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Heating and cooling: Radiators connected to Thermal Store or directly to ASHP

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1 to 28 of 28

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    Due to the madness of MCS, RHI and probably a few other acronyms, I have three lots of people involved in my heating system for my barn conversion. I'm going to be using solar thermal panels, ASHP and maybe immersion backup to heat water, and for space heating, and so its up to me to make the actual design decisions!
    My original model was to use a thermal store that accepted input from both heat pump and solar, and run the CH rads off the body of water in the store. That way I could run the heap pump at night, and take advantage of E7 electricity. One ASHP supplier has stated (without any actual explanation) that the only way to use ASHP for heating is to run directly from heat pump to rads. I suspect the justification is that the heat pump would be more efficient raising the water temperature to, say 45c for rads, whereas at time it will be trying to heat already "hot" water, but will this really kill the efficiency?
    • CommentAuthortony
    • CommentTimeMay 4th 2010
     
    Yes it will kill the efficiency -- ideal efficiency is low as possible temp coming out and as high as pos going in.
  3.  
    Other things to consider. Your heatpump would normally be sized on the small size on the basis that they are more efficient working hard than partly loaded. For this reason they do not have the capaicty to heat up the house from cold unlike a normal gas boiler. You therefore run the heatpump 24 hours a day with may be a small night time set back on temperature so there is not a large morning heat up period for the system. This design approach, that is normal for heatpumps would be in direct conflict with what you are trying to do.
    Your supplier is corrct that you would normally run the system direct to the rads
  4.  
    From my limited knowledge of ASHP's if you run it at night you would have cheaper electricity but your COP would also be lower due to the ambient temp drop. This should also be considered. I looked into ASHP's and have now looked into biomass and GSHP's. More expensive I know but they can heat all winter if set up correctly.
    • CommentAuthorCWatters
    • CommentTimeMay 5th 2010
     
    Don't forget to use rads sized for low temperature use. Sorry if that's already been covered on another thread.
  6.  
    Posted By: jemhaywardOne ASHP supplier has stated (without any actual explanation) that the only way to use ASHP for heating is to run directly from heat pump to rads. I suspect the justification is that the heat pump would be more efficient raising the water temperature to, say 45c for rads, whereas at time it will be trying to heat already "hot" water, but will this really kill the efficiency?
    Not if designed correctly.

    The COP of the ASHP will be determined by the temperature of the flow & return to/from the radiators &/or thermal store. It doesn't care whether that water is actually going through the radiators or not as long as the temperatures are in the right range.

    A thermal store will tend to have cooler denser water at the base & warmer less dense water at the top. If you take the cooler water from the base as the ASHP return, feed the flow to the middle of the store, connect the radiators in parallel with these connections & arrange for the ASHP to cut-out when the return get above 45 degrees then it should all sort itself out without any loss of efficiency.

    An immersion heater will be needed to get the top of the store up to the temperatures required for domestic hot water, but you need that in any case if you're not going to take the ASHP much above 45 degrees. Think of the bottom of the thermal store as a pre-heater for the top of the thermal store which can be fed by the solar thermal or the ASHP, with the immersion heater providing the hot water top-up.

    The only downsides I see are the loss of ASHP COP when using E7 overnight due to outside air temperature drops, the potentially larger capacity of ASHP required because it has to provide the daily energy input during the E7 period & the potentially larger thermal store volume required because most of the store is limited to ASHP temperatures.

    David
    • CommentAuthorTerrier
    • CommentTimeMay 5th 2010
     
    Why tread a fine line, with a good biomass boiler it will do what it says on the tin, i.e. 15kw gives you 15kw what ever the outside temperature, and coupled with the solar for summer hot water, with the option to top up with the boiler via a correctly size accumulator, and with the RHI next year it's win win.

    Terrier.
  8.  
    jemhayward

    Is there any reason why you must have radiators? The flow temperatures for underfloor heating are significantly lower, the floor can help provide additional storage & you get the option of a higher COP from the ASHP.

    The difference in flow temperatures also gives you more flexibility. For example, by placing a mixing valve between the thermal store & the underfloor circuit, you can independently set the temperatures in the thermal store & underfloor circuit. When the ASHP can provide all the daily heat during the E7 period while running at underfloor heating temperatures (30 degrees or so) you can maximise the COP by keeping the store at 30 degrees. When you neeed more heat to see you through the next day you can crank the ASHP/store temperature up to 45 degrees & use the mixing valve to bring this back down to 30 degrees for the underfloor heating. There are "weather compensation" options for ASHPs which can manage all this for you.

