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Do I keep the thermal store

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Do I keep the thermal store

Postby RogerS » 05 Mar 2017, 07:27

I know that MikeG is not a great fan but the heating engineer suggested that I considered keeping the thermal store which would mean I could get away with a smaller capacity oil-fired boiler. Just wondered what people's thoughts were?

The system is vented, no UFH (or plans to have any) and no mains water pressure.
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Re: Do I keep the thermal store

Postby Mike G » 05 Mar 2017, 09:08

Keep the tank for something useful. Just not for storing hot water in.
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Re: Do I keep the thermal store

Postby 9fingers » 05 Mar 2017, 09:45

I have come to realise that storing hot water is a mugs game unless the source of heat is free. Even with huge amounts of insulation the heat escapes as my data logger shows. For maximum efficiency instant point of use heating is the way to go, but I accept, not always practical.
I will dig out some graphical info later but a bit busy today.
If the sun every shines up your way keep the store as a basis for harvesting free energy in the future. Looking at this February. We had seven days where all our daily hot water needs came from the solar systems. Typically from mid March we will use no gas to heat water until mid October.

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Re: Do I keep the thermal store

Postby Andyp » 05 Mar 2017, 11:58

Whats the problem with storing hot water?
I have One of these, sorry it is in french. But it is basically a large emmersion heater. It heats the water on an overnight tariff between 10pm and 6am to a temperature of about 50-60 degrees ( too hot for a bath without adding cold water). The outside of the cylinder never feels hot, in fact not even warm, so the insulation must be good. The water is as hot to the touch at 6am as it is at 9.30 pm.

I appreciate that pumping stored water around a CH system must reduce the temperature to a greater extent than just using it for hot water but hot water can be stored once heated, surely.
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Re: Do I keep the thermal store

Postby RogerS » 05 Mar 2017, 12:12

The thing is that regardless of the amount of insulation those heat stores will leak heat to some degree or another. It becomes a cost-benefit scenario. Andy, in your case, you can take advantage of cheaper electricity and the heat losses in comparison are not that relevant.

But thinking about it, I agree with Bob. Unless you have a 'free' source such as solar heating to store any surplus heat then it's just plain daft to use something like oil or gas fired boilers to heat an intermediary device.
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Re: Do I keep the thermal store

Postby RogerM » 05 Mar 2017, 12:41

9fingers wrote:.............Looking at this February. We had seven days where all our daily hot water needs came from the solar systems. Typically from mid March we will use no gas to heat water until mid October.
Bob

Thanks to one of your previous threads, I fitted a solar immersion to our PV system last summer, and when we were away for 3 days at the end of Feb, with everything turned off, we came home to a tank of free hot water too hot to put your hands under. Even in the middle of winter, if there is some bright overcast, the solar immersion does the "heavy lifting" before the DHW system cuts in during the late afternoon. So a belated thanks for drawing our attention to it. Saved us a fortune.
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Re: Do I keep the thermal store

Postby Doug » 05 Mar 2017, 12:45

Beware of plumbers Rog their advice is usually tailored to the easiest solution for them.....not you :eusa-whistle: :lol:

Personally I can't see a point of any system that is going to take up 20-30 minutes of your day for the best part of half the year just to provide heating unless as you say that heat source is free.
With the limitations you have I'd go for as simple a system as possible whilst still giving good control over your hot water & heating, the fancier the system the more there is to go wrong which is great if you can sort it yourself but not if you are paying an engineer.
Hell I can sort mine myself but I still have a very straight forward system that heats the house, detached workshop & provides hot water as I don't want to spend time buggering about with the system.
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Re: Do I keep the thermal store

Postby Woodster » 05 Mar 2017, 12:54

At work they switched from a centralised system to point of use for hot water. The units are cheap to buy and install and you get virtually instant hot water so it can save on your water bill as well. As said, storing hot water is the old way of doing things, it makes no sense these days.
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Re: Do I keep the thermal store

Postby chataigner » 05 Mar 2017, 13:15

I have a unit similar to Andy's. Just to sumarise for those who did not read the small print, the insulation is VERY effective. 300litres at 65°C loses heat at the rate of 0.1W. ie a loss of 2.4Wh per day or about enough to run a 40W lamp for 3.5mins.

Of course, for us in France, the real benefit is the cheap electricity tarif overnight.
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Re: Do I keep the thermal store

Postby RogerS » 05 Mar 2017, 13:36

Doug, my heating guy is first rate and old school. He suggested using the heat store in combination with a smaller oil-fired boiler as he'd been asked to put one into a similarly sized old house. The owner is supposed to have looked into the pro's and con's in detail but he does have UFH which may be factor.

