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Author Topic: Maximise the iboost dhw input – a variable output inline water heater?  (Read 2524 times)
Mudman
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« on: May 11, 2017, 12:35:42 PM »

I am working with a neighbour to help them reduce gas usage by maximising the amount of solar generation going into the hot water via their iboost and smallish tank.
The problem is that being vulnerable, they don’t cope well with turning on the hot tap and getting cold out so I am wondering if there is a way around this.

What I have noticed is that they often have the gas working on the dhw on early morning but then frequently don’t use any (or much) hot water in the morning so the tank is hot and the iboost cannot use excess pv production to heat it. I find it very frustrating to check the iboost on a sunny day and see that pretty much nothing has gone into the tank when it could have provided a fully heated tank from cold by the middle of the day and save them quite a bit on the gas bill.

When I explain that it would be better not to use the gas to heat the water in the morning (or evening for that matter), they point to the occasions when they have found themselves without any hot water in a morning when they do need it- I guess that is every week or so, certainly not every day.  I guess any time without hot water brands itself on the emotions and they feel they do find it hard to deal with that for reasons I will not go into. Also I have seen them suffer days when, following my advice, they did not use the gas in the morning but it was a dull day so the pv / iboost combo couldn’t raise the temperature much anyway.
From experience I don’t think I will get anywhere encouraging them to do what I would do and check the weather every day and manually tell the boiler to do dhw when it will be a dull day.

One way around this I have imagined would be to have some sort of variable thermostatic  electrical inline heater on the tank output that brings water up to temperature  on the few occasions it has not been fully heated. Anyone know if one exists? I am no electrical engineer, but I imagine that the electronics needed to vary the amount of heating going into the water would be fairly meaty- thinking maximum would be roughly what is needed in an electric shower (?7kw) for cold water from a cold hot tank but if it was taking half heated water, maybe 30degrees, it would need to add only one or two kilowatts to the water to get it to usable shower/dishwashing level.

As for the economics of this it is hard to imagine it – such an inline electrical heater would almost always import electricity rather than using anything the PV is producing. And with gas being cheaper per kwh than electricity, it is important that the heater is not used too often or it would have been cheaper for them to have used the gas as they are currently doing…

What are your thoughts? Anyone else see a market for this kind of device?
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Countrypaul
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« Reply #1 on: May 11, 2017, 03:01:01 PM »

Surely what you are talking about is just an electric boiler or instant hot water heater. I'm not sure if they accept hot water or require just cold water (can't imagine they can't cope with HW though) which are available from many places including, for example, Screwfix as an one:  http://www.screwfix.com/p/strom-mini-instant-water-heater-3-5kw/8035P?kpid=8035P&cm_mmc=Google-_-Product%2520Listing%2520Ads-_-Sales%2520Tracking-_-sales%2520tracking%2520url&gclid=CJeX9-n459MCFRQ6GwodBKEFhg&gclsrc=aw.ds&dclid=CNqiger459MCFQWr7Qod_GoL4g
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A.L.
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« Reply #2 on: May 11, 2017, 04:43:10 PM »

hello,

Stiebel-Eltron instantaneous water heaters can accept heated water as input - https://www.stiebel-eltron.co.uk/en/products-solutions/dhw/instantaneous_waterheater/compact_instantaneouswaterheater.html

suppose the problem might be capital expenditure
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Iain
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« Reply #3 on: May 11, 2017, 04:45:25 PM »

Hi
Quote
What I have noticed is that they often have the gas working on the dhw on early morning but then frequently don’t use any (or much) hot water in the morning so the tank is hot and the iboost cannot use excess pv production to heat it. I find it very frustrating to check the iboost on a sunny day and see that pretty much nothing has gone into the tank when it could have provided a fully heated tank from cold by the middle of the day and save them quite a bit on the gas bill.

