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Heating and cooling: Thermal store/unvented cylinder questions

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    We are renovating a Victorian three storey house and are adding a bathroom on the top floor. Amongst other points, we are considering replacing the two boiler setup with a single boiler/tank combination, with possible extra heat input, probably from solar-thermal, so we are thinking of using a thermal store or an unvented tank on the first floor, equidistant from bathrooms and kitchen. We have many questions. To keep things simple, I am keeping some of the questions about solar thermal out of this thread and I'll ask them separately. I'm sorry if some of these questions have been asked before. I've been reading around so much that I've sort of gone cross-eyed and fuzzy-headed!

    If we use a thermal store, how does that compare with an unvented cylinder in terms of the flow rate/water pressure that it can supply at a given temperature? Is it the same in each?

    Given a thermal store, is it possible to upgrade it at a future date by adding more inputs? Put differently, do we need to buy a thermal store with all of the anticipated possible inputs in mind right now, or could we buy a thermal store with, for example, just solar thermal & gas boiler input, and then add another input at a future date, e.g. PV-T?

    Similarly, could a thermal store or unvented tank on the first floor supply a bathroom on the second? Is that simply a question of sufficient mains pressure?

    How do we judge how large a thermal store/unvented tank to get if we are just doing hot water?

    If we wanted to run our heating system off the tank, presumably that would most easily be done with a thermal store. How do we go about calculating the size of heat store that we would need to heat the house, assuming that we knew the overall heating requirements of the house?

    Is it possible to have a boiler supply the tank AND directly supply a shower & set of taps? In this way, if we ran out of hot water in the tank, we could just use the shower that ran directly off the boiler. Perhaps this behaviour could be switchable?
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      CommentAuthordjh
    • CommentTimeMar 3rd 2018
     
    Posted By: VictorianTomIf we use a thermal store, how does that compare with an unvented cylinder in terms of the flow rate/water pressure that it can supply at a given temperature? Is it the same in each?

    I'm not completely sure but I think they can both deliver flow delivery limited by the cold supply. You'd need to check specifications to be sure - particularly as to the rating of the coil or PHE in a thermal store - or I'm sure somebody will correct me if I'm wrong.

    Given a thermal store, is it possible to upgrade it at a future date by adding more inputs? Put differently, do we need to buy a thermal store with all of the anticipated possible inputs in mind right now, or could we buy a thermal store with, for example, just solar thermal & gas boiler input, and then add another input at a future date, e.g. PV-T?

    It's a lot easier to buy it with all the possible inputs you might want, because then you just have to remove a blanking cap and plumb a new input in. Otherwise, you might be able to share an input in some cases, or it might be possible to make new bosses in the tank, but if you needed another coil then things get very difficult.

    Similarly, could a thermal store or unvented tank on the first floor supply a bathroom on the second? Is that simply a question of sufficient mains pressure?

    Yes, for either.

    How do we judge how large a thermal store/unvented tank to get if we are just doing hot water?

    Not sure but I'm sure somebody else will know. Our thermal store of 250 L works well for just two of us and has coped adequately with four but I don't remember how to size them.

    If we wanted to run our heating system off the tank, presumably that would most easily be done with a thermal store. How do we go about calculating the size of heat store that we would need to heat the house, assuming that we knew the overall heating requirements of the house?

    Most people seem to think having separate tanks for DHW and central heating is the best way to go. What size buffer tank you need for central heating depends on what kind of heat source you have and also on how big the heat demand is. You may need it to stop a boiler cycling, or to store the entire output of a batch log boiler, or to buffer a heat pump.

    Is it possible to have a boiler supply the tank AND directly supply a shower & set of taps? In this way, if we ran out of hot water in the tank, we could just use the shower that ran directly off the boiler. Perhaps this behaviour could be switchable?