My heating guy is very, very sensible and pragmatic and came up with one suggestion which has saved me an awful lot of buggering about and around £1000.

The one advantage of keeping the beast is that the background heat keeps the garage toasty unless it is very cold outside. The heat comes from the wood boiler. Since the garage will become my workshop, then that is a plus. But.....I am looking forward to Sunday when I turn off the Beast. We move into rental accommodation on Mon 13th although I am toying with the idea of sticking an immersion into the hot water tank.

I do adhere to your mantra of KISS, Doug.
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Re: Do I keep the thermal store

Postby Rod » 05 Mar 2017, 13:53

Does that mean you are going to start knocking the house about?

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Re: Do I keep the thermal store

Postby RogerS » 05 Mar 2017, 15:10

Rod wrote:Does that mean you are going to start knocking the house about?

Rod


LOL! Oh yes, indeedy. The boiler replacement is a standalone exercise and will be sited in the garage and a fire-proof oil tank in the log store. Originally the flue was going to be put inside one of the six flues in the gable end. But my heating guy pointed out - quite rightly - that we hadn't got a clue where the flue would be plus the cost of the special flue needed these days.

So he suggested resiting the boiler to take advantage of using a balanced flue.

We're putting the tank inside the wood store as that gives LOML freedom to plan her outside area without worrying about a great big green oil tank cluttering up the sight lines.
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Re: Do I keep the thermal store

Postby Mike G » 05 Mar 2017, 15:38

Andyp wrote:Whats the problem with storing hot water?
I have One of these, sorry it is in french. But it is basically a large emmersion heater. It heats the water on an overnight tariff between 10pm and 6am to a temperature of about 50-60 degrees ( too hot for a bath without adding cold water). The outside of the cylinder never feels hot, in fact not even warm, so the insulation must be good. The water is as hot to the touch at 6am as it is at 9.30 pm.

I appreciate that pumping stored water around a CH system must reduce the temperature to a greater extent than just using it for hot water but hot water can be stored once heated, surely.


A thermal store is an altogether different beast from a normal hot water cylinder. The water in a thermal store is not the water which comes out of the tap, or circulates around a central heating system, both of which are heated via heat exchangers within the thermal store. In other words, instead of heating water up in a boiler and storing it for later use in a hot water cylinder, the boiler is constantly topping up the heat in a thermal store via a flow & return circuit and heat exchanger, and this thermal store then gives up its heat, via further heat exchangers, to the heating and hot water circuit. That store is kept at the same temperature all the time, 24 hours a day, every day. It's a wonderfully inefficient way of storing and using energy.
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Re: Do I keep the thermal store

Postby RogerS » 05 Mar 2017, 18:14

But surely if you have 'free' heat such as solar then it is worth storing it for later use ?
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Re: Do I keep the thermal store

Postby 9fingers » 05 Mar 2017, 18:28

RogerS wrote:But surely if you have 'free' heat such as solar then it is worth storing it for later use ?


Well I think it is worth it but as I designed the system I am capable of fixing any issues with it. During the summer months one good day will give us more than 3 days hot water should the following day be cloudy.

I do have to have two different control regimes. In winter the store is kept at a maximum of about 55 degrees which due to a very efficient thermostatic mixer will give me DHW at 50 degrees. This is paid for energy and I aim to have the bare minimum capability in the store from mid evening onwards.

Come the "Free Energy" season - mid March onwards, the store is allowed to rise to about 93 degrees and the wastage goes up accordingly. If we get consecutive sunny days , the thermal dump comes into action and heats the bathroom radiator and prevents the store getting anywhere near boiling.

A note on the leakage from the French style water tanks (that are not thermal stores). They are specified without pipes connected and it these pipe that conduct the heat out of the water in the tank. My store is high spec on paper and only gently warm to the touch when the content is at 93 C.
Experimenting with thermal breaks in the pipe are on the round -2-it list mainly for academic interest.

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Re: Do I keep the thermal store

Postby 9fingers » 05 Mar 2017, 19:54

RogerM wrote:
9fingers wrote:.............Looking at this February. We had seven days where all our daily hot water needs came from the solar systems. Typically from mid March we will use no gas to heat water until mid October.
Bob

Thanks to one of your previous threads, I fitted a solar immersion to our PV system last summer, and when we were away for 3 days at the end of Feb, with everything turned off, we came home to a tank of free hot water too hot to put your hands under. Even in the middle of winter, if there is some bright overcast, the solar immersion does the "heavy lifting" before the DHW system cuts in during the late afternoon. So a belated thanks for drawing our attention to it. Saved us a fortune.