    David
    • CommentAuthorDantenz
    • CommentTimeMay 6th 2010
     
    Posted By: jemhayward. I suspect the justification is that the heat pump would be more efficient raising the water temperature to, say 45c for rads, whereas at time it will be trying to heat already "hot" water, but will this really kill the efficiency?

    No, why. if the store is at setpoint temperature then the heat pump won't try to heat it further. Many heat pumps operate off weather compensation control i.e. a varying flow temp according to out side temp. Your proposed set up sounds ideal to me and will allow you to take full advantage of tariff electricity. What you want to be able to do is set back the heating but set forward the heat pump during the tariff period. In other words lower your internal house temp by 3-4'C overnight but increase the set point flow temp off the heat pump (10'C or so); some heat pumps have the facility to do this. You are then able to run the heat pump at higher temperatures and load the store cheaply. The drop off in efficiency is more than covered by only paying around a third for the electricity. You need a fairly large store though to really maximise tariff rates.
    • CommentAuthorjemhayward
    • CommentTimeMay 10th 2010
     
    Can't use underfloor heating for many reasons, mainly the fact that the heated bit is mostly first floor and its old wooden floors which we don't want to lift, or dry out. The smaller downstairs area already has a concrete floor we can't take up, and limited headroom... biomass we can't install as nowhere for a chimney, we'd have problems running it in summer to heat water as it would leak heat, and cost is too high anyway. I think I may well set the system up so that the ASHP can run either the rad circuit or the tank, and have the rad circuit linked to the tank as well, so I can use it in either mode. We have peculiar sporadic use requirements too, so we're going to need pretty sophisticated control systems in any case, but I can do control systems DIY, so as long as I have the options, I should be able to route the water where I want from my laptop!
    • CommentAuthorjemhayward
    • CommentTimeMay 11th 2010
     
    I've just has an email from my ASHP contact, and things are making a bit more sense now. Basically, a thermal store of the type specified by my solar panel man needs a higher temperature of stored water, as it has to transfer that heat to the internal hot water coil, so the stored water needs to be somewhat hotter than the hot water itself. The hotter the store, the more losses of energy, and ASHP can't make very hot water efficiently in the winter. So, we need a system that expects lower overall temperatures all through. As my ASHP chap is a good email communicator, I've asked him to come out and design the whole system including the solar thermal aspect.
  12.  
    Posted By: jemhaywardBasically, a thermal store of the type specified by my solar panel man needs a higher temperature of stored water, as it has to transfer that heat to the internal hot water coil, so the stored water needs to be somewhat hotter than the hot water itself.
    If the DHW heat exchanger is at the top of the thermal store then it's only the top of the thermal store which needs to be above the DHW temperature. As long as the thermal store is big enough to provide DHW temperatures in, say, the top third of the store & ASHP temperatures in, say, the bottom third of the store then the ASHP performance will not be affected. Either the thermal store specified isn't big enough or he's overlooking the temperature gradient in a thermal store.

    David
  13.  
    Perhaps a tank-in-tank TS would be better, where the mains water feeds in and out of a small HW tank inside the TS tank - no coil, no losses. I'm not a pro so you need someone to review this idea for your situation (mine works well for my situation).
    • CommentAuthorevan
    • CommentTimeMay 11th 2010
     
    Yes, or a separate DHW tank.
    • CommentAuthorjemhayward
    • CommentTimeMay 20th 2010
     
    I've come up with a much more cunning plan. I'm getting the ASHP people to design the system in totality, and comission the solar panel install as well, that way the whole system is under the control of one designer, and if it doesn't work, we know who to blame.
  16.  
    I like this approach.
    •  
      CommentAuthorDamonHD
    • CommentTimeMay 21st 2010 edited
     
    Yep, definitely would be my plan too.

    Rgds

    Damon
  18.  
    Absolutley the best thing to do: my man was responsible for the TS, Wood Burner, Solar Panels and all major items in between oh and the PV aswell though they are unrelated! Point was he was/is always around, and as I was doing all the prep work myself and most of the plumbing (iaw his diagram) he could advise me etc.
    • CommentAuthorandy500
    • CommentTimeMay 22nd 2010
     
    I'd be wary of trying to heat hot water via ASHP.
    COP will be way down trying to get the thermal store up to 75-80 degrees.
    In winter you will either be using immersion as backup daily, or the ASHP will not be anywhere near as efficient as it should be. Remember E7 is during the night - colder air temps.
    You need specific exact numbers, not 'oh, the solar tubes will help' etc - don't assume anything.
    You need to ask the supplier what the COP is to provide 65 degree hot water via a store that needs to be 5 (plate heat ex) to 15(coil) degrees above the 65 of the required hot water assuming incoming cold water of 5 degrees at the flowrate of your shower - it won't be pretty.
    • CommentAuthorRobinB
    • CommentTimeMay 25th 2010
     