    Yes, you could do that. In practice its more usual simply to arrange 'hot water priority' for the boiler so that as soon as you start to use hot water from the tank, it starts to recharge the tank and you don't run out of hot water, unless you're using it faster than the boiler can generate it, in which case you're going to run out at some point whatever you do.
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    Posted By: djhMost people seem to think having separate tanks for DHW and central heating is the best way to go. What size buffer tank you need for central heating depends on what kind of heat source you have and also on how big the heat demand is. You may need it to stop a boiler cycling, or to store the entire output of a batch log boiler, or to buffer a heat pump.


    As asked - what is your boiler fuel - mains gas, wood (pellets or logs) or oil or LPG?

    Most people seem to think having separate tanks for DHW and central heating is the best way to go. +1
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    Boiler fuel: natural gas.
    We have a wood stove in the room below, but it's branding has come off and the chimney is unlined. So we do have to get the chimney lined and we may have to get the stove replaced (HETAS installer may be reluctant to sign off on an installation of a flue liner with an unknown component). The chimney shades the best roof for any solar thermal. So... we could get a store that could accept input from gas boiler, solar thermal and a wood stove. If we upgrade the stove so that we have a stove that can supply heat to the thermal store, run the stove pipe straight up through the house so that it can come up by the roof line, so allowing us to take the chimney down and install solar thermal. It all starts to add up to a lot of money!
  5.  
    BTW - thanks for all your comments. We are still bewildered, but are considering getting Thermal Mangement Solutions (TMS) http://www.tmsthermalstore.com/ in to have a look at our whole setup. Conversations like this are really helping to frame our thinking, so thank you very much!
  6.  
    Today the perceived wisdom is that PV is better than solar thermal.

    IMO if mains gas is available there has to be (a very) good reason to use something other than mains gas for space heating.

    IMO unless you have your own wood supply and sufficient space to store 2 years wood for proper seasoning you will be better to use natural gas for your heating and not use wood for prime heating but for aesthetic purposes (i.e. occasional use). If you can keep your existing stove you can use the money saved to improve the thermal efficiency of your house.

    Perhaps get a standard gas boiler with a hot water tank run as a priority over heating (a normal set up) or a gas combi boiler - either option should be cheaper than getting a thermal store and a DHW tank. Whether a DHW tank or a combi boiler would be a choice based water usage. Mains pressure DHW tanks have particular regulations in the UK that may be a consideration.

    It would be reasonable to propose a DHW tank heated by the gas boiler with provision to use PV to when installed or just use the PV to mitigate the grid use, which would be better is outside my comfort zone (and probably depends how much PV you can get, FIT options and other issues).

    In any event if you get a DHW tank get provision for any future use as it will be cheaper to future proof at the outset.
    • CommentAuthorbhommels
    • CommentTimeMar 5th 2018
     
    Although I agree that PV is easier to manage, if there is going to be a heat store/(un)vented tank of some sort anyway it would be a shame not to have solar thermal as it is so much more efficient than PV & immersion controller. And one does not exclude the other.
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      CommentAuthorSteamyTea
    • CommentTimeMar 11th 2018 edited
     
    Posted By: bhommelsAlthough I agree that PV is easier to manage, if there is going to be a heat store/(un)vented tank of some sort anyway it would be a shame not to have solar thermal as it is so much more efficient than PV & immersion controller. And one does not exclude the other.
    As there is already natural gas, ST would not be an economical viable option in my opinion. Pipes, pumps, pressure vessels, plumbing, controllers, servicing etc would soon take away any advantage. It is only efficient when there is a large temperature difference between collector and store.
    PV is just about fit and forget and any excess electricity can be used locally, usually to heat water, or exported. You get paid to export energy either though the 'deemed' usage or via a smart meter (maybe oneday).

    Have you considered a Sunamp for your DHW needs. They are basically a thermal store, but smaller for the equivalent energy storage. You could have 2, one for each bathroom. So you only need to run a mains pressure cold feed to each bathroom. They are about the size of a kitchen cabinet for 5kWh of storage. 200 litres of water heated from 35°C to 65°C will store 7 kWh.

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