Big tank, extra coils?

[kristen]

I seem to remember reading that one should not have a thermal tank that is too big - otherwise it just gets lukewarm when there is insufficient solar gain.  OK I understand that!

However, if I had a big tank and it was arranged with a coil from the boiler "high up" and a coil from the Solar "low down" could I have a valve that pushed the solar return through the top coil [instead of bottom one] when the top of the tank was cold enough?

I foresee a couple of obstacles:

I wasn't planning any coil from boiler - the Boiler, Thermal store and CH pipework would all use the same "muddy water". I presume it is bad to share that with the Solar circuit and maybe prevents the solar circuit being pressurised?

OK, assuming I've got that right how about two coils for Solar return, so the valve can divert to the "higher" one when the top of the tank is cold enough? (I'm thinking Summer time when the boiler doesn't come on at all, and we have a few overcast days)

Thanks,

Kristen

[lightfoot]

Hi Kristen,

Yes you can use a upper and lower coil for your solar and room permitting I think it is a great idea to maximize solar gain.  You could use a Termovar AF Bypass Valve  http://www.acaso.se/english/index.html or a motorised valve to divert the flow to the bottom coil when the top of the tank is up to temp.

What size of store etc are you considering and what other heat sources do you have.

Lightfoot

[kristen]

Thanks.

We are looking at 8 panels initially (we have a swimming pool, so I'm looking for the maximum that we can lose the heat from  in the Summer, and thus have as many panels as possible for Winter). 

We have a large flat roof above the boiler/utility room, so no practical limit to the number of panels we could have, but the pool is in a "greenhouse" type cover, so doesn't really need much heat from Jun-Sep, hence start with 8 panels and make sure we can actually dump the heat without the pool becoming unbearably hot! and then decide if we can add more later.

I was planning to work back from that to decide what size thermal store I need.  Plenty of room in the boiler room, ceiling is 260cm

I would like to heat-exchange to pool during the night for economy-7 as much as possible, so I'm thinking to heat Thermal store during the day, dump to pool [during day] if thermal store gets too hot, and then dump to pool during the night (running the pool filter at the same time), until the thermal store falls to "minimum" temperature for DHW

Kristen

[lightfoot]

Kristen,

How many bedroom and bathrooms do you have ?

Do you have any idea what your space heating requirements are in terms of kW or BTU's ?

What is the output of your main boiler, is it oil, gas, wood etc and is it a open vented or sealed system ?

How big is the pool, do you just heat it with off peak electric & solar or is there another source ?

Do you have UFH or rad's or both ?

Do you have a good mains water supply ?

Do you suffer with limescale or dose your kettle fur up ?

lightfoot

[kristen]

"How many bedroom and bathrooms do you have ?"

5 beds, 3 bathrooms.  Family of 4, I work from home and my wife too about 1/2 the week, another 4 people come in during the day.  We often have 4-6 extra staying at weekends.  House footprint is 270 sq.m, 2 floors, so total of 540 sq.m.  Build in 60's, lots of concrete! very solid - block&beam, solid floors, was flat roof, pitched roof has been added on top.

"Do you have any idea what your space heating requirements are in terms of kW or BTU's ?
What is the output of your main boiler, is it oil, gas, wood etc and is it a open vented or sealed system ?"

Current boiler is oil fired, 250,000 BTU.  That was installed before double gazing, roof insulation and cavity fill.  Its knackered now! but presumably could be replaced with something with less output [due to insulation added].  We ran it on average for 2 hours morning and 2 hours evening during last Winter.  Boiler room and hot water tank downstairs, open vented, header tank and separate expansion tank just below upstairs ceiling.

"How big is the pool, do you just heat it with off peak electric & solar or is there another source ?"

70 sq. m, 110-ish cu.m. New-ish Oil fired boiler, ran for about 2~3 days to get the pool up to temperature at Easter.  Now pool temperature is around 78F boosting to 80F if I run the heater for a couple of hours (e.g. on Saturday mornings!).

"Do you have UFH or rad's or both ?"

Cast iron skirting rads.  Single circuit up & down.  Plumber says that it looks fairly challenging to convert into two separate zones.

