DIY hot water system needs replacing - Sunamp?

[Nelliekins]

So... a couple of years ago, following advice from various sage individuals in these parts, I built my own "DIY thermal store" to provide hot water (and a heat buffer for the 250m² of UFH). For the first year, it worked pretty well, but it's become less and less effective over the past year. Now, the hot water doesn't work at all.

 

I have a 140 litre RM Aquastel cylinder, with dual stats (half way up, and at the bottom). It is heated by a 30kW Worcester Bosch system boiler (with the stats providing for a single long burn), and has a 2.8kW immersion heater powered by the solar diverter (which never fires - the cylinder is always hot when it tries). A single indirect coil in the cylinder provides heat for the UFH. We are perfectly happy with the heating side of things, but the heating requirement is so low that I am intending to use a couple of Willis heaters powered by our Solar diverter for the few days of the year it is required (heating hasn't been on once since middle of February, and was turned on less than 60 days in total last year).

 

On the hot water side, I have what is normally the direct connection to the tank being pumped through a 100kW PHE, with the second side of the PHE being fed by mains cold. The water gets hot for maybe 45 seconds, and then goes (and stays) cold.

 

SWMBO has had enough (and to be fair, I really don't blame her). I don't understand why it is so much worse than it was 2 years ago (when it worked perfectly well for 3-4 showers back to back without a hiccup), but I don't have the energy left to try and solve it. The cylinder is being kept at approx 70C, but the PHE temperature drops rapidly from the incoming cold mains and never recovers.

 

So, as per the title, I am considering either

 

(a) replacing our gas boiler and cylinder with a Sunamp, or

(b) adopting a KISS approach and converting what we have into a "traditional" hot water cylinder (i.e. remove the PHE entirely, connect the boiler to the coil connection on the cylinder, and have the contents of the cylinder as the actual hot water that comes out of the taps).

 

(a) seems a more appropriate long-term solution (since gas boilers are so 20th century now, and gas is probably going to skyrocket over the coming years). However, there is a large capital outlay on the Sunamp (£3-4k for the biggest models, plus installation etc). Plus, if we go the Sunamp route, there are a couple of questions that spring to mind:

 

1. If we are thinking of getting a monobloc ASHP in due course, are we better off getting the correct Sunamp model (e.g. Sunamp Thermino 300 hp-SG) now?
2. If we get the HP model, do we need to install the ASHP from day 1, or can the Sunamp HP models work with just electric heating as well?

 

The KISS approach (b) would probably entail replacing the cylinder (since it has has primary circuit water in it for 2+ years, so inhibitor and whatever else inside it, and I don't know if I would trust just flushing it out a few times to make the water safe). But other to that, it's a bag of elbows and tee fittings and a bit of copper pipe, and I would be done...

 

Help! 😢

[PeterW]

(B) using a UVC and the boiler but put a heat pump cylinder in so when the boiler packs up it is a straight swap for the boiler and the cylinder lives on. 
 

I wouldn’t fit a Sunamp if it was free and @Nickfromwales offered to fit it in his lederhosen live on YouTube…..

 

Before you rip it out though have you checked both the PHE pump and the flow switch are working ..?? Would be odd for a PHE to fail that quick so I would check you’ve got decent flow before you rip the lot out … 

[Nelliekins]

I even bypassed the flow switch at first and set the pump to "always on" as an experiment (in case the PHE was getting cold too quickly). Pump seemed to run fine, but PHE still goes ice cold within a minute. Flow switch fires as soon as a hot tap opens.

 

I will remove the pump and check that there is nothing reducing flow internally - I even found the handle for the gate valves the other day, so that shouldn't take long.

 

Clearly, if the cylinder is hot enough (70°C), then:

 

1. The PHE isn't getting enough flow from the cylinder, or 

 

2. The PHE isn't operating correctly, or

 

3. The blending valve isn't working as it should

 

@PeterW the current cylinder is vented, with an 18 gallon header tank. Any reason to switch to a UVC? And last time I was on this forum, the Sunamp appeared to be almost the Holy Grail - what have I missed?

 

Thanks!

[Nelliekins]

Here is a sketch of the hot water and heating system, please ignore the green link - that's how it was originally plumbed, but I have since remade as per the drawing.

