Wanting to reduce gas usage - New Heating Design Circuit - will it work?

Freepost

Hello,

I'm looking at installing a new central heating system with UHF on the ground floor and normal rads on the 1st Floor. The design was initially to have at its heart an System Boiler and a Telford Tempest 500l Indirect Cylinder but I would like to reduce my Gas consumption and have been particularly impressed by Solarchaser's "wee project" https://forums.moneysavingexpert.com/discussion/6377030/ditching-gas-going-electric-immersion-only-a-wee-project/p1 and in light of that I'm considering changing/modifying this design to use cheap rate electricity from Octopus Go/Agile to heat up the cylinder during the night and/or solar PV in the summer months with the System Boiler giving back up in perhaps the depths of winter. Octopus Go/Agile provides very cheap rate electricity for a 4 to 6 hour window in the night and, although not now, has in the past paid the consumer to take the energy during that period. I'm sure when we get through the present difficulties those days of cheap rate will return.

Anyway back to the system. The system is composed of;

System Boiler
Telford Tempest 500l tank (used as a heat store)
2 x 3kw Immersion Heaters
2 Plate Heat Exchangers
2 Way Motorised Valve (Hot water supply to Plate Heat Exchangers)
Pump (Hot water supply to Plate Heat Exchangers)
2 x Non Return Valves (for Plate Heat Exchangers)
Flow Switch/Detector (for Domestic Cold Water supply to Plate Heat Exchanger)
Pump (DHW Loop Pump)
Pump (Central Heating)
Motorised Valve (Radiator Circuit)

Motorised Valve (UFH Circuit)

Operation;

The water in the Telford Tempest 500l Cylinder is heated by 2 x 3kw Immersion Heaters using Octopus Go/Octopus Agile cheap rate tariff, Energy from the Solar PV System and the Ideal System Boiler. The hot water from the System Boiler is on a closed loop, passing through the heating coil in the Cylinder before returning to the System Boiler. The Cylinder has an expansion tank attached! I understand that condensing of the boiler will be easier to maintain with the return loop at a more stable temperature, I believe the goal is 55°C.

The hot water, ~ 70°C, is pumped through a two way Valve, either to the DHW Plate Heat Exchanger or the Central Heating Plate Heat Exchanger depending on which circuit is calling for heat. The DHW circuit has a flow switch to detect the flow of water when a tap is opened. The Central Heating Circuit has a Thermostat for the Radiators and a Control Signal (Boiler Enable) for the Under Floor Heating Circuit. There is also a pump for the DHW loop circuit for instant Hot Water at the taps, although the pump will be on a timer/clock, this will also activate the flow switch.

Is this an idea worth pursuing and will it work?

Your thoughts would be most welcome;

F.

70sbudgie

I will be watching this thread with interest. I already have a system boiler with a (dual core) 300l cylinder and two zones of wet UFH with radiators. So I am very interested to learn how I may be able to reduce my gas requirements. (I appreciate that my current configuration utilises the cylinder differently to what you are trying to achieve.).

Ectophile

Your heat exchanger for the DHW is going to have to be really good if you want anything other than vaguely warm water coming out of the taps.  Unless your system is really well insulated, and you're going to circulate it round and round until it eventually reaches an acceptable temperature.

Freepost

Hi Ectophile,

Thank you for your comment, I don't know much about how to size a Plate Heat Exchanger for DHW but understand that "A typical plate heat exchanger measuring only 20 x 7 x 12cm can transfer heat at over 100kW - enough to heat 45 litres per minute of hot water from 12°C to 42°C" http://www.heatweb.com/techtips/hx/ta.html. Further. I understand that this is a typical size of PHE used in combi boilers but I really don't know what I'm looking for/how to source the PHE for my use .... can you advise?

F.

Solarchaser

Hi freepost, it's great to see you progressing this.

I like your diagram, it's something I need to do ,I've looked for apps etc to do it, but nothing jumps out.

Questions.
Why have you went with plate exchangers rather than coils?

Why a 500l tank, and why horizontal?
And by that I guess I mean, I assume you are height restricted as I am, but for the size of tank, I'm guessing you have went with some calculations about what your gas usage is currently to decide on a 500 rather rhan a 250 or 300l tank etc.

Do you have a really large house?
I'm wondering why you would have hot water on a pumped loop rather than just supply pressure?
Or perhaps your supply pressure is poor?
A loop is positive as you will have instant hot hot water rather than waiting for it to come through your pipes, but the negative is a fairly large amount of heatloss and the pump for the loop and the heat exchanger will need to be on all the time.

I dont have actual ufh,  but my understanding is it runs at a lower temperature than radiators,  so I'd think you'd need something else in that circuit to blend the temperature down for your ufh.

Freepost

Hi Solarchaser,

Thank you for your comments and questions, I'll try and answer as best I can, taking note that I am a novice and my reasoning may not be sound;-)

The drawing is just hand drawn on A4 paper and then photographed with my mobile.