There are three gravity-fed towel rails that ensure that hot water is available promptly.  These use up all the current tank's stored heat in about 8 hours.  I think we need to replace with a main-feed hot water system (coil in top of thermal store), and a circulating pump.  Run the pump at key times in the day - e.g. just before evening and morning ablutions, and when a tap is in use, plus perhaps on a push-button-timer prior to using dishwasher?

"Do you have a good mains water supply ?"

Yup, I reckon.  Pressure seems better than other properties we have been in.

"Do you suffer with limescale or dose your kettle fur up ?"

The water is hard (we filter water for drinking / kettle)

Kristen

[lightfoot]

Hi Kristen,

Thanks for the useful answers,  Let me have a think about it, do some sums and I will come back with some suggestions later.

In the meantime can you give me a better idea of the pressure and flow rate of your incoming mains ie how many buckets can you fill in a minute and dose the flow rate drop off much when you open other taps on this main ??

As for the kettle furring up I am trying to work out weather you suffer from temporary hardness ie the mineral deposits come out of the water when heated a stick to the inside of the pipes, cylinders etc.  Do you get deposits around the hot tap outlets etc or ask your plumber as he should know.

One other thing I wouldn't scrap the lovely cast iron rads unless you really have too.

Lightfoot.

[kristen]

"Thanks for the useful answers,  Let me have a think about it, do some sums and I will come back with some suggestions later."

Many thanks for your help.

"In the meantime can you give me a better idea of the pressure and flow rate of your incoming mains ie how many buckets can you fill in a minute and dose the flow rate drop off much when you open other taps on this main ??"

20 seconds to fill 10L bucket (tap in garage).  I opened the garden tap full and it didn't take any longer to fill the bucket from the garage tap.

"As for the kettle furring up I am trying to work out weather you suffer from temporary hardness ie the mineral deposits come out of the water when heated a stick to the inside of the pipes, cylinders etc.  Do you get deposits around the hot tap outlets etc or ask your plumber as he should know."

I'm in what is regarded as a hard water area.

"One other thing I wouldn't scrap the lovely cast iron rads unless you really have too."

The plumber thinks they are the dogs danglies, so they are staying!  Plus wife not too keen on ripping up all carpets, digging an inch out of the floors, and cutting an inch off all the doors to put in UFH!

Kristen

[lightfoot]

Hi Kristen,

As a ballpark figure allowing say 30 W/m3 for fabric heat loss and 1.5 air changes per hour your peak heating load would be around 60 kW or 200'000 BTU's .  This is just a educated guess and may well be less than this with the work that you have done.  On this size property I highly recommend you have a detailed heat loss survey done before you shell out too much cash as it could save you money in the long run, any heating engineer worth his pay should be able to provide this and don't be fobbed off by some  ' I've been doing this for thirty years' rule of thumb nonsense !

It's also a good idea with a heavy mass house to have the heating (boiler) on low 24/7 in the winter months, because if you allow the house to cool too much you will have to reheat the mass before you feel the benefit.

I will get back with some DHW info later.

Lightfoot.

[lightfoot]

Hi Kristen,

I have had a bit of a think and crunched a few numbers and going on the info you provided this is what I have come up with so far.

I would install a 500 litre Thermal Store, something like a Akvaterm AKVA 500 EK (sorry Ivan) they come fitted standard with a 35 L/min (90 kW) DHW coil in the top hatch (you may need to upgrade to a higher flow rate coil), and you can have a suitable solar coil fitted in the bottom hatch, they are very well designed and insulated and reasonably priced (around £1400 with solar coil + two immersion heaters delivered) It is 2050mm high and 800mm in diameter and will weigh just shy of 700 kg in service. http://www.akvaterm.fi/Default.aspx?id=529581

My thinking is you connect the solar panels either all in parallel or maybe connect 4 pairs of panels in parallel with each pair in series if that makes sense ?  This may be easier to balance and give the best compromise between high temps for the thermal store and maximum solar gain. Connect to the solar coil on the store. Tee off the main solar flow and return between the panels and store and run pipework to another heat another heat exchanger for your pool, again I could give you a drawing for this setup. Fit a zone valve on the flow to the store between the store and the tee and another off the tee on the flow to the pool heat exchager.  The store will get priority over the pool and when the store stops calling for heat the zone valve for the store will close and the zone valve to the pool will open and divert the flow to the pool heat exchanger.

