Navitron Renewable Energy and Sustainability Forum

General Renewable Topics => Inventions, Ideas, Innovation, Bodges etc => Topic started by: Solarchaser on May 14, 2019, 06:56:13 PM



Title: Thermal store with gas ch
Post by: Solarchaser on May 14, 2019, 06:56:13 PM
Hi all, I'm considering buying a thermal store and an I boost type of thing to control the heating of it.
As I've noticed I'm exporting alot more solar than I realised.

My problem is that the combi boiler is downstairs and the thermal store would have to be in the loft, and I dont fancy running two sets of hot water pipes.

I want a maintenance free solution,  as I'm away with work quite often and want it to run without the family having to do anything.

Essentially what I'm thinking is connecting the outlet of the thermal store directly to my hot water pipe, with a thermo static controlled valve. so that when the store falls below 40c, the valve closes stopping water exiting.
This in turn would mean that any hot water requirement would trip the hall sensor on the combi and it would fire up to produce the hot water as it does right now.

My mains water comes in at 3.5bar, so say 3 bar in the loft.
The combi produces hot water around a bar.

My hope would be with the store at hot water temperature the 3 bar of hot water would hit the house taps, and because the combi can only produce 1bar of hot, nothing would be drawn from it, and the 3 bar against the hot water outlet on the combi, would keep the non return valve closed and so it would do nothing until the thermal store was all used up and its valve closed.

Would this work? I feel I'm missing something obvious.

My fall back here would be a couple of motorised valves connected together, so when one opened, the other closed, so hot water can only come from one of the two.

Has anyone tried anything similar, or runs anything similar?

Also for those who have hot water tank, roughly how much energy is used to heat how much water?

On the bright days I'm exporting 30+kWh and I'd like an idea of how much of that I can use to heat the thermal store?

This needs to be a system that requires nothing switched on or off on a daily basis, it has to just work.


Title: Re: Thermal store with gas ch
Post by: nowty on May 14, 2019, 07:14:21 PM
I do the simplest, run the cold into a thermal store, heat that with a solar diverter then straight into the combi as if its cold water, if its not hot enough the combi heats it up, if its hot enough the combi does not bother.

And in Summer when there is hot water a plenty in the thermal store I simply turn the combi off completely.

No complicated valves or anything.


Title: Re: Thermal store with gas ch
Post by: Solarchaser on May 14, 2019, 07:44:05 PM
Unfortunately I cant really do that.

Combi is in kitchen.
Kitchen was completely gutted and redone a year ago.
Wife will not go for me pulling a wall to run another hot water pipe.
What you have done would be MY preferred option tbh, but not possible.

Do you know how many watts you put into the tank in a "normal" day?
And what size is your tank?


Title: Re: Thermal store with gas ch
Post by: rogeriko on May 14, 2019, 08:03:06 PM
Where is the combi fed from? If the mains pressure is 3.5 bar how come the combi is only 1 bar?


Title: Re: Thermal store with gas ch
Post by: pantsmachine on May 14, 2019, 09:04:41 PM
I can help on the hot water tank side of things. Solar I boost is putting about 12 to 13 kwh solar into our 210 ltr tank water every day. The immersion coil is set to cut out when tank reaches 70 degrees ands it's reaching that around 3 pm.

Have a play with this.
http://processheatingservices.com/water-heating-time-calculator/


Title: Re: Thermal store with gas ch
Post by: Philip R on May 14, 2019, 09:05:54 PM
 Many combis do not like hot water fed into the DHW cold water inlet unless the temperature has been blended down a bit. Some are compatible like some Alphas, but most combis out their do not! You could end up flooding your house by splitting the internals of the boiler.

Are you referring to static pressure or dynamic pressure? sounds like you are using both out of context.  

You could use the combi for the local (kitchen) tap outlet. Then use the upstairs thermal store for the other appliances upstairs.

Philip R


Title: Re: Thermal store with gas ch
Post by: A.L. on May 14, 2019, 09:17:23 PM
hello,

Also for those who have hot water tank, roughly how much energy is used to heat how much water?

volume of tank in m3 x 1.16 x temp lift in C is energy required/stored in kWh.


