Heating engineers? Going electric

Solarchaser

Hi Bendy_house, I have about 10kw of solar, and 20kwh of batteries already.
It was built up over the last few years with a view to going all electric in time.

The thermal store principal for hot water is appealing as the water never sits in a tank.
The tank is full of *dirty* water and a coil of pipe runs through that water, your cold water runs through the coil, being heated by it, and so at the end of the thermal store you have clean hot water which could be drank if you so wished, with virtually zero risk of standing water microbes, legionella etc, 

A buffer tank is just a tank full of water, though you can also get them with internal coils.
Upstairs is where the majority of hot water is consumed, and thats where the hot water part will go.

The crawl space under the ground floor level is where the central heating buffer tank will go for a couple of reasons, not least as it will be very easy to connect to the larger heating pipes that run under the house here.
This means a vertical height restriction meaning I'll need a horizontal cylinder/buffer.

I could reinforce the loft to carry another tank, but that would mean there is a combined load 0f 3/4 Ton of scalding hot water above a bedroom. And I'd only be able to tap into smaller bore pipes there which would lead to a hotter Upstairs than downstairs,  which isn't ideal.

I'll have a look at the forum, thanks 👍

Bendy_House

Wow, a hefty solar/battery system, then! Lots of potential, surely. Do you get any FIT from this? Is it connected to the grid, or stand-alone (it's big for a 'domestic' setup, I understand?)

Anyhoo, I think I understand the principles of a T-S, but that's why I was confused by your first post, where you were wondering how to make your 'double-tank' system work, with pumps and valves and stuff.

Surely the T-S system is as simple as it gets - the water content is 'system' water, I understand, so is shared by the boiler and radiator circuits. So that is circulated very simply by the usual pump. The DWH doesn't even require a pump, but should have a thermostat blending valve in order to keep the delivered hot water temp at a safe level. This is heated by an exchange coil on demand. Again, super-simple.

So why not just a larger T-S? Or two - I presume they can be fitted in parallel? I don't get the buffer tank malarkey - it just sounds like a complication?

ComicGeek

This is a good but very simplistic tool for initially sizing thermal stores (energy storage) - https://www.heatweb.co.uk/design-tools/

This gives the stored volume and the storage water temperature above the required heating flow temperature, and based on the heating duration and the required heat output. You can see that the store size starts to get really large once you're running the heating for more than a couple of hours.

Bendy_House

ComicGeek said:

This is a good but very simplistic tool for initially sizing thermal stores (energy storage) - https://www.heatweb.co.uk/design-tools/

This gives the stored volume and the storage water temperature above the required heating flow temperature, and based on the heating duration and the required heat output. You can see that the store size starts to get really large once you're running the heating for more than a couple of hours.

I guess that's when SC's batteries come in.
It'll be great to follow this thread as SC gets their system up and running. Basically, how large an array + batteries is needed.

Solarchaser

The thermal store I linker is 210L, thats the largest one that particular company do.

From a hot water supply point of view there are no moving parts,  cold water gets pushed in and pushed out again as hot.

However if im to use the thermal store for heating I wouldn't have the heating plumbed into the tank water, it would be to a heat exchanger on the side,  for a few reasons, system pressure for a start, but mostly for thermosyphon reasons.
I dont want the hot water to be drawn out until I decide, and don't want to lose it during the day when I want to use it at night.
With a heat exchanger you need to pump the water to it and back out of it  to keep it hot in order to exchange the heat. 

In regards to why not have two tanks upstairs, for the reasons I said in the last post, larger pipework is downstairs, as living areas are mostly in need of heat, so buffer tank goes downstairs to feed the living area before the bedrooms.

It's probably easier if you think of them as two separate systems, 1 for hot water, and one for heating.

I was thinking about linking them and having the hot water thermal store be used as a top up for the main heating tank, that's the complication.
So perhaps worth separating the two when you ghink about it.

As tbh the more I think about it, the more likely I am to have them as two separate systems.
The £600 difference for the simpler system is another 300l tank for the heating really if I need it.

I'm basing the energy demands on the actual data from my smart meter /energy bill of the demand per day this winter

I have 2015 fit on one of my solar systems, nothing on the other.

ComicGeek thanks for the link to the heating calculator,  but its hard to know how much of the time I'll be pumping round water, the thermostat decides when the combi kicks on and off, as it will do with the electric tank.
I know what my usage is from gas on an old combi, so the electric can't use more than that, and should use about a quarter less.... should

ComicGeek

Bendy_House said:

ComicGeek said:

This is a good but very simplistic tool for initially sizing thermal stores (energy storage) - https://www.heatweb.co.uk/design-tools/

This gives the stored volume and the storage water temperature above the required heating flow temperature, and based on the heating duration and the required heat output. You can see that the store size starts to get really large once you're running the heating for more than a couple of hours.

I guess that's when SC's batteries come in.
It'll be great to follow this thread as SC gets their system up and running. Basically, how large an array + batteries is needed.

But normally the battery has a maximum output through the inverter - mine is only 3 kW, the OP may well have a higher output but there is still a limit on the rate of electricity drawn from the battery.

Key questions:
What is the peak space heating demand (kW), including the heat up factor when the heating first comes on in the morning?

What is the space heating demand (kW) to maintain temperature?

What is the total space heating demand for a winter heating day (kWh)?

What is the design flow temperature for the heating systems?

