My solar/woodburner/gas combi/thermal store plan for scrutiny

[mistadave]

I have now prepared a plan for the heating and DHW for my renovation and would appreciate peoples comments.

The house is a 1860's end of terrace cavity wall which we have pretty much completely rebuilt short of knocking it down and starting again. The floor area is just over 100m2 with a 15m2 conservatory being added on the back but this will be rarely heated. The cavity had already been filled with yellow fluff but the sections of wall that we had to rebuild (of which there were a fair few) have been filled with celotex. We are then dry lining with insulation - the narrower back of the house with 20mm marmox board to save space and the main part of the house with about 30mm of celotex behind a metal gypliner system because the walls are so wonky. The roof will have 150mm of celotex and the floor 70mm on a concrete slab. I'm using the celotex and marmox as my air tightness layer and getting an MVHR unit and with this i hope to achieve at worst 5.5-6kw heat loss (according to the dps room loss calculator). You will probably not that this is going to work like a house with relatively little thermal mass and this is deliberate because we are in and out all the time so needed a house that could heat up quickly on demand.

So, here is my plan for making and using that heat:


Sorry if I haven't used all the right symbols for various valves! the dashed lines are all mains HW, the header tank is up in the loft and the black lines are the feed and vent for the tank and also the waste pipe for the combi condensate, overheat waste, and also for the drain at the bottom of the store. There are gate valves to isolate the DHW coils so that i can flush them with scale remover if needed.

The thermal store which I already have in position is an Akvaterm solar 300L, it is sat on an rsj upstairs between two bedrooms so pretty central. The woodburner is a dunsley yorkshire which has 9.2kw to water and 4kw to air. It is room sealed and has thermostatic control and is in the lounge/dining room below and 1-2 m to the right of the store. I am connecting it with a ESBE load unit. The max output of the stove to air is way more than I will ever need for the lounge but I plan on running it on a far lower setting for longer which is is what dunsley say is the way to get the best efficiency figures anyway. Also I can leave the door open so the heat can go upstairs too.

The gas combi which I don't have yet is going to be an intergas. These are pretty special in that they have very few moving parts, can fully condense in dhw mode, but most importantly they are the only combi that I know of which can run open vented and where there is no temperature limit for the cold DHW feed to the boiler.

I'm planning on having 30 or 40 58mm navitron tubes (as long as they eventually get back to me with an installer and a quote hint hint!)

All of downstairs will be a thin modular screen, fast reacting UFH system http://bekotec-therm-uk.schlueter.de/bekotec-therm.aspx and all of upstairs is radiators but sized to run off the same temperature as the UFH. This will probably be a flow of 40 and return of 30, or a flow of 42 and return of 27. There will be programmable thermostats in every room and everything will come off the central manifold. I am planning to use a smart pump and draw off the top of the tank via a mixer, but also on the return use a thermostatic diverting valve to make sure water can't return to the bottom of the tank unless it is below 30, otherwise it gets diverted back around to reduce mixing.

I feel that the thing that makes this thermal store setup different to your "normal" TS setup is that I don't plan on using the backup gas boiler to maintain a temperature in the tank at all. I figured that it is a waste of energy to heat the top section of the tank if you aren't going to use it all so my store is really only for the wood and solar energy, and perhaps to allow the combi to run at its most efficient, but not really as a store for the combi.

If there is a call for central heating and the water in the top of the tank is below 40 the combi will fire. It will use the colder water from the bottom of the tank to aid condensing but there will be a thermostatic diverter after it to divert water back around that isn't up to temperature. This is to prevent mixing in the tank not just when starting to heat, but also when the boiler auto cycles the pump every day for a minute or so as most boilers do these days.

The hot water from the boiler is fed straight into the pipe that feeds the CH (but before the mixer valve) so we won't be waiting for it to heat up the top of the tank, heat should be instant. I will also wire it in with some kind of timer relay so should it fire but the heat demand cycles off, the combi stays on for at least a reasonable period (not sure how long this is, 5 mins?) regardless before turning off. Any excess heat would obviously spill back into the top of the store and so wouldn't be wasted while maintaining full condensing and no cycling - gas boiler heaven! I have put in a motorised valve by the boiler which will be closed when not operating, this is to prevent the CH pump drawing water through the boiler from the bottom of the tank when the boiler is not in use - not sure if i really need this or not though.