Title: Re: Thermal store with gas ch
Post by: regen on May 14, 2019, 09:30:50 PM
MY insulated Gledhill thermal store is in the upstairs bathroom and looses enough heat to keep that room about 5 deg higher than the rest of the house. Unless there has been a considerable improvement in quality of store lagging then you may have a very warm attic!


Title: Re: Thermal store with gas ch
Post by: RIT on May 14, 2019, 10:59:51 PM
Hi all, I'm considering buying a thermal store and an I boost type of thing to control the heating of it.
As I've noticed I'm exporting alot more solar than I realised.

My problem is that the combi boiler is downstairs and the thermal store would have to be in the loft, and I dont fancy running two sets of hot water pipes.

I want a maintenance free solution,  as I'm away with work quite often and want it to run without the family having to do anything.

Essentially what I'm thinking is connecting the outlet of the thermal store directly to my hot water pipe, with a thermo static controlled valve. so that when the store falls below 40c, the valve closes stopping water exiting.
This in turn would mean that any hot water requirement would trip the hall sensor on the combi and it would fire up to produce the hot water as it does right now.

My mains water comes in at 3.5bar, so say 3 bar in the loft.
The combi produces hot water around a bar.

My hope would be with the store at hot water temperature the 3 bar of hot water would hit the house taps, and because the combi can only produce 1bar of hot, nothing would be drawn from it, and the 3 bar against the hot water outlet on the combi, would keep the non return valve closed and so it would do nothing until the thermal store was all used up and its valve closed.

Would this work? I feel I'm missing something obvious.

My fall back here would be a couple of motorised valves connected together, so when one opened, the other closed, so hot water can only come from one of the two.

Has anyone tried anything similar, or runs anything similar?

Also for those who have hot water tank, roughly how much energy is used to heat how much water?

On the bright days I'm exporting 30+kWh and I'd like an idea of how much of that I can use to heat the thermal store?

This needs to be a system that requires nothing switched on or off on a daily basis, it has to just work.

I looked at this type of solution for my home as I also have a combi.

Your best option is motorised non-return valves from the store and the combi to the output so that you know exactly what is going on and in which direction water will flow at all times. The valves get switched based on the temp of the store.

Any other configuration and you have to worry about the exact design of the combi. For example, my combi's operate by using a simple diaphragm that causes the switch to activate if the pressure on the output drops. This is still likely to activate when a tap is turned fully on even if hot water is coming from the store, and while the majority of the flow should be from the store the combi may still supply enough water to keep it active. Alternatively, the combi could end up starting and stopping as it detects a drop in pressure and so opens its output valve, just to raise the pressure enough the cause the value to shut. Such a cycle would not be good for the combi.





Title: Re: Thermal store with gas ch
Post by: brackwell on May 15, 2019, 07:32:32 AM
Very doable but-

500kg of water in the loft?

This was very heavily talked about some yrs back on this forum and if my memory is correct not many combi boilers will take hot water and then only up to a lowish temp. To mess up a boiler for a bit of hot water.

One is not able to save the amount of energy you might think. The calculation above is correct but that could only be achieved with tank recirculation to upset stratification effects. And then there is the effect of drawing down hot water and so in the end perhaps you can only store overnight 50% of what you might think.

Then you have very large heat losses which for a tank this size at elevated temp in the loft could be of the order of 5kwh per day. If you couple that with pipe runs/dead legs you will only be saving about 40% of the energy input.

You will likely not have enough excess PV to power this for 6 months ? /yr

Compare that to using a static battery and/or EV  which is a far better idea.


Title: Re: Thermal store with gas ch
Post by: Solarchaser on May 15, 2019, 06:16:56 PM
Rogeriko, combi is fed from cold mains, as far as I'm aware all combis have fairly poor pressure output, I assume it's an internal restriction to have the maximum hear transfer to the water... I dunno.

I can help on the hot water tank side of things. Solar I boost is putting about 12 to 13 kwh solar into our 210 ltr tank water every day. The immersion coil is set to cut out when tank reaches 70 degrees ands it's reaching that around 3 pm.

