ProDave

A few ASHP / UFH bits of information.

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I was working today on a new build that was having the "professionally" designed and installed heating system commissioned,so I took some time to have a look and talk to the guys.


 

It was a Mitsubishi ASHP coupled to a "pre plumbed" Mitsubishi cylinder and controls.


 

The nuggets of information I gleaned, in no particular order were:


 

The tank was an unvented cylinder for hot water only. Heated to 45 degrees by the heat pump. Once every 3 weeks, on a day of the week of your choosing, it automatically uses the immersion heater to raise the tank to 60 degrees for an hour as an anti legionella measure.


 

Because of the low grade heat input, they said it would require too large a heat input coil, so it used a plate heat exchanger to heat the tank.


 

The UFH runs directly from the heat pump. No buffer tank. It either runs DHW or heating, never both together. When in heating mode, the heat pump runs at a low temperature which you can set. They say this obviates the need for UFH mainifolds with temperature control, instead you can use a dumb manifold (cheaper) and just dial up the flow temperature you want for the UFH in the heat pump  controller.


 

The outside unit ran very quietly, and I didn't see it icing up or needing to defrost, it was about 3 degrees today.


 

I was particularly interested in the no buffer tank, and direct operation of the UFH with a dumb manifold. Certainly something to think about.



 

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I've gone for one of these packages, as in addition to the benefits you have detailed (albeit my understanding was that you could set your own DHW temperature), you can get bolt on kits for the cylinder to run a second heating circuit at either UFH flow temp or a different flow temp, it can be programmed to provide cooling, and it's seemingly possible to set dip switches to keep the heating pump running regardless of whether there is a call / demand for heat.

 

I bought mine direct from a wholesaler, 15 % less than I was being quoted for elsewhere. 

Edited by Stones
Typo

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How much concrete do you have in your warm slab?  I can see that this approach could work for a typical house, but I personally would be a little more concerned as the house spec moves towards the passive / zero-energy type performance.  If you have the water to 30°C, say going into the slab, the how does the ASHP know when the demand is off?  Your house has quite a large footprint, hasn't it? So if your slab gets up to 10°C above room temp and you've got 100m² heated area, then the slab will be pumping roughly 7kW heat into the house.  Can you cope with that rate of heating without cooking?

 

In our house if our slab gets much more the 3°C above room temp even in December, then we will start to overheat.

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Yes, but in a really well insulated house, you would not set the UFH temperature to 10 degrees above room temp, you would set it lower.

 

I think the main point is no need to go via a buffer tank between the ASHP and UFH, and no need to heat the water any hotter than the target floor temperature (so the heat pump can run at a lower temoperature and hopefully better COP.

 

One think I did notice, which I will probably be asked to correct later, is the installer did not use the "call for heat" from the UFH controller. So at the  moment it looks like the ASHP would still be left idling heating just a short loop if the UFH was satisfied.  So in any event the UFH controller with it's room stats would stop the house cooking.

 

In case it's not clear, the system I am describing is not in my own house, but the new build I have just about finished wiring today.
 

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4 minutes ago, Barney12 said:

Was it the packaged Ecodan product as described here:


 

https://heating.mitsubishielectric.co.uk/Products/Pages/Monobloc_Packaged_Cylinder.aspx


 

Of all the reading I've done it ticks the most boxes for me.

Almost.

 

It is actually this cylinder https://heating.mitsubishielectric.co.uk/Products/Pages/Monobloc_Pre-Plumbed_Slimline_Cylinders.aspx And I believe it's an 8KW monoblock heat pump/
 

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3 minutes ago, PeterW said:

Which wholesaler did you go through as there seems to be some wild variation on price on those ...

 

+1

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1 hour ago, ProDave said:

Because of the low grade heat input, they said it would require too large a heat input coil, so it used a plate heat exchanger to heat the tank.

 

Not sure I understand what is happening here.

As it is an unvented store, does the mains water just enter it cold, get heated, then comes out the top to the tap (I don't think this is what happens).

Or does the water in the cylinder get pumped though the plate heat exchanger, heating the the cold supply that then goes off to the tap (I think this is what happens).