    Andy500, not pretty but it will always be better than an electric immersion heater though won't it? So if you've no gas or oil or sun on your solar collector it's the best you can do? I guess a way of only heating what you really need at these times is key.
    • CommentAuthorjemhayward
    • CommentTimeMay 27th 2010
     
    I think that the ASHP people are going to recommend a twin coil pressurised tank, rather than a thermal store as such, so that we store water at the temperature we will be using it, rather than at a higher temperature. As RobinB says, we're off the gas grid so we have no viable alternative to electricity (we did think about wood pellet, but the escaping heat would be a problem in the summer), so even a COP of 1.1 would be 10% saving for us. We are expecting to be using an immersion heater for the really cold days of the winter.
    •  
      CommentAuthorDamonHD
    • CommentTimeMay 28th 2010 edited
     
    Eco-Cute style ASHP should have no trouble getting up to even 90C as far as I know. CoP won't be great at -25C outside, but still better than 1 with the right unit.

    Rgds

    Damon
    • CommentAuthortony
    • CommentTimeMay 29th 2010
     
    Er, yea but er but efficiency is greater the lower the temperature difference so use the lowest possible temperatures.

    It will have great difficulty getting to high storage temperatures and the heat losses from the store become very significant at 90 making the whole system very inefficient

    I have never stored hot water at 65 and the other side of the pond they routinely store at 50 or 55 -- the lower you go the less the heat loss and the lower the cost.
    •  
      CommentAuthorDamonHD
    • CommentTimeMay 29th 2010
     
    Some of the Eco-Cute units are 'instant', ie no storage needed at all, though the Hitachi model that I had my eye on is not available in the UK, allegedly due to internal feuding...

    So, have the hot water delivered at 50C with no scalding or legionella or CoP worries, and feed water at 60C to existing rads...

    Rgds

    Damon
    • CommentAuthorhatmandu
    • CommentTimeJul 22nd 2010
     
    jemhayward: Hi - can I ask you what the end result of all this was? I'm in a similar situation and my (amateurish) instinct says a thermal store is what we need (I'm not keen on the unvented cylinder plan, which my ASHP supplier is also recommending), as it gives us the option of adding other heat sources later if the ASHP doesn't work out. Did you go for the unvented in the end? And how's the ASHP doing?
  26.  
    Still getting quotes in at present - and each potential supplier comes up with new ideas, but one important fact has emerged. If you use a thermal store type tank and the pass water through it via heat exchanger coils inevitably the coild will not be in the hottest part of the tank (the very top) and the heat transfer cannot be perfect, so, if the ASHP doesn't heat the water up quite as much as you want, what comes out of the tap may not be hot enough, wheras if you use an unvented cylinder, taking the hot water from the very top of the tank, you will get hotter water. You can still have lost of coils in it to capture heat from other sources.
  27.  
    Posted By: jemhaywardIf you use a thermal store type tank and the pass water through it via heat exchanger coils inevitably the coild will not be in the hottest part of the tank (the very top) and the heat transfer cannot be perfect, so, if the ASHP doesn't heat the water up quite as much as you want, what comes out of the tap may not be hot enough, wheras if you use an unvented cylinder, taking the hot water from the very top of the tank, you will get hotter water. You can still have lost of coils in it to capture heat from other sources.
    If using an ASHP/GSHP the temperatures will always be marginal for DHW & it's likely an immersion heater will be required as a boost. Given these marginal temperatures, it makes sense to minimise the number of heat exchanger coils between the ASHP/GSHP & the DHW. If using a thermal store this means directly connecting the ASHP to the water in the thermal store, so the only coil is one at the very top of the tank for heating the DHW. If using an unvented DHW cylinder then you have a heat exchanger on the input side, but one is not required on the output side. Either way you should only have one heat exchanger coil in the path from ASHP/GSHP to DHW.

    Not all thermal store providers put their DHW heat exchanger coil at the very top of the tank. Consolar & others do, but it is not always necessary. For example, TiSUN have a heat exchanger which runs the full height of the tank so the cool water at the base acts as a pre-heater & the top section only has to lift it that last few degrees.

    David
  28.  
    Indeed I have one installer who wants me to use an "Ecovent" cylinder to heat the DHW, which is another ASHP, but dedicated to the water heating job. I must admit that this does have its appeal, at least in theory. Not sure how they propose to integrate it with the Solar Panels though...

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