Have a play with this.
http://processheatingservices.com/water-heating-time-calculator/

Great info, thanks Pantsmachine :genuflect

Philip R, as I said, I'm not feeding the combi with the store, when store is good, then combi would potentially be isolated with motorised valves, if the pressure differential doesnt provide isolation, which thinking about it as RIT says, wouldn't work, so would probably have to be motorised valves.
Static pressure measured with a pressure gauge.
Why do you think I'm using pressure out of context?
The idea of the store is to get rid of gas heating things during summer/shoulder, and so come winter I still need to be able to heat up all sinks etc, so I'd rather not split, appreciate the idea though :thumbs:

Thanks A,L, gives me something to play with.

Regent I've been looking at advance appliances, which has I think a 60mm jacket, however if its not enough then I could wrap it in more I guess... in the fullness of time loft will be converted to a room, so it might actually be a plus to have some heat loss.

RIT, yeah I think you are right, I think it will have to be the valves. Which then means I need something to control them also.
Having never used motorised valves, I also have a concern about them being stuck in a half open position, if they dont cycle fully... any experience?

Brackwell, yeah I have to admit that 500+ litres in the loft does make me a tad nervous. however the house was extended a few years ago, so I have a new section of loft I could store the store in, and it gives me a big sturdy double brick wall I can punch through and use straps to spread the load.
As the store will have an overflow,  I will bring a 40mm drain into the loft (should help the other house drains with more air behind water) and then have an aluminium collector under the wall mounted store feeding the drain, so that anything except an explosive failure should be safe enough to trickle down the drain... well that's my thinking anyway.

Again not planning on feeding the combi with hot water, though appreciate that's what Nowty has done, primarily because I've done a bit of reading up and found that most boilers dont seem to like a high input temp... though full disclosure I havent checked mine... but still, running that way isn't possible for me anyway.

As for losses, yeah I have absolutely no idea.
I've seen lots of figures banded about for heat loss. Though tbh none as high as 5kw, but then none as big as a 500l tank, so I dont doubt what you are saying.

If I get 6-8 months out of the year doing this, I'd be delighted, but not sure its possible.
I accept it's not a year round solution which is why I dont want to disable the combi completely.

I agree completely about batteries and EV.

However I have 10kwh of batteries that for the last week have been full by 10am at the latest.
The EV is also at 70/100% each day, and I'm still exporting 25-30kwh pretty much every day.

However come 7-8pm the wife and kids decide to use the 9.6kw showers, which the batteries cant ramp to.
So I still end up buying a few kWh when I've exported 30 today.
Using a thermo static shower fed from a thermal store would mean I could be pretty much 0kw use for the 6 summer months .... or at least, that's my theory



Title: Re: Thermal store with gas ch
Post by: pantsmachine on May 15, 2019, 07:10:38 PM
Our central heating system is split into two zones and has a motorised valve for each  zone. One failure in over 10 years, that valve replaced. Good kit.


Title: Re: Thermal store with gas ch
Post by: Countrypaul on May 15, 2019, 07:21:42 PM
Quote
Brackwell, yeah I have to admit that 500+ litres in the loft does make me a tad nervous. however the house was extended a few years ago, so I have a new section of loft I could store the store in, and it gives me a big sturdy double brick wall I can punch through and use straps to spread the load.
As the store will have an overflow,  I will bring a 40mm drain into the loft (should help the other house drains with more air behind water) and then have an aluminium collector under the wall mounted store feeding the drain, so that anything except an explosive failure should be safe enough to trickle down the drain... well that's my thinking anyway.

One problem you may have with the drain idea is that it should not normally have any water through it, as such any trap that lies on water is useless. You would need to use a waterless trap that will deal with boiling water (such as a HepVo) and need polypropylene pipe with push fix connections supported frequently. Also bear in mid that building regs limit the length of waste pipes that are allowed before discharging into a soil pipe/drain. Irc it is someting like 4m for 40mm and 5m for 50mm.

As has been mentioned on here previously, you are better off with a taller thinner tank than a fat wider one as it makes stratification easier.  If you are using a PV diverter to heat the TS, then if you only heat from the bottom you can end up with a tank full of lukewarm water - one solution might be a Willis (?) type approach rather than just immersion heaters in the store andother apporach is to use a pump to take water from the bottom of the store and push it into the top of the store with a themostat (or 2) to control when the pump operates. The larger the store the easier it is to end up with luke warm water if you don't design a solution in from day 1.








