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Advice on thermal solar and boiler system for DHW and UFH

  • 09-02-2022 2:18pm
    Registered Users Posts: 18

    After researching ATW heat pumps and getting a heat loss/air tightness test, I have given up for the short term on getting ATW but I do need to replace my hot water tank soon and need help optimizing the systems I currently have and possibly future proofing it for ATW in the future.

    the system was installed in 2006 and isn’t really working optimally. I bought the house so I don’t fully understand it’s design and how everything works.

    I have a very large 3 coil unvented hot water tank. I was told it is 300l but I’d be surprised if it wasn’t bigger. The bottom coil is connected to a 4 panel thermal solar system which doesn’t seem to contribute very much.

    The middle coil is a loop that goes to the ufh system, with a manually operated mixing valve to control temperature.

    the top coil is heat from a condensing boiler, the theory is the boiler heats the top to 60c, and then the solar heats the bottom of the tank. I’m guessing the middle coil would be closer to 40 making it ideal for UFH.

    DHW is from the top of the tank and the radiators upstairs are heated directly by the boiler.

    in reality the boiler ends up heating the whole tank to 60 and prevents any significant contribution by the solar. I was at a loss to find a way around this and was considering some type of system that would shut off the boiler during the days when solar thermal should be working.

    I am waiting on a site visit from an expert in solar thermal and smart controls to discuss options, from our conversations so far he was surprised that the solar system didn’t have an obvious heat dump and queried if I have a continuous hot water return loop as this would act like one. Turns out I do and he reckons the return of this into the tank is probably causing turbulence that affects the stratification layer and hence the boiler heating the whole tank. He reckons it might have been designed as a hybrid thermal store and hot water tank.

    if I go for a replacement 3 coil tank, would it be crazy to have the middle coil heating the water for the UFH as currently but also the radiators upstairs which I would have to update!

    so the boiler would just be heating the top half of the tank via a coil and the system is boosted by solar thermal on the bottom.

    what solutions are there to prevent the hot water return causing turbulence upsetting the stratification layer?

    Would having the return higher up in the tank work or maybe a second smaller tank for DHW that contains the return and is heated by drawing water from the main tank, possibly boosted from the boiler?

    The intention is when I have dealt with the air tightness issue and the boiler needs to be replaced in the future that the system is designed so I can just replace the boiler with a heat pump with out any other major modifications, ie the top coil is designed to work with a heat pump but is first run by a boiler running at a lower output temp?

    I can’t find any designs online similar to this, I have seen similar ones where the DHW is heated via a coil and the water in the tank goes directly to the heating system. I’m concerned that this would not provide enough DHW for our requirements. (Regular baths) or would the continuous hot water loop work well with this system?

    any help or advice would be greatly appreciated!


  • Registered Users Posts: 5,115 ✭✭✭homer911

    There is no way your boiler should be heating the whole tank, unless the connections are wrong

    Is the solar system PV of SHW?

  • Registered Users Posts: 18 Thewires

    SHW, unfortunately that is what seems to be happening, so there is no temperature differential to allow the solar to contribute.

  • Moderators, Home & Garden Moderators Posts: 5,975 Mod ✭✭✭✭graememk

    The long and the short of it is

    Top coil, heated by oil

    Middle coil is to take heat out of the tank for under floor heating

    Are these completely separate in height or are they roughly in the same space,

    Bottom coil is solar.

    In my system the bottom foot or so is heated by the solar, the solar is always heating the coldest water. (Does the solar system have any control box where you can see the temps?) As it's always heating the coldest water you'll never really see it working.

    The mixing of water could be very possible. But without eyes on it's gonna be hard to explain. If you figure out where every pipe is going and returning might get a better grasp

    Also when's the last time the solar was looked at, usually it needs a topup or something every few years.

    How is your oil boiler controlled does it only heat the hot water tank and that is used as a buffer for the under floor heating?

    That's bound to have a control box where it can be set when to turn on and off

    Also a stat on the tank so it can shut off when it's hot or turn back on when it needs heat.

  • Registered Users Posts: 18 Thewires

    Sorry just realised I was wrong, both the solar coil and the UFH coil are similar height on the bottom of the tank. With the boiler heating coil on the top half.

    the boiler is controlled by timer dials one for hot water one for UFH and one for radiators. It switches on when there is a demand for heat from the radiators or from my best guess a temperature demand from the tank! It directly heats the upstairs rads and heats the tank via the coil.

    the solar has a control box but as the bottom of the tank is too hot from the boiler heating the full tank it doesn’t switch on.

    luckily we switch the boiler heating off in the warm spring to autumn so the solar then heats the tank without the interference from the boiler. I’m guessing of we hadnt been doing this we would have had serious problems with the solar system overheating.