On the DHW side the mains will pass through the preheat and upper coil as is standard on akvaterm tanks but there will be a stat on the pipe just as it leaves the store which I am hoping will reflect the water temperature in the coils even if there is no DHW being drawn off. This will control a motorised valve and divert straight to the taps if above 42ish and to the combi if not. There is a mixer valve for reducing the water temperature but the exit of the combi will bypass this to cut down on unnecessary restrictions, intergas advised that it wouldn't be needed as long as we used their external stat to tell the combi what temperature the water was as it left the store and it would modulate to suit the output temp we set.

Because I am not maintaining the top of the store at a certain temperature I can gain more energy from the solar coil and have a bigger buffer for the wood burner - approx 22kw which is 160 mins at max on the woodburner. Overheat protection will come in the form of a watts sts20 thermostatic safety valve - once the top of the store hits 97 it opens the valve and mains water passing through the coils (rated at 35kw each) brings the temperature down. I have been told that this is enough to satisfy regs since the stove also has thermostatic control and it all works without power including the load unit which has a gravity bypass.

So, what does everyone think?! I have spent a LOT of time trawling various posts on this forum gathering ideas so a big thank you to all contributors. This is a mix of all those ideas so I welcome any constructive criticism!

Ta

Dave

[HalcyonRichard]

Hi Dave,
            I am at the thinking stage for my system. But as I am moving it's very much in the air at the moment. Dunsley have various system diagrams on their website :-

http://www.dunsleyheat.co.uk/images/LINKUP_LEAFLET_July_09.pdf

They use a "NEUTRALIZER" for connecting system elements together which save on valves/pipework I should think. It's probably worth a look as you may save pipe runs and use less valves.

Good Luck Richard

[mistadave]

Hi Richard,

I did consider the neutralizer when I first bought the stove, however these are designed to work with more traditional systems and have no way to store the heat. The thermal store effectively replaces the need for a neutralizer as it is now the neutral point between all the heat sources, the difference is that it also stores un used heat. Most of the pipework and valves I have added is because of a desire to make the best use of that stored heat.

Certainly though, if you don't have space for a thermal store or don't need/want to buffer the heat then the neutralizer seems to be the simplest way to do it.

[knighty]

that's looks like a good / well thought out setup...


not much I can think of other than... well done !


are you a plumber ?  not many people get it right first time !

[mistadave]

Thanks Knighty!

I'm not a plumber, as I said I just spent a LOT of time reading up various threads and sites, picked all the bits that looked good from other peoples plans and added a few bits of my own.

Luckily I have a couple plumbers in the family to come and fit it for me but when I originally approached them about designing it like most plumbers they wanted to stick to what they knew which was unvented cylinders or normal combis etc. So when I started talking about thermal stores they politely suggested I design it myself as they hadn't done anything like it in the past. I just hope they don't find it too much of a pain to fit it all into the store room, especially since I have already annoyed them by insisting on using plastic pipe everywhere possible!

[Solal]

Remember a thermal store  will  have  inferior  dhw water  delivery rates in comparison  to an unvented cylinder.
They also utilise less  solar  enery due  to  the inefficiency  of instantaneous  heat transfer  across a heat exchanger.
They  sludge up and encourage  sludging  build up in ch/dhw integrated systems.

Sealed systems  are the way to go. DHW delivered directly  from the cylinder  and solid fuel connected to  unvented cylinders. The europeans have been  doing it for years. They know best. Especially the Germans.

[mistadave]

I wanted to go for a sealed thermal store originally, using a woodfire stove which was designed to run on pressurised systems, and the akvatherm stores are all designed to run sealed anyway. The problem was really that even though you can buy the kit it still seems it is illegal to run a solid fuel stove directly on a pressurised system in the UK, a shame really as it would open up a whole load of new possibilities.

That was an unvented thermal store though, I assume you are talking about an unvented DHW cylinder? How do you propose i use that for space heating?