I have my hot water(boiler) come on in the morning but only allow the gas to heat just enough water for our morning showers. This then always allow the solar thermal to heat the tank up during the day.
Similar situation?
The thermostat (electronic one) is 2/3 the way up the tank and is only set to 42 deg C.
The gas only heats up the top of the tank to 42 deg C, 2 showers and then a cool tank for the rest of the day.
Even if we didn't have showers there is plenty of water still to heat.
Might be an easier solution.
Most people have the boiler heating all the cylinder to a high temp. The mechanical thermostats have too high a hysteresis (normally 5 - 15 degrees) so an electronic one works better, mine has a hysteresis of .5 degree


Iain
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brackwell
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« Reply #4 on: May 11, 2017, 05:27:58 PM »

http://www.toolstation.com/shop/Plumbing/d20/Water+Heating/sd3105/Electronic+Instantaneous+Water+Heater/p41729

Also have you checked the immersion temp setting.  You probably need a higher temp setting so that you can carry a good day over to the next without any risk  of it being to cool.  I also wonder how modern the tank is and how good the insulation is - for the storage idea to work you do need a well insulated tank and pipes and i suspect that is the crux of the problem.

Ken
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Mudman
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« Reply #5 on: May 11, 2017, 10:50:41 PM »

Great thank you for all replies
Thank you A.L. and Ken, that Stiebel-Eltron product may well be what I am after- it says it keeps the output temperature constant. The data sheet is a bit sparse- I guess it varies its heating no matter what the input temperature is, though it only seems to have two power levels. Has anyone used one?

Iain I like your idea of only heating half the tank with the boiler but I don’t know that this is applicable here because the tank is small. – only about 1200 tall so all the water can be used in one shower – and a few loads of washing up will deplete it considerably.  

Ken, the immersion temperature is set higher than the tank stat so there is a bit of PV heating in the morning but not a lot. I found that the tank stat is pretty hard to do any fine adjustments.  It has foam insulation on it and I installed a jacket over that to increase the insulation so heat loss is not too bad. But being so small a tank, you cant expect to carry much if any hot water over, it will be used up. Also, higher temperatures mean more scale deposition and considering this is in London…

« Last Edit: May 11, 2017, 11:00:47 PM by Mudman » Logged
gnarly
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« Reply #6 on: May 12, 2017, 07:47:40 AM »

One other thought - if their hot water is cold in the morning, how well insulated is their tank?  I have an (old) pre-insulated tank with two additional red jackets on, and some pipe insulation around where the pipes come out (e.g. top of tank)

If their airing cupboard is quite warm, that might indicate a poorly insulated tank...
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brackwell
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« Reply #7 on: May 12, 2017, 09:10:24 AM »

Mudman,

I have lived many years with a 120L,which i believe is/was the norm.  To get a feel for things -this a bath full or a 20 mins shower at 10L/min (a good shower) if the tank temp is maintained at the minimum poss say 42C.  One can double this if the water temp was c75C.

I have lived many years now with just heating the water with the immersion heater and at a temp which hardly requires cold water for a shower with only one issue and that is it will give a adequate bath but not a full one.

The unit i directed you to allows the temp output set to any temp you reasonable want and adjusts the electrical heating amount to maintain this temp. So if the tank water is hot it just does not come on. At the other end the tank temp needs to be c21C in order the heater can provide water at 40C and 10L/min.for a shower.  If the tank temp is lower you could say the unit is behaving like a normal electric shower and you have to decrease the flow rate to maintain temp. I believe these heaters provide the most efficient solution to the intermittent solar issue and is a trick widely missed partly i suspect that most eco heads have a problem with leccy full stop-how wrong.

Wrong because--
In the use of a gas boiler heating a tank the overall efficiency measured by gas energy in /hot water out IS LESS than 50% and more like 30% for many and this is for modern condensing boilers so think what it is for some old systems!  The figs are improved if the DHW (Domestic hot water) is only run at the same time as CH.   Further the gas boiler heats all the tank,frequently set to 60C and therefore the heat losses are significantly more than say 1/2 a tank to 42C

Ref,
  ref http://tools.energysavingtrust.org.uk/Publications2/Housing-professionals/Heating-systems/In-situ-monitoring-of-efficiencies-of-condensing-boilers-and-use-of-secondary-heating-trial-final-report.    Also see the efficiencies of just hot water production in that report.

