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Author Topic: Thermal store with gas ch  (Read 769 times)
Solarchaser
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« 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.
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nowty
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« Reply #1 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.
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Solarchaser
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« Reply #2 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?
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rogeriko
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« Reply #3 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?
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pantsmachine
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« Reply #4 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/
« Last Edit: May 15, 2019, 08:07:36 AM by pantsmachine » Logged

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Philip R
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« Reply #5 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
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A.L.
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« Reply #6 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.
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regen
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« Reply #7 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!
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RIT
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« Reply #8 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.



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brackwell
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« Reply #9 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.
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Solarchaser
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« Reply #10 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

« Last Edit: May 15, 2019, 06:34:48 PM by Solarchaser » Logged
pantsmachine
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« Reply #11 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.
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Countrypaul
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« Reply #12 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.






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RIT
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« Reply #13 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.
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brackwell
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« Reply #14 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.
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