You could I suppose run another coil to take off heat for space heating, but then your argument about the heat transfer on a heat exchanger applies in the thermal stores favor, as does the fact you can't heat a unvented cylinder directly with a wood stove so there would be losses there too. Then the argument for the solar efficiency is lost when you have to use gas every day out of the solar season to heat the entire tank to 60 degrees to kill legionella - a complete waste if you aren't going to be using that water and something you don't have to do with a thermal store.

I'd assume it is easier to descale the inside of a DHW coil in a thermal store, than to fill a whole unvented cylinder with descaling chemicals in order to remove scale from the outside of the boiler coils. I know I'm going to have to put a lot of inhibitor into the store, but this with a some magnacleans will keep any sludge problem at bay. You can't really use that argument for an unvented though as the reason they don't have any sludge problems is because they aren't dealing with the space heating water which thermal stores do.

I'd agree that unvented flow rates are generally better if you were filling a bath, but from experience of my parents solar thermal store which is nowhere near as good as the akvaterm (its a square gledhill thing) the flow rates are more than I could ever need, certainly better than any combi. The two coils in the akvaterm are rated at a total of 180kw so the temperature of the water as it leaves the coils isn't going to be too far off the temperature of the tank.

More disadvantages of unvented? I can't buffer the stove over the whole cylinder, or I can't buffer the gas boiler to keep it condensing. If I use a coil off it to run the heating I have to heat up a lot of the DHW in it first before the heating will work. Or if I have a second cylinder to run the heating I will then have to choose which one to heat first, and will ultimately end up not using it all, and you have more losses from two tanks instead of one. That's the beauty of the thermal store, no matter what heat you use, it goes into one place and it doesn't matter if you need DHW next or space heating, it will come out of of the same place until depleted, no waste.

[Solal]

no waste

Theres' always  waste. Kristens  thread  from  the archives  data logging the heat loss   in his  Akvaterm  clearly  shows the waste.

The two coils in the akvaterm are rated at a total of 180kw
Are you sure?

Their info  says....
DHW = Domestic hot water or DHW preheat
 coil 9,4 m, 22 mm finned copper

I don't know the area but if you calculate it  at  say 1.2 sq/m x 80c delta  it  only gives 54kw.

[mistadave]

I know it is always going to loose heat somewhere, but you know what i mean, you are going to waste more by heating it up when you don't need it and you are going to get more losses at a higher temperature. Heating up a tank full every day to 60 degrees to kill legionella is a BIG waste, as is heating up a tank of dhw when the next thing your system wants is CH.

I got the 180kw figure here(2x90kw): http://www.akvaterm.fi/eng/Other_products/Coils.54.html

I know that never represents the amount of heat it can put into the water in practice, but my point is that they are pretty good coils, certainly a world apart from the ones in my parents gledhill that still manages to provide a better flow rate than we need. So what advantage am I really going to get from unvented, other than a bath might finish running a minute sooner? Not really worth all the other disadvantages to my heating system?

[dhaslam]

The only question is  the heating output from the combi, this seems to go directly to the  heating circuit rather than the  store but the return  comes from the store.   Was this just to avoid storage losses?     

[guydewdney]

it is illegal to run a solid fuel stove directly on a pressurised system in the UK,

the word that is relevant here is DIRECTLY. Use a heat exchanger to separate wbs from cylinder, and you are laughing. well - I am anyway....

[mistadave]

Dhaslam: I think the best way to describe what I have done there is that top left tapping is both a return to the store from the combi, and a flow to the central heating combined.

With the usual setup you would have say the flow to the CH on the left, and then the return from the gas boiler on the top right tapping. The problem with this is because I only plan on heating the top of the tank on demand with the gas boiler, it could take quite a while for the heat to build up enough in the top of the tank for it to actually get to the CH flow on the LH side meaning slow response times.

By combining the two in one tapping it is putting the heat exactly where the CH is drawing it off so a fast response, but if the boiler is producing more heat flow than is needed by the ch (say the thermostatic mixing valve closes down) the hot water flows right and out into the top of the tank just like it would on the more normal setup.

Guydewdney: That was what I found, a shame as you can run them directly on a pressurised system in europe but I'm sure we will catch up eventually and the good thing is when we do my setup can easily be converted to a sealed system. I didn't want to do it indirect though because the stove can't buffer the whole store that way.