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Author Topic: Thermal Store sizing  (Read 1792 times)
Bikerzz
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« on: February 03, 2020, 10:57:26 AM »

Yes I know its nearly impossible task, but you guys will have an idea.
I have been told 400L will be big enough by many plumbers etc.... but I never trust anyone.
It will be fed by oil boiler and PV immersun to help during the day.

It will feed 130m2 of UFH (6kw they say) rads upstairs, and DHW. Its a 4 bathroom house, say family home one day with 2 adults and 2 children.
Will 400L be enough? What have you guys got? 400L is 37kwh roughly at 80C?
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brackwell
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« Reply #1 on: February 03, 2020, 12:26:43 PM »

why are you looking to store so much energy? instead of using the oil for heating. During the heating season solar will contribute very little to heating and most of the time will only just keep up with the DHW if at all.  During the non heating season you are in great danger of creating a Large tank of warm water not hot enough for use without using oil and this in my book would be a real nono with solar. I personally think that heating demand and DHW demand are not good bed fellows if you want to maximise solar.  How big is your solar?

Your EPC energy rating for your house would be a good starting point for heating.  Whats changed that you are looking at this now?  I would be thinking hard about getting locked into oil and i would want to see how oil compares with cheap overnight leccy regardless of pollution and the eco future.

Ken

PS some sites suggest oil is as cheap as gas.
« Last Edit: February 03, 2020, 12:48:13 PM by brackwell » Logged
Countrypaul
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« Reply #2 on: February 03, 2020, 12:52:22 PM »

We have a 430L Thermal Store which is used to supply 125m2 of UFH in a 70mm S&C screed downstairs, plus about 12m2 of UFH using fitted from below spreader panels for the upstair bathrooms in a 4 bedroom house.  The TS is heated by an ASHP along with PV (3.3kWp) surplus using an Eddi - it has fittings and pipework already installed for a WBS and Solar Thermal. Probably never use the ST option, as any pipework would be difficult as would finding space for the ST panels/tubes. The WBS we would like, but is unlikely short term (18 months at least). The TS size was limited by the space available and was made by Newark to fit the space we had - sloping ceiling, so 60cm diameter copper + 75mm PUR spray on insulation, open vented (WBS)

The ASHP has been set to heat the water to 40C which is sufficient for the UFH at present. The ASHP was only installed at the start of January so has not had a signficant cold spell to cover. The DHW is usually provided by the Eddi either with PV or simply on a timer.

Prior to the ASHP we went one winter using just immersion heaters to heat the thermal store and used about 40kWh per night in winter (E7), with no additional input during the day except during a significant cold spell where we had an additional immersion on during the day/evening - about an additional 40kWh - location Yorkshire dales.

The ASHP still has some problems, like the meter is not wired up to it yet and is missing a thermostat to stop the ASHP running when the TS is "full".

We are also 2 adults plus 2 children.
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Bikerzz
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« Reply #3 on: February 03, 2020, 01:58:21 PM »

Thanks Paul

So house is undergoing renovation hence looking into it now.
I cannot have ASHP really (conservation area and sooo many restrictions make it daft), village has no mains gas and even if it did I live too far away so oil it is.
PV not installed yet but I think would make financial sense too (its allowed in my conservation area) and I have a large roof so could accommodate 9kw I would think if not more.

I have UFH 130m2 with insulation from 30mm - 150mm depending on room with 50mm liquid screed, 50mm cavities that are filled, triple glazing and 400mm loft insulation.
New over sized radiators going in upstairs.

« Last Edit: February 03, 2020, 02:02:58 PM by Bikerzz » Logged

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Countrypaul
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« Reply #4 on: February 03, 2020, 05:50:01 PM »

It was as we renovated that I took the decision to put extra insulation etc. in. The UFH sits on top of 120mm PUR, the walls vary from 170mm PUR, 100mm PUR +50mm XPS down to 70mm PUR +50mm rockwool. The roof is made from 175mm SIPS panels as are the gable ends and a few of the walls.

We too are in a conservation area, but has the ASHP on the plans we submitted and nothing was said about it, like you we have no gas. I believe ASHPs are now alowed as part of Permitted Development even in conservation areas. The house previously had a combination of oil central heating, open coal fire and 3 x 3kW storage heaters - but very little insulation. There were 2 x 300 gallon oil tanks under the bedroom floor - swmbo not happy they had to go regardless.

I put tripple glazed windows on the north side of the house, but double for the rest. Very few penetrations of the walls - 7 I believe all for wires, so sealed on the outside with silicone and on the inside with Siga tape, plus 2 for the MVHR - inlet and exhaust. The vent for the soil pipe is currently the only roof penetration (OK - plus one for the PV wire) eveything else goes through the ground floor.

Getting the air tightness tested is still on my to do list!

I would definitely go with the PV as much as you can.

Have you done a heat loss calculation (and not just an EPC one) as it might be a lot lower than you expect. In our case we used about 9000 kWh for heating Aug18-Aug19 which cost about 900, with the ASHP that should drop to 300 ish, but without the RHI I could not make it worthwhile as the payback became 25yrs+ (and that is compared to using immersion heaters.)
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Bikerzz
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« Reply #5 on: March 09, 2020, 10:41:50 AM »

Thanks Paul

300L Telford I have with 2 x 3kw immersions and a oil boiler feeding, with a high gain coil so can swap oil boiler for ASHP if rules change in future.
My UFH manifold states maximum for 70C for the manifold so must make sure my thermal store only goes to 70C.
What temp would I need my TS at to make sure I get 40C water at my taps do you think?
I understand your ASHP only does to 40C and them immersion goes higher from free electricity, whats that immersion stat set to on yours? Does your manifold have the same do not go over 70C warning on it?