You won't need a pump to supply the taps with a thermal store of the mains but if you have very long pipe runs you may have a problem with dead legs. The towel rails sound like they may already be on the secondary return which would normally use a small bronze pump but could of been done by gravity. You could reuse this circuit again but you will need a bronze pump for this on a thermal store, You have to connect it all up differently but I could send you a drawing to explain how to do this as it would save a lot of typing.

With zero solar gain available your boiler should be able to recover this store to working temp within a hour, plus you have always got two immersion heaters to fall back on.

I have tried to balance demands on the system with the potential inputs to give a fast response with good overall efficiency.

Feel free to ask questions and will try my best to answer them.

It would be good to hear other peoples opinions on this too, as there are some smart people on this forum.

Hope that's some help, I'm off to bed.

Goodnight,

Lightfoot

[lightfoot]

Hi Kristen,

Having slept on it I think you may get away with using 4 (30 tube panels) or 6 (20 tube panels) you can always add more if needed.  As you are going to get most of your solar gain when your central heating is least required, your pool is the limiting factor and you don't really want to be over heating it and this depends on how well insulated the pool is and how much DHW you use.

Installing a larger or additional store may help to usefully buffer heat during times of peak solar gain, the other option is to cover some panels in the summer if the pool over heats or use a quench coil off the mains.  I hate to see energy wasted.

I am assuming the panels will be in a optimum position, where abouts in the country are you.

You mentioned the towel rails consume a tank-full of hot water in about a 8 hours, I am guessing your tank is between 200 and 300 litres as I am trying to get a idea of the output of the towel rails.

Lightfoot

[kristen]

Tremendously helpful, many thanks.

I can visualise the layout, so don't think I need drawings, thanks!

I would prefer to have more panels so that I can get some gain in the Winter to supplement the CH.  I'm assuming this isn't completely nuts?!

January is 1.25kWh per 20-tube panel.  But by the time we get to March or October that is 3kWh per panel.  8 panels would be like running a kettle for 8 hours

More panels would help me with the initial heating of the pool around Easter time too.

Infrastructure cost of the plumbing (thermal store, pump, pipework up to the roof) is constant.  So more panels is just incremental cost.  So even if I have to shade them its not too bad.

"Tee off the main solar flow and return between the panels and store and run pipework to another heat another heat exchanger for your pool"

This works for Heat Dump.  However, I would prefer to use heat from the Thermal Store to heat pool at night for Economy-7 when the pools filter circuit is running.  What do you think?  (So this may mean that the heat dump becomes override-activation of the night-time-circuit that transfers heat from Thermal Store to Pool; also means that in-extremis I can use the CH boiler to heat Pool i.e. via Thermal Store)

Would a heat exchanger on the pool pipework run by gravity from the Thermal Store (so just need a mechanised valve to turn it on when the Thermal Store is hot enough), or would it need a pump as well? (Some energy-usage calculation to be done there perhaps).

Alternative might be to have a second circulation pump on the pool, bypassing the filter, and thus needing less horsepower.  That could come on when the divert from Thermal Store was activated.

I'm also a bit concerned if the pool pump is going to be on-off if controlled by the availability of heat from the Solar Panels. I don't suppose those sorts of pumps are well suited to on-off operation, so that might favour heat exchanger direct from the Thermal Store too.  If the Heat Dump function cuts in the pump could stay on until the thermal store has dropped by, say, 10 degrees.

"if you have very long pipe runs"

I think I do!  If I turn off the gravity-feed towel rails it takes more than 5 minutes running the taps to get hot water.

"plus you have always got two immersion heaters to fall back on"

The boiler broke this year.  Falling back on gas stoves etc. was a poor substitute, although it was quite mild at the time.  Boiler man took a few days to get to us and sort the problem out.  I wonder if I should consider having enough electric heating to be a substitute for the boiler?  Am I right in thinking that there are electric "tube shaped" heaters that would be good for this type of use, and are relatively cheap to buy (dear to run of course!).  Would single-phase be enough? (Again, I suppose this would be Economy-7, so not much else would be on in the house)

"your pool is the limiting factor"

Yes, that was rather what I was expecting.  We didn't heat the pool at all (well, just the odd day here & there) last year from July to end September.