 

So this system will always need a working pump and a power supply then.

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32 minutes ago, ProDave said:

Yes, but in a really well insulated house, you would not set the UFH temperature to 10 degrees above room temp, you would set it lower.

 

I think the main point is no need to go via a buffer tank between the ASHP and UFH, and no need to heat the water any hotter than the target floor temperature (so the heat pump can run at a lower temoperature and hopefully better COP.

 

One think I did notice, which I will probably be asked to correct later, is the installer did not use the "call for heat" from the UFH controller. So at the  moment it looks like the ASHP would still be left idling heating just a short loop if the UFH was satisfied.  So in any event the UFH controller with it's room stats would stop the house cooking.

 

In case it's not clear, the system I am describing is not in my own house, but the new build I have just about finished wiring today.
 

 

My plan is to use the call for heat function from the master controller / stat, but keep the heating pump circulating so that the slab temp is nice and even.  As per your comments, I plan to run the slab as near as I can to room temperature.  It will no doubt take a bit of trial and error to work out the optimum settings.

 

20 minutes ago, PeterW said:

Which wholesaler did you go through as there seems to be some wild variation on price on those ...

 

http://www.seconsolar.com/ekmps/shops/seconsolar/resources/Other/secon-heat-pumps-v8.4-a4x6pp-jan16.pdf

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25 minutes ago, SteamyTea said:

Not sure I understand what is happening here.

As it is an unvented store, does the mains water just enter it cold, get heated, then comes out the top to the tap (I don't think this is what happens).

Or does the water in the cylinder get pumped though the plate heat exchanger, heating the the cold supply that then goes off to the tap (I think this is what happens).


 

So this system will always need a working pump and a power supply then.

My understanding is this is an unvented water tank, so cold mains water (via PRV) goes in at the bottom and hot water to the hot taps comes out of the top.

 

You would normally heat this type of tank with a heat exchange coil at the bottom. But because you are heating it with low temperature water from the HP, it would need a particularly large heat exchange coil.  So instead it is heated by a plate heat exchanger. So the water / antifreeze / inhibitor mix of the closed circuit heating circuit passes through one side of the plate heat exchanger, and water from the tank is pumped through the other side via two tappings to the tank.  So it only needs power and a pump running while the cylinder is being heated.  Once hot, you can draw hot water from it in the same way as any other unvented cylinder without any power.

 

There are 3 pumps on this tank. One is to circulate the hot water via the PHE. One is to circulate the heating circuit to the ASHP whenever that is running, and the third is to circulate heating water to the heating circuit, in this case UFH.

 

I did like this system for what it does and how it does it. but my feeling is the "controls" side of it lack integration.  There is a very complicated controller, which I strongly suspect is beyond the ability of most normal home owners to understand or adjust. Then in this case there is a completely separate under floor heating controller with it's own timer clock etc. There is almost no integration between the two.  That is probably the fault of the home owner who in this case had originally planned to heat the house with an oil boiler, until he found that it was not possible to get a SAP "pass" with just an oil boiler so he rather reluctantly I think chose the ASHP system.  I am still minded to put my own system together along these lines, but built from individual components with a simpler and perhaps better integrated control solution.
 

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52 minutes ago, Stones said:

That is a VERY interesting price list.  To buy this 8KW mitsubishi air source heat pump and the pre plumbed cylinder would cost £3715 from that price list (presumably plus VAT which on a new build you would get back.)


 

I know for a fact, in this case, the owner has paid in the region of £16K for a local renewable heating contractor to install it. Some of this he will get back via the Renewable Heat Initiative (I will attempt to find out how much)


 

It has so far taken 3 man days to install it, one man yesterday, and two today. I believe he is coming back again tomorrow to plumb in the solar thermal bit, so lets say 4 man days total to install.  So lets work on a Plumber in the Highlands charging about £200 per man day, that should be about £800 in installation costs. bringing the total to about £4515. Lets say £5000 total to allow for some pipe and fittings and to make the sums simple. No lets round that up to £6K in case I have under estimated just how much a plumber charges.


