versa in winter.

Also you get more noise in your radiators because you need a high flow rate through the heatpump heat exchangers to maintain efficiency. Also if you have TRVs on the radiators you need to leave a few on fully open to maintain a minimum flow rate.

You can get round this by having a buffer tank but they can add a permanent step change to the temperature so the heatpump runs hotter and reduces efficiency. I recall there is some good advice on the Kensa heatpump website about this.

UFH with a large thermal mass is best but retrofitting is difficult in most houses. One of my crazy ideas was to grind out large grooves in some of my solid internal walls to fit UFH piping but I doubt that would have passed the SWMBO test.

[Ken]

I never tire of reading this guys website - he is a very long time experieced guy with nothing to sell !
https://heatpumps.co.uk/technical/press ... low-rates/

[Joeboy]

I never tire of reading this guys website - he is a very long time experieced guy with nothing to sell !
https://heatpumps.co.uk/technical/press ... low-rates/

That is an excellent resource. Cheers!