aroTHERM Plus DHW Flow Temperature

[BenY]

I'm using the vaillant dew point monitoring at the moment as well to ensure the flow temp isn't too cold 

[Dan F]

3 hours ago, BenY said:

I'm using the vaillant dew point monitoring at the moment as well to ensure the flow temp isn't too cold 

 

If your pipework is insulated and it's fan coils you are using I'm not sure you need dew monitoring. Do your fan coils have drainage for condensate?

[JoeBano]

What temperature do your cooling the slab to with Ufh Dan? I’ve been playing about with mine, the lowest I could get the air temperature inside was 23c when it was 30c outside. I put it on the evening before. 

[JohnMo]

@JoeBano what temp are you flowing?

 

I am doing 12 degree flow temp, running from 9am until 5pm. We still get some overheating from about 4 til 7 due to big windows and lots of solar gain. But way better than it used to be.

[JoeBano]

I’m running at 12c, any lower I’ve been getting a tiny amount of condensation on the floor not pleasant on the bare foot. It was on for 24hrs but I do get solar gains in part of my kitchen (south facing) 

[Dan F]

10 minutes ago, JoeBano said:

What temperature do your cooling the slab to with Ufh Dan? I’ve been playing about with mine, the lowest I could get the air temperature inside was 23c when it was 30c outside. I put it on the evening before. 

 

Normally runs at about 17C I think.  I cool the slab to 20C on hot days, and 20.5C on less hot days.  Room temperature follows slab temperature roughly (depending on temp outside), but I don't try to get it any colder than 22C.

 

My UFH is in the slab itself and I have automated shading, so don't need to cool the slab much to make a difference.  (of course, cooling the slab 1C takes about 6hrs though).

 

The Vaillant controller has automatic dew-point monitoring and will ensure that flow temperature doesn't go beyond this (if enabled), but given floor surface is never below 19C this is a non-issue.

[JohnMo]

I found the pipes were just getting slightly damp if I went any lower, so settled on 12 also.

 

Running in day is mostly PV powered, our floor will never get very cold as we have 300mm pipe centres in 100mm concrete and I only run in the day time.

[Dan F]

3 minutes ago, JohnMo said:

I found the pipes were just getting slightly damp if I went any lower, so settled on 12 also.

They'll get damp at higher than 12, depending on humidity.  A question though; if the majority of pipework is insulated, pipes don't go inside walls/ceilings (where dampness could be an issue) and floor surface itself doesn't get cold is there a concern?

[JoeBano]

I get quite a bit of condensation on

my Ufh manifold but I’m not concerned as it’s in the garage just drips on the floor. I get water through the door on a heavy downpour. Dan have you got normal Actuators or the auto balancing ones? 

[Dan F]

1 hour ago, JoeBano said:

I get quite a bit of condensation on

my Ufh manifold but I’m not concerned as it’s in the garage just drips on the floor. I get water through the door on a heavy downpour. Dan have you got normal Actuators or the auto balancing ones? 

Auto-balancing.  They work with cooling as well as heating, but I'm not sure exactly what they do in practice, have just let them do their thing.

Edited June 29, 2023 by Dan F

[JoeBano]

13 hours ago, Dan F said:

Auto-balancing.  They work with cooling as well as heating, but I'm not sure exactly what they do in practice, have just let them do their thing.

That’s nice to know the auto balance ones works with cooling. Thanks for your info, not that much information on the underfloor cooling out there. I’m just in the process of getting a new stat which does cooling and then setting up weather compensation on my heat pump then I can just leave it alone. 

[BenY]

All my pipes are lagged yes and I have drainage on the fan coils.  When I ran it at 8 degrees once there was a load of condensation in places especially the manifolds so I keep the temp higher now.  I probably don't need the dew point monitoring I agree.

[markocosic]

I'm resurrecting this @Dan F based on the "could you rock a small heat pump with a small cylinder" discussion here; and in particular the comments form @HughF on "but Glyn's 5 kW Samsung setup into a 1m2 coil works ok" etc prompted by @JamesPa and @sharpener comments too.

 

 

That is a 150 litre Vaillant / OSO slimline cylinder with a 1m2 coil.

 

He "holds off" the reheat of the hot water to two time periods each day; in order to only "reheat from cold" rather than "topping off" the tank. The nominal setpoint is 43C. It gets to about 45C at the top of the cylinder with stratification. That is giving a COp of >4 when outside temperature is >7C outdoors. Which us most of the year:

 

https://youtu.be/kkNx2oSO-S4?t=808

 

 

Why is @Dan F's Mixergy setup so pants in DHW preparation then? Light bulb. Coil location. Mixergy are forced to put the coil in the stupidest possible place so that they can do the "only partly fill the cylinder" party trick. Like; right at the bloody top:

 

 

 

 

Whereas a regular cylinder will have it at the bottom. 

 

This means that the heat pump into a Mixergy primary coil is going whammo; straight into the hottest part of the cylinder; hence being unable to dump the heat from the compressor into the cylinder at anything other than some deltaT above the hotter part of the cylinder.