Thanks Roger it is very good to hear that PV diversion is working for you. My home made version is still going strong too and adding its contribution to the thermal store along with the solar thermal tubes.

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Re: Do I keep the thermal store

Postby Andyp » 06 Mar 2017, 08:22

Mike G wrote:
Andyp wrote:Whats the problem with storing hot water?
I have One of these, sorry it is in french. But it is basically a large emmersion heater. It heats the water on an overnight tariff between 10pm and 6am to a temperature of about 50-60 degrees ( too hot for a bath without adding cold water). The outside of the cylinder never feels hot, in fact not even warm, so the insulation must be good. The water is as hot to the touch at 6am as it is at 9.30 pm.

I appreciate that pumping stored water around a CH system must reduce the temperature to a greater extent than just using it for hot water but hot water can be stored once heated, surely.


A thermal store is an altogether different beast from a normal hot water cylinder. The water in a thermal store is not the water which comes out of the tap, or circulates around a central heating system, both of which are heated via heat exchangers within the thermal store. In other words, instead of heating water up in a boiler and storing it for later use in a hot water cylinder, the boiler is constantly topping up the heat in a thermal store via a flow & return circuit and heat exchanger, and this thermal store then gives up its heat, via further heat exchangers, to the heating and hot water circuit. That store is kept at the same temperature all the time, 24 hours a day, every day. It's a wonderfully inefficient way of storing and using energy.



Thanks Mike,
It was Bob's quote
9fingers wrote:I have come to realise that storing hot water is a mugs game unless the source of heat is free.
that prompted my post. As David pointed out
chataigner wrote:.. the insulation is VERY effective. 300litres at 65°C loses heat at the rate of 0.1W. ie a loss of 2.4Wh per day or about enough to run a 40W lamp for 3.5mins.
it is how it is used that causes the inefficiencies.
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Re: Do I keep the thermal store

Postby RogerS » 09 Mar 2017, 17:44

Just come off the phone to a chap who's put in an oil-fired boiler with a heat store. This is his rationale. It actually makes a fair bit of sense to me but interested to hear others thoughts.

1) Condensing boilers are only at their most efficient (the 90%+ that the makers claim) when they are condensing. But condensing stops around 65 degrees which is way below were most boilers are set. Ergo oil-boilers setup with a high temperature than 65 degrees are much less efficient. Significantly, he claims.

Anyone else agree with this ?

2) Boilers also work best when they are working hard.

3) Boilers wear and tear improves when they run constantly and not cycling on and off.

So he has a heat store that is heated by an oil boiler. The heat store then supplies the heat required by the underfloor heating and radiators. He has two sensors on the tank. One at the top set to around 70 degrees or so and one at the bottom set to around 50 degrees. When the top temperature is reached the boiler switches off. It restarts when the lower temperature limit is reached. So between those two 'events' the boiler remains off. (He did admit that he got a little bit of cycling towards the end of the heating phase).

The size of the boiler is small. Much smaller then the one specified for a 'traditional' system and he's thinking that even the one that he has is a bit over-sized.

It does mean that his radiators are running cooler than their design temperature and I've not got my head around that one yet...seems to suggest that larger radiators in each downstairs room would be needed in the absence of UFH. So there's a trade-off there to be worked out.

For DHW (domestic hot water) he has a coil in the bottom part of the tank and this forms a complete insulated secondary return system which constantly circulates his DHW. So no dead legs. No wasted water and instant hot water.

Of course his heat store is only three years old and so significantly better insulated then my 2002 fibreglass wrapped monster.

Thoughts anyone ?
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Re: Do I keep the thermal store

Postby 9fingers » 09 Mar 2017, 18:39

Most of this is what I was trying to say earlier in the radiator sizing discussion.

1) If you set your boiler to run at best efficiency which is a return temperature of 40 or so then you must oversize the radiators to get enough heat out.

2) With a flow temperature of say 55 (to get return of 40) you should get a DHW temperature of say 50. We run at this temperature and it is still too hot to put your hand in for either washing up and certainly too hot for a bath.

3) Pumped circulation of DHW is all very well but will be more wasteful of energy than having dead legs (this is for a similar level of insulation on the pipes. The hot taps will act like little radiators pi88ing away your energy. Plastic piping will be better than copper if you go this route being lower conductivity on the run up to the tap.

4) Running the boiler at near max output for longer periods is far better than cycling so yes I agree with him

5) As for wear and tear caused by cycling maybe this is more significant for oil as the ignition sequence is working less. Not convinced it is a big deal.

6) Your guy then shoots himself in the foot. For his top stat set to 70, the boiler will have to run at say 75 to achieve that so it will spend all its running time not running in condensing mode.