 

The immersion heater is in the top of the cylinder, though.

DHW Design - draft 1.pdf

Edited March 24, 2022 by Nelliekins

[JohnMo]

You say you have the thermostats set for a long burn, what are they set at?

 

Hot water is pretty simple system.

 

Open tap, flow switch detects flow, pump starts, pump water through heat exchanger.  Cold water gets heated goes to taps via TMV.

 

When you open tap, does pump start?

If so flow switch works.

 

What are the water temperatures from thermal store, going into and out of heat exchanger, when pump running.  Are they in line with your drawing?

 

What the DHW temps in and out of the heat exchanger.  Are they in line with your drawing?

 

Finally TMV, temp into and out of, based on your drawing do the temps match?

[Nelliekins]

The stats are set at 50°C and 70°C. The immersion is set to 70°C and every time the solar diverter fires up to try and heat the cylinder, it tells me the cylinder is already hot.

 

Yes, flow switch works as expected.

 

Temps going in must be right, because the cylinder stats all say they are. Stats are operating correctly.

 

The temp coming from the taps reaches design temp of 45°C within seconds (which means ~50°C at PHE as expected), but drops off rapidly (within 60s it is below 25°C). This is only slightly mitigated through reduction of HW flow rate.

[dpmiller]

Is there a TMV to temper the hot water?

[Nelliekins]

21 minutes ago, dpmiller said:

Is there a TMV to temper the hot water?

Yes, why do you ask?

[Nelliekins]

This is probably a silly question... If I am switching to using Willis heaters for the UFH side of things, is there any reason I can't try using the indirect coil for preheat of DHW?

[dpmiller]

Is the DHW pipe coming out of the PHE dropping in temp immediately, or just the taps? If just the taps, a sticking cartridge in the TMV could do this

 

Have you removed and backflushed the PHE?

[SteamyTea]

Are you in a hard water area and are you using a water softener? 

[Nelliekins]

49 minutes ago, SteamyTea said:

Are you in a hard water area and are you using a water softener? 

 

Not hard water - "moderately soft" according to United Utilities (5.32ppm on last check for our area, which was apparently 1 week ago today).
We aren't using a water softener.

[SteamyTea]

Just now, Nelliekins said:

 

Not hard water - "moderately soft" according to United Utilities (5.32ppm on last check for our area, which was apparently 1 week ago today).
We aren't using a water softener.

So hopefully that will discount any serious scaling up.

 

Does seem odd that is has got gradually worse.

Check the thermostatic valve.

[Nelliekins]

58 minutes ago, dpmiller said:

Is the DHW pipe coming out of the PHE dropping in temp immediately, or just the taps? If just the taps, a sticking cartridge in the TMV could do this

 

Have you removed and backflushed the PHE?

 

I haven't removed the PHE - that's next on my list. But it's the PHE itself dropping in temp very rapidly, which suggests restricted flow rate through on the primary side.

 

The TMV appears to be operating as expected, because if I increase the temperature on it, I can get absurdly hot (>60C) water at a tap for a few seconds before everything goes cold again.

[Nelliekins]

Ok so there is some extensive scaling inside the pipework. Given there was 1 litre of inhibitor in the water, I am not entirely sure how or why it is this bad after a little over 2 years...

 

Is it possible that the cylinder was too hot at some point and this is the result of kettling?

 

This elbow was between the PHE and the cylinder, so I am expecting this throughout now...

[bassanclan]

Is there a non return valve that might be malfunctioning?

Edited March 29, 2022 by jack
How do I delete an accidental quote? - quote deleted by mod

[PeterW]

That is really bad for 2 years - you need to do a descale (safely) and then get it rebuilt. Not sure a single litre was good enough - inhibitor is cheap and I would add a lot more. 

[SteamyTea]

4 minutes ago, bassanclan said:

How do I delete an accidental quote

On an ordinary browser use Ctrl+a to select everything in the text box, the del to get rid of it.

Not found a way to do it in my Android phone except by closing the thread, then going back to it and starting a new reply. The misquote will appear with the option to clear the editor.

 

[Nelliekins]

20 hours ago, PeterW said:

That is really bad for 2 years - you need to do a descale (safely) and then get it rebuilt. Not sure a single litre was good enough - inhibitor is cheap and I would add a lot more. 