 

You could manually run the Mixergy charging / destratification pump; to stir up the cylinder and make it all the same temperature. That would mitigate the effect; at the expense of the water at the top of the cylinder being unusable until such a time as the whole lot is heated.

 

But what you want is a coil either at the bottom or rising through the entire cylinder. Even a modest area then gives you an ok COP if the heat pump that's charging it has a modest output. 

 

I had in my head that "a small coil like a Mixergy would be no good even on a 7 kW class heat pump" without having clocked where that coil was and the effect that this would have.

 

 

 

I'm now wondering out loud about these:

 

Was previously toying with plate-loading a direct cylinder that would fit under an oven:

https://osohotwater.co.uk/product/super-xpress-hot-water-cylinder/

 

Now toying with an absolutely boggo coil cylinder (0.8 m2) under an oven. (1160 high even with all the expansion and inlet group etc in "180L = 163 litre actual capacity) 

https://osohotwater.co.uk/product/super-coil-hot-water-cylinder/

 

Not ideal by any means (why don't they make one of these with a say a 2m2 coil in this form factor...) but perhaps not horiffic.

 

 

The large coil variants all have a ruddy great T&P relief stuck out the side; in addition to being 595 mm not 580 mm width. (too tight even for the bravest of squeezing it in place of a kitchen unit jobs)

 

https://osohotwater.co.uk/product/delta-geocoil/

 

And the Vaillants have the T&P relief and an immersion heater cover in addition to everything being side exit/needing external expansions etc 😕

 

https://www.vaillant.co.uk/downloads/product-manuals/unistor-1/unistor-pre-plumbed-heat-pump-cylinder-domestic-installation-and-maintenance-instructions-1956775.pdf

 

 

 

 

 

[JamesPa]

15 minutes ago, markocosic said:

I'm resurrecting this @Dan F based on the "could you rock a small heat pump with a small cylinder" discussion here; and in particular the comments form @HughF on "but Glyn's 5 kW Samsung setup into a 1m2 coil works ok" etc prompted by @JamesPa and @sharpener comments too.

 

 

That is a 150 litre Vaillant / OSO slimline cylinder with a 1m2 coil.

 

He "holds off" the reheat of the hot water to two time periods each day; in order to only "reheat from cold" rather than "topping off" the tank. The nominal setpoint is 43C. It gets to about 45C at the top of the cylinder with stratification. That is giving a COp of >4 when outside temperature is >7C outdoors. Which us most of the year:

 

https://youtu.be/kkNx2oSO-S4?t=808

 

 

Why is @Dan F's Mixergy setup so pants in DHW preparation then? Light bulb. Coil location. Mixergy are forced to put the coil in the stupidest possible place so that they can do the "only partly fill the cylinder" party trick. Like; right at the bloody top:

 

 

 

 

Whereas a regular cylinder will have it at the bottom. 

 

This means that the heat pump into a Mixergy primary coil is going whammo; straight into the hottest part of the cylinder; hence being unable to dump the heat from the compressor into the cylinder at anything other than some deltaT above the hotter part of the cylinder.

 

You could manually run the Mixergy charging / destratification pump; to stir up the cylinder and make it all the same temperature. That would mitigate the effect; at the expense of the water at the top of the cylinder being unusable until such a time as the whole lot is heated.

 

But what you want is a coil either at the bottom or rising through the entire cylinder. Even a modest area then gives you an ok COP if the heat pump that's charging it has a modest output. 

 

I had in my head that "a small coil like a Mixergy would be no good even on a 7 kW class heat pump" without having clocked where that coil was and the effect that this would have.

 

 

 

I'm now wondering out loud about these:

 

Was previously toying with plate-loading a direct cylinder that would fit under an oven:

https://osohotwater.co.uk/product/super-xpress-hot-water-cylinder/

 

Now toying with an absolutely boggo coil cylinder (0.8 m2) under an oven. (1160 high even with all the expansion and inlet group etc in "180L = 163 litre actual capacity) 

https://osohotwater.co.uk/product/super-coil-hot-water-cylinder/

 

Not ideal by any means (why don't they make one of these with a say a 2m2 coil in this form factor...) but perhaps not horiffic.

 

 

The large coil variants all have a ruddy great T&P relief stuck out the side; in addition to being 595 mm not 580 mm width. (too tight even for the bravest of squeezing it in place of a kitchen unit jobs)

 

https://osohotwater.co.uk/product/delta-geocoil/

 

And the Vaillants have the T&P relief and an immersion heater cover in addition to everything being side exit/needing external expansions etc 😕

 

https://www.vaillant.co.uk/downloads/product-manuals/unistor-1/unistor-pre-plumbed-heat-pump-cylinder-domestic-installation-and-maintenance-instructions-1956775.pdf

 

 

 

 

 

I'm not sure I follow what you are suggesting, a normal coil bog standard cylinder with a heat pump so that it's an easy fit under an oven?