    But on a day like today we should be getting some gains from the solar and the system is running very inefficiently as the boiler heats the whole tank.

    im less concerned with what is wrong with the current setup but interested in advice on a new set up to improve efficiency.

    the current tank had to have a lot of welds recently to temporarily fix leaks so needs to be replaced soon enough.

  • Moderators, Home & Garden Moderators Posts: 5,975 Mod ✭✭✭✭graememk

    But By understanding your current system you'll know how to make improvements!

    I have a theory,

    The top coil is simple it's DHW, at 60c

    The coil connected to the UFH and rads doesn't take heat from the cylinder it puts heat in! And that it's on the same level as the solar..

    As the solar and ufh coil isn't separate, vertically it could have been a compromise at install. And generally when you need heat the panels won't be doing much and when you don't need heat the panels would be working well.

    If the UFH didn't go into that coil, in the winter you'll only able to heat half? of the tank

    (This time of year my panels do very little)

    The bottom third of my tank is only heated by solar, and that sits idle really for the majority of the winter.

    On my hot water tank, solar coil is by itself, at the bottom.

    Coil 1 and coil 2 are at the same level. One is heated by oil and the other is via the stove and solid fuel.

    Maybe your ufh coil is to get rid or balance the heat going to heat the house.

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  • Registered Users Posts: 18 Thewires

    Thanks for all your input so far. On further examination my tank is unlike anything i have seen online whilst researching options. I assumed the lowest pipe is always the cold water feed and this was confusing me when i was trying to figure out what was going on with my own tank.

    In my tank the cold water enters into the tank on the very top via the pipe with the expansion vessel, this pipe continues straight down into the lower third of the tank. The DHW outlet pipe exits on top beside this and they are connected via a mixing valve.

    The solar coil is in the bottom third of the tank.

    This is where it gets very wierd, the boiler feed comes in splits between going directly to radiators and into the top coil of tank. On exiting the top coil it then goes to the UFH! There is a mixing valve between the UFH feed and the UFH return. The return from the UFH then goes into the top of the third lower coil, and exits and returns to the boiler, meeting up with the radiator return. i will upload a diagram to show exactly whats going on.

    Has anyone seen anything like this before? i think the way the cold water is fed in and possibly whats happening with the third coil are leading to the whole tank being heated from the boiler and ruining any solar gain when the boiler is running.

    I forgot to mention im on well water and this must be the reason my last tank is rotting.

    Any recommendations on what kind of tank i should replace this with? I like the idea of a buffer tank contributing to hot water and heating but im not sure if my current one is doing anything for heating.

    Would a tank in a tank option work well? or having the whole tank feed directly into the heating systems and using an external heat exchanger to heat the DHW?

  • Registered Users Posts: 18 Thewires

    Here are a few pictures of the set up. The first is the cold water feed and DHW outlet which are connected to a lid/flange attached to the top of the tank. This has failed from rust and has been welded to an inch of its life to keep it going untill i can replace the whole tank.

  • Moderators, Home & Garden Moderators Posts: 5,975 Mod ✭✭✭✭graememk

    Cold water to the mixer valve is normal prevents the hot water from being too hot at the taps

    Coming in 1/3 of the way up is for sure unusual. But should sink down....

    Your current setup is interesting, having the solar and the return of the ufh at the same level. The solar tubes wouldn't be heating much (if any in the winter) and if it was it would heat quickly up to the temp to circulate. - eg my bottom third basically sits cold all winter.

    Well water could be an issue if it's quite hard, mains water can be just as bad! Go for a well insulated and maybe stainless steel one.

    When my dad replaced his tank, he went for one that would be compatible with solar tubes, never actually installed them. In the end up, the solid fuel stove heats a top coil, exits and enters what would be the solar coil.

    Although by the looks of it, you can't heat the water from oil without heating the under floor?

    The mixing valve on the UFH is to prevent the ufh from getting too hot.

    The hot water tank isn't providing much, if any buffering as far as I can tell. And with oil, it isn't needed.

  • Registered Users Posts: 18 Thewires

    I’m leaning towards a thermal store system 300-500l tank. A lower solar coil and high gain main coil that I can switch from boiler to a heat pump in the future.

    The tank directly supplies the heating system water. As this would then be a closed system it would reduce the corrosive effect of my well water.

    Domestic hot water is supplied via an external plate heat exchanger.

    Are there any flaws to this design? I’m wondering if the return flow from the heating system and the heat exchanger to the bottom of the tank might upset the stratification if it’s too high a rate? Also with thermal solar there is no obvious heat dump.

    Im thinking I could run this system at a lower temp as legionella would not be an issue, maybe boost with an immersion in the morning and evening for showers/baths.

    Would you go for a 300, 400 or 500l tank for a 3,500 sq ft house with 8 under floor zones and 8 radiators upstairs?

    Also due to my well water corroding my current tank do I need to worry about all the piping in the house?