Title: Re: Thermal store with gas ch
Post by: RIT on May 15, 2019, 08:28:58 PM
RIT, yeah I think you are right, I think it will have to be the valves. Which then means I need something to control them also.
Having never used motorised valves, I also have a concern about them being stuck in a half open position, if they dont cycle fully... any experience?

Sorry no, as I worked out what was needed for such a project I was then able to work out the likely pay back period of the costs vs the gas saving, which for my situation was sometime never so I dropped the idea.

I did consider the issue of stuck valves, but as the water only flows when you turn a tap on I did not consider it a major issue - I just added 4 manual valves, 3 to fully isolate the pumps and the last to allow a bypass, with all the costs involved these extra valves were only an extra few pounds and a little more pipe. Such a configuration would allow you to switch back to just using the combi while you fix any issues and provide easy access to anything you need to replace.

If you go down this route the most important thing is to leave enough flexibility in the pipework so that any of the valves can be replaced in the future.


Title: Re: Thermal store with gas ch
Post by: brackwell on May 16, 2019, 07:41:01 AM
There has to be significant advantage in not using the combi boiler out of the heating season. The energy saving trust did an investigation into the efficiency of modern condensing boilers (it does not seem to be available on their site anymore) but it showed that in summer without heating that it was very difficult to get better than 50% efficiency (energy out/in ) due to the stop start losses and dead legs etc.

I permanently heat my HW by the immersion/PV so i fully understand the need for maintenance free use but i think you are setting yourself up for disappointment in this area because the English weather is just not reliable like that and then the vagaries of use eg. visitors.

My suggestion would be that you proceed with a more modest tank size ( how did you get to 400L anyway) and then fit a inline leccy heater which controls the preset water temp by adjusting its leccy use. This will take hot water in summer (and not switch on) and if the water is not quite hot enough or is running out it will draw down enough leccy to control the water temp.  This system is less expensive than the gas and 100% reliable being even able to heat the cold water. ref https://www.toolstation.com/stiebel-eltron-electronic-instantaneous-water-heater/p41729

You could probably use your leccy showers, with the leccy disconnected, as mixer valves if you needed to turn the water rate down when heating from totally cold water as per normal leccy shower.  I have no experience of doing this.

At this time of the yr. May is the most productive, i to have 20kwh excess but i do not worry about it as this is par for the course for a sensible sized PV system.


Title: Re: Thermal store with gas ch
Post by: Solarchaser on May 16, 2019, 08:04:34 PM
I already have the gas combi boiler, its downstairs in the kitchen, it does what its supposed to, but it uses gas to heat things, and on days of excess solar, I could use electricity instead.

For me, the simplest thing is to put a thermal store in the loft, right above the main bathroom, as then all I'm doing is adding a loop between the bath cold water feed and the bath hot water feed.

If I then close the combi hot water valve, I get hot water from the store.

In a nutshell, that's my plan, but it has to pass a couple of tests.
The first being cost.

CountryPaul, the drain normally wouldn't have any water in it, it would really be a vent, but I could use it as an emergency overflow also.
Normal wastepipe survives the odd boiling water deluge,  and tbh if it was getting boiling water down it, then the tank has completely failed, I'd expect it to be dry unless the tank is burst.
Re building regs. I'll have a look, but I'll likely just ignore them if they dont suit, though if you are right about 4m for 40mm, I'll probably be fine anyway.
Good points re the stratification,  I've been looking at the harvi which heats one element till it cuts out, then can heat another (and then another) so should hopefully deal with that issue, though if I'm honest I think I'd prefer a little pump to circulate the dirty water inside, if that's even possible, as it's a pretty warm environment for a water pump.

RIT, that's kinda what I was hoping you WOULDN'T say.
I think motorised valves will be prohibitively expensive.

Fair point about faulty valves, pressure differentials means it couldn't actually cause any harm, just nuisance cycling of the combi I guess, or the odd bit of cold water in the hot worst case.