Edited August 6, 2023 by JamesPa

[DanDee]

13 minutes ago, markocosic said:

 

Why is @Dan F's Mixergy setup so pants in DHW preparation then? Light bulb. Coil location. Mixergy are forced to put the coil in the stupidest possible place so that they can do the "only partly fill the cylinder" party trick. Like; right at the bloody top:

 

 

That doesn't happen anymore since the HEX version

[JamesPa]

Silly question (maybe), but if the level of stratification that Mixergy rely on is achievable, why not make a direct TS with a hp operating at say FT 65, plus a water pump and external contraflow HEX to heat mains DHW to say 60 (or 55).  Circumvents G3 but delivers mains pressure DHW with no requirement for legionella protection.  

 

Edited August 6, 2023 by JamesPa

[DanDee]

19 minutes ago, JamesPa said:

Silly question=a hp operating at say FT 65

 

[JamesPa]

2 minutes ago, DanDee said:

 

Not really.  It's back to the 'good enough' discussion.  For DHW only, FT65 is not at all silly if it avoids other expenses /compromises.  For space heating I agree it is arguably silly, but for dhw not so.  

 

Ps I the idea is not supposed to answer @markocosic's question, it's a related idea connecting back to earlier discussions on dhw solutions for retrofits.

[dpmiller]

A 9kW HP with a setpoint of 54c running into a smallish (250-is active) TS with a 3m2 coil, hasn't yet run out of hot water for us.

[JohnMo]

1 hour ago, JamesPa said:

Silly question (maybe), but if the level of stratification that Mixergy rely on is achievable, why not make a direct TS with a hp operating at say FT 65, plus a water pump and external contraflow HEX to heat mains DHW to say 60 (or 55).  Circumvents G3 but delivers mains pressure DHW with no requirement for legionella protection.  

 

That is pretty much what RED do, they use a Harlequin cylinder, 1.2m2 heating coil and a big DHW coil, the cylinder is vented, the water in the cylinder just stays there to carry heat like a thermal store 

[JamesPa]

Interesting.  Nothing to stop  a TS having a rectangular footprint AFAIK = 27%more volume for the same useful footprint occupied.  If I am forced to replace my dhw tank for no good reason then I think I might prefer this to all the G3 stuff.  

[Beelbeebub]

My worry about Thermal Stores is this.

 

Say you have a 100l cylinder at mains temp (say 10C) and you want 45c water (let's ignore blending valves)

 

You heat it to 45C. You now have 100l of 45C water to draw on.

 

If you have a TS you have well below 100l of 45C hot water. The moment the max temp of your store drops below 45C you can no longer achieve 45C water at all.

 

To achieve a good volume of 45C water, you need to either store your water at a higher temp or have more water.

 

A PHX system with a variable speed pump might do better, but at the cost of complexity.

 

The unvented store has the merit of high efficency and simplicity. The one problem is the G3 regs.  It would be useful if that could be disregarded when connected *only* to a HP as there is absolutely no chance of it ever boiling. Therefore the risk of explosion is zero.

 

The other option is a vented cylinder with a booster pump for pressure. very simple thermally and a small amount of complexity (off the shelf booster)

Edited August 6, 2023 by Beelbeebub

[JohnMo]

9 minutes ago, Beelbeebub said:

The one problem is the G3 regs.  It would be useful if that could be disregarded when connected *only* to a HP as there is absolutely no chance of it ever boiling. Therefore the risk of explosion is zero.

That is only true without an immersion heater. Not seen any cylinders without an immersion as a heat pump cylinder is fitted with them for legionella...

[JamesPa]

27 minutes ago, Beelbeebub said:

A PHX system with a variable speed pump might do better, but at the cost of complexity.

That was what I was thinking of, some complexity but not insuperable.  With decent stratification, if that is possible (many claim it is and mixergy rely on it) this could work very well, particularly if the thermal store was direct heated.

 

28 minutes ago, Beelbeebub said:

The other option is a vented cylinder with a booster pump for pressure. very simple thermally and a small amount of complexity (off the shelf booster)

Which perfectly describes my current system that I am almost certainly forced to rip out grr!

 

[sharpener]

18 minutes ago, Beelbeebub said:

To achieve a good volume of 45C water, you need to either store your water at a higher temp or have more water.

 

Yes but nothing to do with the heat pump per se. I would say you need to have 50l per person min at not less that 50C. One installer has quoted me some rule of thumb of (# of bedrooms + 1) x 50  as the min vol, I think this came from a cylinder mfr.

 

Are you differentiating between a conventional HW tank and TS? The problem with having a TS with HW flow through a PHX or secondary coil is there is no stratification so as you say the temp will fall steadily as you draw off.

 

5 hours ago, dpmiller said:

A 9kW HP with a setpoint of 54c running into a smallish (250-is active) TS with a 3m2 coil, hasn't yet run out of hot water for us.

 

Is this a TS as just described where the temp falls, or a conventional HW cyl in which case I would expect 250l should be adequate stored volume, and 9kW input would provide a (not very generous) shower on a continous basis.