 

So (me being very cynical) thinks they are charging something like a £10K "RHI premium" to install this system.  So the big question has to be, will you get £10K back in RHI payments?  If the answer is no, then I would personally say, avoid getting a packaged, installed system at RHI prices.


 

This confirms my rather cyincal view, that the only people that benefit from the RHI scheme, is the installers who can bump up their prices and charge a premium. This also confirms the view of a poster on ebuild who had concluded much the same thing that it was a lot cheaper to self install and forget the RHI altogether.


 

When the time comes for my own system, I am tempted to get the same renewables company to quote, then look up the component prices and an estimate of reasonable installation costs, and ask them to justify their premium price.




 

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1 hour ago, ProDave said:

 

This confirms my rather cyincal view, that the only people that benefit from the RHI scheme, is the installers who can bump up their prices and charge a premium. This also confirms the view of a poster on ebuild who had concluded much the same thing that it was a lot cheaper to self install and forget the RHI altogether.


 

 

From the limited research I have done this is exactly the case. RHI simply puts money in the installers pocket! 

There (again from my research) is an unbelievable arrogance from these RHI installers. Snake Oil, Inflated Pricing and Distain towards people with a bit of knowledge all seem to come as standard! I guess they're never going to like a Self Builder who is well informed!! :(

 

Just as bad as MCS accredited solar installs. 

 

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The plumber we used wasn't MBC registered so couldn't do the RHI.  He said he looked into it but the paperwork and costs of maintaining currency in the legislation itself wasn't worth the hassle.  He may not have realised how much of a premium could be involved!

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4 hours ago, Stones said:

My plan is to use the call for heat function from the master controller / stat, but keep the heating pump circulating so that the slab temp is nice and even.  As per your comments, I plan to run the slab as near as I can to room temperature.  It will no doubt take a bit of trial and error to work out the optimum settings.

 

What concerns me with a conventional controller is that the time constants for slab and the master stat (if based on a room temp) are way out of whack.  It will take about 4kWhr of heat to raise the temperature of my slab by 1°C.  So if you put 4kWhr into the slab when at equilibrium, maybe an hour later the slab will be 1°C warmer and dumping about ½kW additional heat into the room.  The time constants and gains of the two systems are far too separated for a simple on-off control to be stable.   I have some ideas on how to do this, but that's the problem: they are just ideas, because I haven't got my system commissioned and collected enough hard data to characterise the system and enable me to establish a decent control policy. 

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Terry, I think the thermostat issue is one with mechanical thermostats. Fit one with an electronic sensor and it should have much lower hysteresis.

 

Also the situation you describe would only happen if the water flow temperature was too high and it put too much heat into the slab in one go.

 

As with any control system, it will need some tuning to prevent overshoot. It's just that it takes a long time to tune such a system.

 


 

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There was quite a discussion on ebuild about how running a slab at +1c of the desired air temperature would allow a near perfect control scenario however the bigger question is how much warmer does the water need to be to keep X CuM of concrete at YdegC when the losses from the room are whatever. 

 

The logic would be that if you ran the water at 25c then the floor and consequently the room can never be heated to above that by the floor (notwithstanding solar gain etc) The issue being that most UFH manifold mixers don't seem to be able to run this low so your only choice would be to connect the ASHP direct to the floor but that then either loses the ability for you to constantly circulate the floor circuits to even out hot/cold spots and may cause short cycling of the ASHP as the heat load of the room varies and triggers the heat cycle unless you directly link the manifold to the ASHP and risk the potential cooling of the slab as the ASHP initially starts the water heating cycle. 

 

One for more thought - and potentially a separate thread ..?

 

 

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@ProDave the documentation talks about "programming via SD Card". I would be really interested to know what the software app looks like? There really cant be that many parameters? 

 

@Stones were you furnished with the programming software when you bought from the trade co you linked to?

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1 minute ago, PeterW said:

The logic would be that if you ran the water at 25c then the floor and consequently the room can never be heated to above that by the floor (notwithstanding solar gain etc)

 

As it happens, 25C is the lowest temp that my ASHP will modulate down to.  I don't know how common that is.

 

My longer term plan is to measure the return water temperature at the UFH manifold.  That should represent a lower-delay version of the slab temp, because you aren't waiting for heat to move through the concrete and hit your temperature sensor(s).