We can take the thermal cover off, to increase evaporation, but that increases chemical and top-up water usage.

"Installing a larger or additional store may help to usefully buffer heat during times of peak solar gain"

Do I run the risk that I only get a larger thermal store to "tepid" on overcast days?  (Or are we back where I came in of having two Solar Coils, one higher one lower, in the Thermal Store?)

"I am assuming the panels will be in a optimum position, where abouts in the country are you."

Suffolk.  The flat roof "faces" South.

"I am guessing your tank is between 200 and 300 litres"

Existing tank (including installation, can't tell how thick that is) dimensions are 140cm tall, 230cm circumference

I owe you a beer, at the very least!

Kristen

[lightfoot]

Hi Kristen,

Yes instead of diverting you solar to a pool heat exchanger (heat dump) you could just take a pair of flow and returns straight from the store to the pool heat exchanger, you could control it by using a pump on this circuit.  This will give you more options as to heating the pool rather than just using it as a heat dump.  You could use and simple timer/programmer and high/low cylinder stats for this with a manual over-ride option.  Note that if you use chlorine in the pool you will need a stainless steel heat exchanger,  I would install a plate heat exchanger into you filter line.

You could fit more panels but you need to be able to put that heat somewhere useful or you will just be wasting money.

You can get electric flow boilers but it's not something I would consider for your size of house with rads etc.  You can run your central heating direct from the store with the boiler kicking in as needed and you have two off peak immersion heaters but I would not rely on them to heat your house.  To give you some idea (going by your current boiler size and floor area) at a push there is enough energy in 1000 litres of stored water to heat your house for about a hour at peak load ie -1 deg C outside.  You could use the heat stored in you pool to heat your house but you would really need to use a heat pump to step up the water temp to run your rads.

Depending on how much insulation, your existing tank could be getting on for 500 litres which is more than I thought it might be, so if you want to benefit from your towel rails on a secondary return then you go for a somewhere between 750 & 1000 litre thermal store, either one big one or two smaller ones, one main and one slave.  The benefit of having two stores is apart from the flexibility and size is that you have two more hatches to put coils in.

Only temporary hardness causes limescale deposits which will coat the inside of your DHW heat exchangers and hot water pipes, this will reduce their efficiency over time.

Hope this helps.

Lightfoot.

[kristen]

Excellent.  I even think I understand it all, including the "nuance" of the opportunities 

I don't need to heat the towel rails, per se, they are there (I think) to ensure, in a nice old fashioned way, that hot water was readily available from the taps by forcing a gravity-loop.

So I would favour reusing that circuit but with an on-demand pump, which will have a side effect of causing some heating of towel rails too - I don't want to "waste" any more Solar Heat than I have to ...

Just to repeat a bit from an earlier post of mine:

"Run the pump at key times in the day - e.g. just before evening and morning ablutions, and when a tap is in use, plus perhaps on a push-button-timer prior to using dishwasher?"

Kristen

[lightfoot]

Hi Kristen,

Yes that would work fine,  just put the pump on a timer/programmer like your central heating has.  Just to make sure.... you don't need the pump to get the water out of the taps only to stop dead legs and benefit for warm towel rails.  You must use a bronze pump and if you or your plumber are not sure how to fit a secondary return on a thermal store let me know and I'll do you a sketch, a picture paint's a thousand words and all that.

Cheer's

Lightfoot.

[kristen]

"Just to make sure.... you don't need the pump to get the water out of the taps only to stop dead legs and benefit for warm towel rails."

Yup, that's my aim - given that it took me 5 minutes to run the taps hot when I turned off the towel rail circuit as an experiment 

"if you or your plumber are not sure how to fit a secondary return on a thermal store let me know and I'll do you a sketch, a picture paint's a thousand words and all that"

Well, I had a go an explaining what I think it might be in another thread today ... how did I do?   

http://navitron.org.uk/forum/index.php?topic=1630.0

Kristen