When the extension went on to the house I completely redone all the plumbing, so theres isolation valves on every branch, and I'd definitely be fitting full bore valves on both ends of the store.

I'd hope to save quite a bit.
I'm paying around 1200 a year for gas and electricity.
So I think theres a good potential to save 2-300 a year by not using leccy showers and reducing the gas on hot water usage.
So if I look at an optimistic 5 year payback, then 1500 on a store, Solar diverter and valves seems reasonable I think.

As you say Brackwell the weather is not dependable, especially in not so sunny Scotland, but I've consistently got over 3Mwh a year on my 4kw array, so I'm confident that 8 months of 12 I should have excess solar now I've added a further 6.6kw array.
But obviously that wont help November to February, or weeks of full overcast rubbish, and as you mention, visitors etc. which is why I want to be able to seamlessly switch back to the combi if the store is not hot enough.

Hence thermostatically controlled motorised valves.
When store is cold, the supply to it is isolated and combi runs as normal, when store is hot, combi valve is isolated and store is open.

How did I get to 4-500L? I've looked at many specs around the 210L tanks which are good for 2-3 showers, and there are 6 people in my house, so 4-500L seems reasonable.

I note that many people when going down the solar route, first of all look to reduce their usage, some manage it dramatically.
And while I did change most white goods to energy saving equivalents and of course all the house bulbs are leds, my main goal isn't to reduce usage as such, more to create enough excess that I can live my life as usual, while saving money doing it.

Because with the best will in the world, I live with a woman who just loves to use kw's


Title: Re: Thermal store with gas ch
Post by: Countrypaul on May 16, 2019, 08:42:35 PM
Quote
How did I get to 4-500L? I've looked at many specs around the 210L tanks which are good for 2-3 showers, and there are 6 people in my house, so 4-500L seems reasonable.

Are you sure that is only for 2 -3 showers?  If your store is at say 70C and you blend it down to 40C with 10C cold water that would give you close to 400L of shower water. If each shower takes about 8mins** that is close to 60L per shower, so you would easily get 6 showers out of the small store.

I am told that teenage females tend to have much longer showers, I know my wife who definitely is no longer a teenager certainly does, where as me and my lads tend to be less than that.

Do all six need showers at the same time? If not then a smaller TS backed up by the combi might be more efficient, especially if getting a 4-500L store up therewould be very difficult. It took 2 of us to get a 430L TS up one flight of stairs (600mm diameter copper with 75mm PUR - from Newark), so a larger steel one might be a significant challenge!


Title: Re: Thermal store with gas ch
Post by: Solarchaser on May 16, 2019, 10:13:02 PM
I have to admit, since I have no real world experience of a store. I'm purely taling the manufacturer's blurb at its word.

If your real world says a 210 would be fine, then I'm tempted to just go with that, and if it's not enough I could double up in time, and that would likely make more sense, in a few ways, not least the physicality of maneuvering one into the loft.

And if one is enough then if I still have spare pv power I could fit one for the central heating too. Hmmmm


Title: Re: Thermal store with gas ch
Post by: Countrypaul on May 16, 2019, 11:32:05 PM
I can't say that one 210L TS would be enough for you, as I said I don't know if you have 5 heavy users in the house or not  ;D

I know I put in a 430L store which was the largest I could get in and it provides the DHW and central heating year round, but is not heated purely by PV as yours would be.  My store normally runs upto 80C (some eople I believe run upto 90), that extra temperature can provide a significant amount of extra hot water. If the cost of heating the water is negliable (you already have the PV) then again you might get away with a smaller store at a higher temerature.

Whilst 2 smaller stores might give you more flexibility (and starting with one might remain just one) it would have to be expected that the 2 stores would have greater losses than a single store, though putting them close to each other with insulation round the pair might minimise the difference.

I don't know the weight of the stores you have been looking at or the structure of your loft access, but I know a 430L copper store took two of us just to carry it up the stairs and along.  You might be able to hire a "lifter" that builders use for hoisting beams up etc but then that would all be additional expense that might be better spent on other things. Only you will be able to make those decisions, but those of us who have installed similar things can try and give you the benefit of our experience whether good or bad to help you minimise the bad ones.