 

15 hours ago, ProDave said:

Because of the low grade heat input, they said it would require too large a heat input coil, so it used a plate heat exchanger to heat the tank.

 

Interesting.  Do you know the size of the ASHP?  We have a 3.2m2 coil designed for ASHPs, and it works very effectively, but our ASHP is only 5kW heating a 250L tank.

 

16 hours ago, ProDave said:

The UFH runs directly from the heat pump. No buffer tank. It either runs DHW or heating, never both together. When in heating mode, the heat pump runs at a low temperature which you can set. They say this obviates the need for UFH mainifolds with temperature control, instead you can use a dumb manifold (cheaper) and just dial up the flow temperature you want for the UFH in the heat pump  controller.

...

I was particularly interested in the no buffer tank, and direct operation of the UFH with a dumb manifold. Certainly something to think about.

 

This is exactly how we run ours at the moment (in the sense that the mixing valve on the manifold is ignored, and temp controlled by the ASHP).  It works perfectly.

 

With weather compensation (water temp up to 29 degrees at zero external temp or below) kicking in over the last few days, and quite a lot of sun streaming through around 10mof glass on our south-facing sliding doors, our slab temp is a little higher than ideal, having reached 23 degrees this morning.  I haven't adjusted it for the moment though, as delays mean our insulation still hasn't been replaced where we had the leaks upstairs, so it's a bit chilly in those rooms at the moment!   

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18 minutes ago, Barney12 said:

@ProDave the documentation talks about "programming via SD Card". I would be really interested to know what the software app looks like? There really cant be that many parameters?


 

@Stones were you furnished with the programming software when you bought from the trade co you linked to?

I didn't see the installer do any "programming" Yes I saw an SD card slot on the board under that square cover on the cyilider but didn't see anything plugged into it, and I never saw him carying a pc with him.

 

All I saw him do is go through some menu's on the programmer that came with it and set up some parameters.

 

I suspect the SD card is used for software updates and they think putting that in the sales blurb makes it look sophisticated. Sory to me it just makes it lok over complicated.
 

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1 hour ago, jack said:

As it happens, 25C is the lowest temp that my ASHP will modulate down to.  I don't know how common that is.

 

My longer term plan is to measure the return water temperature at the UFH manifold.  That should represent a lower-delay version of the slab temp, because you aren't waiting for heat to move through the concrete and hit your temperature sensor(s).

 

Interesting.  Do you know the size of the ASHP?  We have a 3.2m2 coil designed for ASHPs, and it works very effectively, but our ASHP is only 5kW heating a 250L tank.

 

This is exactly how we run ours at the moment (in the sense that the mixing valve on the manifold is ignored, and temp controlled by the ASHP).  It works perfectly.

 

With weather compensation (water temp up to 29 degrees at zero external temp or below) kicking in over the last few days, and quite a lot of sun streaming through around 10mof glass on our south-facing sliding doors, our slab temp is a little higher than ideal, having reached 23 degrees this morning.  I haven't adjusted it for the moment though, as delays mean our insulation still hasn't been replaced where we had the leaks upstairs, so it's a bit chilly in those rooms at the moment!  

 

I suspect the "heat exchange coil too small" was just the installer pushing this cylinder with a plate heat exchanger. Personally like you I would prefer s simple cylinder with a large area heat input coil as it makes a very much simpler system, and almost certainly one that's easier to put together as a system yourself.

 

I am not sold on this weather compensation stuff. I would try and turn that off, or set the curve flat. The house heating controls should take care of putting more heat in when it's colder.
 

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Back to my question about the store.  It seems a complicated and backward step to me.  But then I like simple things.

 

As for the SD card thing, what happens when they stop making SD cards, or they are not supported any more.

I have Sony camera, it has its own card format, I think I may have an old 'XD' card somewhere.

I did throw out the old floppy disks the other day, including my copies of AutoCad, Office and Photoshop.

 

Heating systems should be designed to last at least 30 years, 50 would be better.  There are no mysteries left at this level of thermodynamics, so we know what is needed and how to do it.

 

 

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