  Daily average losses associated with summer DHW production  p31

Calculations using some of the figs there and adding my own
These are modern condensing gas boilers.
   

                                          Combi  KWh/day         Regular  KWh/day   Immersion heater    Instant Electric HW      50% PV/50% instant
Gas/elec in (kwh/day )                    5.13                        9.89                    7.71                             2.58                           3.58
Cost (Gas 4p/Elec 14p/kwh)         20.52 p                      39.56 p               107.94p                       36.12 p                         25p                   
DHW out kWh                                 2.69                          2.58                    2.58                             2.58                            2.58
Tank Losses                              NA                           5.13                    5.13                               NA                  say       1.0
Boiler Losses                            2.42                          2.22                      NA                               NA                              NA

A lot depends on volume of water used and frequency.but this represents a full tank of water at 60C and the 2.58kWh of hot water out is about 50L out or a 5min shower at 10L/min

You can see from this that the immersion heater is cheaper than gas in your neighbours case

Other ref  http://www.navitron.org.uk/forum/index.php/topic,25295.msg292421.html#msg292421

Ken



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Mudman
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« Reply #8 on: May 12, 2017, 11:23:28 AM »

Hi Ken
very interesting thank you for clarifying what the device does. I would need to get a sparky to put one of those thick (?6mm) cables in to the airing cupboard and mount the device at the back of the cupboard along the line of the hot water run off pipe. With toolstation price of about 200, and I guess a morning from a sparky at about 100 and my plumbing expertise, I guess this means about 300-350 capital cost.

I think the situation my neighbours are facing is pretty common actually – just to clarify, the tank is pretty well insulated and if no hot water was drawn off for hand washing, washing up etc in the hours of darkness, it would hold on to most of the heat. The problem I am describing is more to do with how to keep people happy (ie hot water always available) and minimise gas usage while also starting the day with a coldish tank that can soak up as much PV generation as possible.

Using this electric heater, a coldish tank morning would not destroy the occasional morning shower/washing up and from Ken’s description of the real world efficiency of gas water heating (especially in the summer when it cannot be combined with central heating use), it seems likely that the running costs would be quite a bit cheaper. Hard to work out how much though- probably will take a while to get a £350 payback.

MM
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TheFairway
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« Reply #9 on: May 12, 2017, 12:37:16 PM »

Whats the problem with topping the tank up with hot water in the evening if there is any solar shortfall?

I use to do this, works well. May not be 100% efficient, but is 99% convenient. Doesn't work all year round mind, as my heat in bathroom is via a towel rail running from dhw. So when its cold, we also run dhw in the morning.

Only time manual intervention is needed is if a significant amount of hot water is needed at night and again in morning before solar has topped it up. Just like with a conventional dhw system.

With mine, its semi automatic, as I have two boiler timers, in series, one works at GMT, the other works on local time - ie compensates for DST.

As the days gets longer around the BST switch, the two timers diverge, and the window where they are both on at the same time is only in the evening. So the dhw moves from being on twice a day, to evening only. A cylynder thermostat adjustment also takes place, going from 60C to 40C around this time, and back again for winter. On days where solar helped the tank, we have to set the shower control cooler as solar is much hotter than the dhw - wouldn't be an issue if we didn't turn the tank down.

This way, in the longer solar months, the boiler is topped up only to make up for any shortfall in solar through the day. There will be some losses overnight, but there is still enough hot water in the morning for a shower or two. If you need more than that, the boiler control has an advance button.

The advantage of running the dhw occasionally is that you know if you have a boiler issue before you need it. And it probably does the boiler some good not being left unused for months on end.