Its all being setup on Friday so I dont know yet.
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JohnS
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« Reply #6 on: March 09, 2020, 12:08:34 PM »

If you are worried about the manifold supply from the thermal store going over 70C, you could always have a blending valve mixing in the return to keep the temperature below 70C.

The manifold will have its own blending valve, so I appreciate that this is duplication.
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Countrypaul
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« Reply #7 on: March 09, 2020, 01:08:47 PM »

As JohnS says, you should have a blending valve in front of the manifold that mixes the hot water from the TS with the cold return from the UFH to ensure warm water is ciculated round the UFH. Check to see whether that blending valve is also limited to 70C - nothing in my documentation indicates a limit. The TS came with a blending valve and pump as did the manifold, it made sense for us to remove one, so we removed the blending valve and pump on the manifold so that the HW form the TS travels least distance and loses least heat. The best option is to run the UFH at a low a temperature as you can that way you can make the most of what is in the TS.

I have an Eddi with two temperature sensors in pockets in the tank. The upper sensor is set to 65C with the immersion stat set at 80C, the lower one is set to 80C & 80C. Given that we take off water in the middle of the TS not just the top, I thought it makes more sense to warm the lower portion of the tank which should lose less heat than the top portion being higher than required for DHW. We have TMVs on the TS to ensure the DHW is not too hot, though we also have TMVs on the bath, showers and basins to further limit it, but not the sinks.  Because the way in which the TS works is likely to be different with yours to ours, not sure I can give you any better idea of what temperature you are likely to need in your TS for suitable DHW. I suspect you will have to experiment to find the optimum level - I know we still are.

Bear in mind to get 40C water from a tap might only require 45c in the TS, whereas running a bath might require that the TS is much hotter so there is enough heat to provide for all the hot water needed for a bath. Only way to find out is to try it!

I had some changes made to the ASHP setup which appear to work better for some aspects (like it no longer runs continuously) but worse for others like it insists on generating at 60C despite the target flow temp being set to 45 at present (more work needed obviously).
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Pile-o-stone
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« Reply #8 on: March 09, 2020, 01:34:35 PM »

why are you looking to store so much energy? instead of using the oil for heating. During the heating season solar will contribute very little to heating and most of the time will only just keep up with the DHW if at all.  During the non heating season you are in great danger of creating a Large tank of warm water not hot enough for use without using oil and this in my book would be a real nono with solar. I personally think that heating demand and DHW demand are not good bed fellows if you want to maximise solar.  


I have to agree with all of this. I installed a 300l thermal store in order to combine a boiler stove, solar and gas boiler into the central heating system. After having the stove overheat and crack, flooding the ground floor (and having other issues) I decided to go with a non-boiler wood burner. We now just have a central heating boiler and solar PV connected. As Ken says above, we are basically heating a large volume of water in order for it to go cold again. In summer, at least some of that water is solar heated, but as we have showers in the morning before the sun has gotten going we are still using the gas boiler. While we may be saving a bit of money because the gas boiler is in condensing mode for longer - that would be more than offset  by having a tank of water going cold.

If I were you and only linking in two energy sources (oil boiler and solar), I'd install a standard hot water cylinder for your DHW with PV connected to the immersion heater (if you can get a cylinder with the immersion heater at the base or with 2 immersion heaters then all the better) and have the boiler connected directly to the radiators for space heating.

« Last Edit: March 09, 2020, 01:38:19 PM by Pile-o-stone » Logged

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Bikerzz
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« Reply #9 on: March 10, 2020, 09:21:58 AM »

Thanks guys
This is what I have

https://imgur.com/OTn77nf


I can ask them on Friday, I see the TRV just before manifold that I could turn down to 45C or something but I dont know if that is whats only allowed to "see" 70C.
Pile I was concerned that if I did that PV could never heat the floors or rads. I think with 6kw I will get enough PV in shoulder months to heat the floor and the boiler only needed in evenings for shows and the depths of winter. I do see your point and I guess I will find out and see if I made a costly mistake.
« Last Edit: March 10, 2020, 09:26:35 AM by Bikerzz » Logged

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Countrypaul
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« Reply #10 on: March 10, 2020, 10:17:44 AM »

I can't see why any of the components before the TMV would be limited to 70C, though I can see the thermal readouts only go upto 80C so you do need to ask.  I was told the actual UFH pipes were the main limitation and that they should not be allowed to get too hot, but I don't remember any figures being mentioned and I can't find any in the documentation so far. I know that there is a liit of 55C for most UFH systems so that they do not adversely affect the screed etc.

If you will commonly run your TS at 80C+ you may want a TMV before the TMV on the manifold for safety safe. Should the TMV on the manifold fail and allow 80C water through the manifold it could cause damage that might not be covered by any warrantees to the UFH components or screed, I would suggest this extra TMV is set to below 55C and you put a thermometer on the exit, if it ever fails you would then have teh chance to see the water entering the manifold is above 55C. If both failed at the same time that would be extremely unlucky...
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