 

That was the amount recommended (and indeed supplied) by the plumber who did the boiler side of the system, based on size of the cylinder, the header tank, and the pipework.  But clearly not even close to enough... Even the instructions on the Sentinel X100 say 1%, and the system is somewhere around 200 litres.

 

What are the odds that the level of scaling has completely killed the PHE? Am I better off replacing the whole lot, given I am disconnecting the UFH from it all anyway? 

[ToughButterCup]

On 24/03/2022 at 20:14, PeterW said:

...

I wouldn’t fit a Sunamp if it was free

...

 

Other opinions are available.

Full disclosure, I have one: they aren't for everyone. Properly installed, they work as well as anything else. One person's expensive is another's cheap. Price isn't the only determinant of customer choice. 

 

 

[Nelliekins]

8 hours ago, ToughButterCup said:

 

Other opinions are available.

Full disclosure, I have one: they aren't for everyone. Properly installed, they work as well as anything else. One person's expensive is another's cheap. Price isn't the only determinant of customer choice. 

 

 

Frankly at this point, the £3k outlay is becoming attractive. Showering in water that is less than 35C isn't pleasant to me.

 

@PeterW and others - can I ask a few questions, to make sure i haven't misunderstood stuff along the way...

 

1. Should a 30kW system boiler be capable of replenishing the heat in the cylinder as fast as a single shower (@10 litres/min) can deplete it?

 

2. Should a 100kW PHE be able to heat the incoming cold mains to 50C at 10 litres/min, if it has sufficient supply of hot(ter) water to pass through it on the primary side?

 

If the answers to both the above are YES, then

 

3. if we switch to using the Willis heaters for the UFH, could we reuse the indirect coil in the cylinder as a DHW preheat coil, to reduce the risk of cold showers happening again? (I am assuming that this mitigates the impact of reduced flow on the PHE primary because the uplift on the incoming mains is already reduced)

[JohnMo]

1. yes you would have thought of so, but that depends on the flow rate through the coil.  If it's flowing too fast the hot water will not have enough time to transfer through the coil.

 

2.  If it's scaled up like the photo of union, be lucky if you get flow through let alone heat exchange.

 

your issue may stem from where the scaling came from, if that is not addressed whatever you change could/ likely to end up the same.

 

your system did work well - then over time performance decreased.  From what you have said, may of been down to not enough inhibitor.  So nothing wrong with the system design, just a plumber that's rubbish at maths

[Radian]

On 29/03/2022 at 10:48, Nelliekins said:

This elbow was between the PHE and the cylinder, so I am expecting this throughout now...

 

 

Inhibitor isn't really the issue - if that's on the recirculating side of the PHE then the open side would probably be worse as it's constantly being replenished with minerals from main supply water. A water softener would seem to be indicated.

[Russdl]

On 24/03/2022 at 20:14, PeterW said:

I wouldn’t fit a Sunamp if it was free

 

13 hours ago, ToughButterCup said:

Other opinions are available.

Couldn’t agree more with ToughButterCup.  
 

I’m not sure how many of us on here have Sunamps, quite a few I think but Sunamp grumbles seem to be way lower than ASHP grumbles.
 

Maybe it’s a percentage thing?

[Nickfromwales]

ASHP’s have been around for many decades. “Percentages” would tell even the most inept of individuals to remove their rose-tinted goggles and try actually performing the correct maths, eg so they were making a statement with 20/20 vision instead of one that would gain little or no credit.

 

In the absence of that;

Sunamps for half of one, the current offering less than 1/10 of one. Decades that is. The thing is still on trial. ASHP’s are not. UVC’s aren’t either. 

So :- Actual summery;

 

Most grumbles of ASHP’s are from assholes fitting them incorrectly, or in instances that they were doomed to fail once unsuitability installed in.

 

UVC’s, bombproof these days, it’s basically a stainless steel flask. Zero issues such as micro bore based heat exchangers…👎. 
 

Percentages?

 

How many ASHP’s and UVC’s are currently in operation in the UK ( or the world for that matter ) ? Eh?🤔😉

 

How many SA’s by comparison? 
 

That equation will give readers of this forum an ACTUAL percentage to go on.