I don't think it added much to the gas bill from the times when we went gas free. Looking back over my records, by the time you removed the standing charge component, it looks to have been approx £2-£3 difference per month between doing this and running with boiler off.
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Mudman
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« Reply #10 on: May 12, 2017, 01:32:28 PM »

Hi Fairway
topping up with gas in the evening is ok- and they currently do this- but this household varies the timing of their dhw usage so much that often this gas-generated dhw will still be sitting in the tank in the morning and thus the following day’s solar input will be limited to topping up any overnight losses (tends to be about 1.5kwh to do this). Other evenings they will use the hot water in the evening, which is why they often run the boiler in the morning so that they have the option to have a morning shower- but it is the variability that I am trying to help with by providing a fall back heating option for these different days, but also allowing enough energy space in the tank at the time of day when the PV can add plenty of energy to a coldish tank.
You could say that this is an ideal house for a straightforward combi boiler but seeing as they have 3.4kw of pv and an iboost, it seems worth trying to get the most from that.
MM
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TheFairway
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« Reply #11 on: May 12, 2017, 02:49:36 PM »

What you are saying is fine if you want to minimise gas usage, but if you want to maximise comfort/ease of use for minimal extra cost (in my case about £2-3 per month), then having a tank full of hot water in the morning (less overnight losses) is not the end of the world surely?

Looking back at my figures, when I had zero DHW usage, my gas bill was just over £4/month (the rest was standing charge), when I was doing this, it was just over £6/month - the difference between my Optimmersion saving me £60/yr or £50/yr. If you want to minimise gas usage, turn the tank thermostat down to 40C, enough for a shower, or a bit more if they like it a bit hotter. The only side effect of this is, with a thermostatically controlled shower control, you need to adjust the H/C bias on the shower control depending on whether its a full solar day (run slightly cooler), or a day with dhw topup, run hotter/max.

TBH, I thought it worth couple of quid a month for not getting grief from the misses on the odd occasion that the shower was less than hot and the boiler kicking in on the odd occasion. The year that I ran with no DHW the whole of the summer, I had a suspected T-valve head failure when I first needed to use the boiler which took a few days for me to sort, then a few more days when I found the real reason. So that was a couple of days when we woke up to a cold bedroom, which would have got me grief from the misses.
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brackwell
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« Reply #12 on: May 12, 2017, 03:21:02 PM »

Fairway,
Nothing wrong with what you say and saving £2-3 per month But the inline heater can probably save that amount again because of reduced heating losses and a reduction in dead leg losses as the heater can be situated close to the shower.  eg in a typical house say dead leg time before hot water comes through 10 secs at 10L/min (ie 1.66L)  = c 0.1kWh  or o.2kWh for 2 persons = 73kWh /yr.   I got my dead leg down to 6secs by running a direct pipe from tank to shower -in 15mm plastic.   

Mudman,
Out of curiosity i would be checking the length/time of dead leg and flow rate of shower.  Have you any idea what they consider to be the length of time of water flow for a shower.
Many small tanks have also a shortish length of immersion heater element so the PV aspect could be greatly improved with a longer heater element which would also be measuring the temp lower down the tank. Very cheap to do but if tank very old it is sometimes difficult to get the old one out.

Ken
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Mudman
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« Reply #13 on: May 12, 2017, 03:34:08 PM »

Hi Fairway –yes I understand your peace – generating justification pretty well  surrender

Hi Ken yes the dead leg is very short- less than a meter- as the tank cupboard is directly behind the bathroom (incidentally it is almost above the boiler too so the dead leg for gas water heating is not far either). I insulated the pipes pretty well.
I will do some shower flow rate measurements with a bucket sometime soon- no pump on the shower or anything and the system is indirect so I guess it is a pretty ordinary shower flow rate.
I had been thinking about how long the immersion is- I guess it only heats the water down to a certain level, about as deep as it goes. Is there any way to check the length of the element without trying to remove it, which I am a bit wary of in case it is stuck and I kill the tank! I think the tank is about 9 years old – house is late 1980s but the pipes show evidence that the tank has been changed in that time. I will have a look for a make/model number next time I am around there. I remember seeing a thread somewhere – think it was on navitron – where someone had put a longer sleeve around the element so as to draw up water from lower down and so heat more of the tank. Never saw the end result of that though.
MM
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