I still say that storing expensively heated water is a mugs game. Storing water heated for free makes good sense but it does not sound like you are going to do that.

The ideal situation to me for paid for energy is to have a boiler that at full tilt will supply your max demand for DHW. there are only two of you so that might be filling two baths at once (or have fun and share a bath!) or more likely one bath or two simultaneous showers. This needs no storage.
Having worked out the DHW need, check the boiler can provide your worst case radiator need BUT NOT BOTH. You simply turn off the radiators when there is DHW demand. You won't notice the difference and it keeps the boiler size down and so efficiency up.

Flow temp say 55, "oversized" rads and you are good to go. Keep the store for when you realise renewables can cut your bills and that too can feed in water at 55C when you have it and not fire the boiler.

I think this is all coherent but if not point out any gaps and I'll try and help.

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Re: Do I keep the thermal store

Postby MJ80 » 09 Mar 2017, 18:41

The system the chap has sounds similar to mine. I have a 15kw pellet system boiler that heats the thermal store. The tank itself is a twin coil affair, one coil supplying the hot water and the other connected to the solar water on the roof. There is a sensor at the top and at the bottom and a circulating pump, if it goes below a certain temp then it will reheat the system, this also applies to the temp of the hot water. So quite clever theoretically.
The heating is a mixture of underfloor and radiators and the boiler has a weather sensor fitted. The ufh and rads are on different circuits each with its own mixer and controller. The ufh can run all day at 25 ish degrees and just take a bit of warm when it needs. The rads have set times to come on and the flow temp depends on the external temp.
From a year of observing the system I have learnt the following.

It needs to be a better insulated tank, something I would address in the future, but when its going in winter it can be around -20 and it uses 14.5 to 15 kg pellets a day.

The hot water can run tepid when everyone has been having a shower, the tank will show hot at the top so it hasn't kicked on but at the bottom it it cool. I could rectify this I think by controlling the incoming flow rate so it has a longer time in the coil. Or my daughter could just wash in the stream until she appreciates the luxury of hot water. The boiler in this respect isn't very responsive, I have made the parameters closer, but it is too soon to see how much difference it makes.

Pellets are nonsense, if we could get gas that would have been better. The last owners pulled the gas tank out and were installing a monsterous woodburner and 6000 litre tank,with I think 500 litres on the burner itself. I made them take it out, there is no way it would keep warm for days and I would be feeding it forever. Plus there is no automation. The tank was welded on site, I thought it was something to do with farming when I saw it. Madness

The Solar on the roof is great at keeping it topped up, we are just starting to get some through now the weather is clearing, and it gives all the hot water from May through late September.

Cost - I miss our gas combi in our old house. Quick, efficient, cheap. The system here is expensive even with the grants. Pellets are expensive and not as efficient as gas and I believe they are not a green solution.
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Re: Do I keep the thermal store

Postby RogerS » 09 Mar 2017, 19:06

Thanks for your comments, guys.

Bob..is your boiler a combi fed with mains pressure ?
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Re: Do I keep the thermal store

Postby 9fingers » 09 Mar 2017, 21:02

RogerS wrote:Thanks for your comments, guys.

Bob..is your boiler a combi fed with mains pressure ?


No it is a system boiler and not connected to the mains at all. It heats a proper thermal store ie one where the water NEVER leaves the store. it is simply a store and could be full of oil, water, concrete whatever. It has 3 heat exchangers and two immersion heaters
Heat is added from the boiler, and the sun and taken out to produce DHW only. The store is at atmospheric pressure.

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Re: Do I keep the thermal store

Postby Doug » 10 Mar 2017, 09:54

What you need to do is work out the savings any efficiencies are going to make over the cost of making those alterations.

If your fuel bill is £1000 annually & running your boiler at maximum condensation level will give you 14% efficiency saving that's £140 a year. That will just about buy you a bronze secondary circulation pump, the payback time on nearly doubling your radiators & upgrading pipework will run into decades

I'm not saying don't maximise any potential savings but having worked on many domestic projects the proposed savings are far out weighed by the installation, material & servicing costs
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Re: Do I keep the thermal store

Postby 9fingers » 10 Mar 2017, 10:02

Excellent point Doug. My system goes to extremes regardless of capital cost or my design and installation time. Being retired I looked on it as a form of annuity. ie pay a lump sum up front and have negative total energy bills for 25 years and a huge plus for me, the interest of playing with the system contols in full nerd mode :lol:
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Re: Do I keep the thermal store

Postby RogerS » 10 Mar 2017, 11:24

Doug makes a very good point and is were I got to late last night. For all the aforementioned reasons, I can't see us staying here that long. Could be wrong but somehow I doubt it.
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