Only once the above is known can the size of the thermal store be considered. That then dictates what the heat requirement into the thermal store is, and that is a compromise between time taken and energy input. There's also the failure risk, ie what happens when the store runs out of heat, as  you don't have a larger heat source to kick in, just smaller immersions - normally this is set by a minimum store temp, but you're also trying to minimise peak tariff usage so do you accept it getting colder once a week, once a month, once a year, never?

Yes, there may be times where the solar PV can recharge the store during the day, but the design day would have to assume that it won't as there are a number of days in winter when the PV won't generate anything of value. 

I admire the OPs willingness to have a crack, but my wife would not tolerate being cold while I fiddled with this. I would much rather see a heat pump linked to this that could deliver a larger heat input for less electricity usage.

Solarchaser

On the key questions, to most, the answer is don't know, how can I really?
You can model on floor space and guess about wind factors and external walls, you can guess about how effective the insulation is, and there will be more added, but all I can really say for definite is in the dead of winter I use 50kwh of gas a day, split between heating and hot water.
And no, I don't know what the percentages are.
The combi is 28kw, I'll be replacing that with maybe 12kw.
If I assume 75% combi efficiency then I need 36kwh of heat between the water and heating for the day.

In terms of flow, truthfully I'm likely to lift the existing combi pump and use that to get the same flow as I have now, (it removes any calculations,  its not modulating) and there will be playing with the flow rate of the hot tank heat exchanger pump to be able to get say 60c through the pipes but not 80c.
I could go with a smaller heat exchanger, but most are around 28kw anyway which is the same as the combi, end if the day, it works the same way once the water has been heated, its exchanged and pumped round, so from that point of view the heat up time for the house should be roughly the same.

And so yeah it will take longer to heat if I start from zero, but then I'll never really be starting from zero unless there is a total failure that's lasts over a couple of days.
As the tank should always be above 40c.
And if total failure is the case, it's really the same position I was in 2 years ago when the combi wouldn't start and I had to wait a few days for a new fan.
If we are talking topping up because the tank is cool, then 12kw should be enough because at that point I'm not heating the house from cold, I'm just maintaining.
As for what happens if I run out of heat, the elements go on, and I take the hit for an expensive day of heating, the house cannot be let go cold, I have a young family.

We have an electric fire which the wife uses if she isn't warm enough with the radiator in the living room, and a few 3kw standing heaters of things get critical,  but that would obviously be worst case and very expensive. 

My inverters will put out around 7kw, but in a little under 3 hours they would be flat.
I'm hoping to gain from not using them so much as currently they get thrashed running electric showers whenever the family want showers.... not in the cheap electric period obviously,  its only me uses them then 😤

Even if I could get away with a heat pump, which I can't, it's not there for a 3 hour boost during the cheap period, it's an all day runner, and that kills my opportunity to save some cash.

I'm currently exporting around 2MW a year for nothing, then buying gas to heat the water and house at the same point sometimes.
That 2MW is around a quarter of my annual heating bill which I can save... well save some anyway.

It's a risk, but I'm gonna spend 2-4k doing this, which is not too dissimilar to replacing the combi anyway.

Oh and my wife won't tolerate being cold either, so one way or another it will need to work.



So going right back to the beginning, I need hot water thermal store.
I need a buffer tank for heating with a pump and heat exchanger,  a couple of expansion vessels and some full bore valves to mess about with flow rates a bit.

Aside from the actual size of the tank (s) and the size of the elements, is there anything I've missed in terms of it being a heating system?

Bendy_House

Solarchaser said:

From a hot water supply point of view there are no moving parts,  cold water gets pushed in and pushed out again as hot.

However if im to use the thermal store for heating I wouldn't have the heating plumbed into the tank water, it would be to a heat exchanger on the side,  for a few reasons, system pressure for a start, but mostly for thermosyphon reasons.
I dont want the hot water to be drawn out until I decide, and don't want to lose it during the day when I want to use it at night.

System pressure? The stored water would, I imagine, be at normal system pressure, so 1 bar or just above. It won't be at mains pressure - only the DHW exchanger will be at that.

Thermosyphonic reasons? I don't understand how, since the boiler supply to the TS and the TS's supply to the rads will, I'd have thought, be controlled by 2-port valves, like any normal CH system. I'd have thought - but don't know - that the two would be quite independent, with the boiler charging up the TS when programmed to, and the TS supplying the CH rads when ditto. This'll almost certainly overlap at times, but will otherwise be quite separate, and isolated by their respective 2-port valves. So no thermosyph as far as I can see?

Unless I'm not understanding this properly, I'm at a loss as to why you'd complicate the whole setup with an additional tank with its own set of pumps and valves. 

Bendy_House

ComicGeek said:

But normally the battery has a maximum output through the inverter - mine is only 3 kW, the OP may well have a higher output but there is still a limit on the rate of electricity drawn from the battery.

Even 3kW should be able to maintain a comfy temp in a few rooms, after the stored therms had done their job and were starting to be used up, I'd have thought?

Solarchaser

If I complicate the system by attaching it to the thermal store upstairs, which is now highly unlikely,  then plumbing in the water to the tank would mean that tank would now have to sit at the radiator circuit pressure of 2 bar.
Any heating or cooling of that vessel would directly affect the whole CH system.
By using a heat exchanger only one side of that plate is under pressure and the only heat point for thermosyphon would be the heat exchanger itself.

Valves shouldn't be needed, the whole system should be free to move water, why add in an obstruction and failure point. 
It's the old favourite of KISS
Keep It Simple Stupid that I'm trying to apply.
The less moving parts, the less there is to break