1) Why Plate Heat Exchangers - Two fold, if we consider that heat rises, then water in the top of the tank will be the hottest and my design has the feed to the PHE from the top of the tank. Even as the tank cools, the hottest part is always the top. With regard to coils, the bottom of the coil is in the cooler part of the tank, consequently the coil will never live up to potential unless of course the water content is mixed by (I think you call it) a desaturation pump. I was also having problems trying to find a tank with the necessary number of coils in the top of the tank.

2) Why a 500l tank, and why horizontal - My current gas consumption in the depths of winter (January) is around 70kW/day. With 2 6kW immersion heaters running for a 4hr period I can get near enough 50kW into the tank and the gas boiler will have to make up the short fall until I install batteries. Further to this and to give some idea about my arithmetic I used a formula of Volume of tank x 4 x temperature rise / 3412 = Power (kW) required from https://www.boilerguide.co.uk/articles/immersion-heaters where the tank is 500l and my available energy in a four hour period from the immersion heater is 48kw then a little bit of algebra delivers approx 80°C. .... The tank will be mounted vertically.

3) Do you have a really large house - Ground floor = 115m² and 1st floor is a similar size with radiators but I don't envisage them being used very often, the house is well insulated

4) Hot water on a pumped loop - The DHW is at mains pressure, the loop pump is to provide instantaneous hot water to taps and shower. The pump will be on a timer. It is not installed yet and may not be but I have plumbed the loop, will I use it, I don't know!

5) UFH - The house is a new build, or it was when I started out, it seems to have taken a long time to build but I installed UHF and it has it's own manifold which is not shown on the diagram.

As I said above, I am a novice in regard to heating and I am concerned that I may not have enough heat in the tank to supply my needs and unfortunately I don't know how to calculate that. I did ask the question on a plumbing and heating forum but the only answer that I received indicated that the 500l would last about 45mins but this wasn't quantified.

When I compare my theoretical design and your actual system, I think they are very similar, albeit that I propose to use PHE rather than coils, and your system is working and I presume lasting longer than 45mins?

I hope that I have answered your questions and that the system design is clearer.

Regards

F.

QrizB

Freepost said:

Further to this and to give some idea about my arithmetic I used a formula of Volume of tank x 4 x temperature rise / 3412 = Power (kW) required from https://www.boilerguide.co.uk/articles/immersion-heaters where the tank is 500l and my available energy in a four hour period from the immersion heater is 48kw then a little bit of algebra delivers approx 80°C. ....

I can't comment on most of this but that formula looks a bit weird. The heat capacity of water is 4.2 kJ / kg K and there are 3600 kW-seconds in a kWh, so I'd expect it to be (vol of tank) x 4.2 x (temp rise) / 3600.

Their numbers work out almost the same (0.5% different) but I'm puzzled why they chose those in particular, rather than the usual ones!

Freepost

Hi, Thank you for checking the formula, I don't have the knowledge to back it up but appriciate what you say and the result is almost the same. May I ask, in the equation "heat capacity of water is 4.2 kJ / kg K" what is the final K after kg? Kelvin?

Thanks

F.

QrizB

Freepost said:

Hi, Thank you for checking the formula, I don't have the knowledge to back it up but appriciate what you say and the result is almost the same. May I ask, in the equation "heat capacity of water is 4.2 kJ / kg K" what is the final K after kg? Kelvin?

Yes, Kelvin. I could've written "degree C" instead but that's seven extra keystrokes

Solarchaser

Thanks for your information,  I realised I read 500L telford and just assumed horizontal, my mistake.
So assume you are going for this?
https://www.cylinders2go.co.uk/shop/renewable-energy/telford-tempest-500-litre-twin-coil-solar-indirect-unvented-cylinder-twin-immersion/?gclid=Cj0KCQjw7KqZBhCBARIsAI-fTKKX4GfLjbbSVCzpu3R8IBMLy8mv4SjBcG_Dlbd4yhPHwogLVSXSb7saAh3ZEALw_wcB

Or some variation of it?

My coils are looped to around 1/3 of the bottom of the tank, but thats a horizontal tank, with a vertical tank that you specify, you could easily have the coils at the top, or say one at the top and one 1/3rd of the way down...  well I say that, I assume you could, my questions were all around the horizontal tanks, but I don't see why you couldn't.

For reference I used this guy on ebay
https://www.ebay.co.uk/itm/193247849462?mkcid=16&mkevt=1&mkrid=711-127632-2357-0&ssspo=AlZ0LTyyRca&sssrc=2349624&ssuid=DtbsXk1pRsq&var=&widget_ver=artemis&media=COPY as he has a direct line to telford and their custom build service 

This is what my coils look like inside the tanks, 6.9m long of 28mm coils (biggest and longest they did) equivalent to 25kw of heat transfer.



What you will hopefully see from those is that there are various straps going from the coils to the tanks to hold them in place.
So if you imagine your coils coiling around the top circumference of the tank,  they will always be at the hottest part,  say you use that one for central heating, and one closer to the middle for hot water, as your hot water doesn't need to be anywhere near as hot as your central heating radiators.

The top of the tank being hot and the bottom being cold is called stratification,  to stop this, or mix it up, you use a pump to destratify, a destratification pump, or in my case a central heating pump doing that job, but I need that really because of the cylinder being horizontal,  you probably won't.

2x 6kw heaters for 4 hours will get you 48kwh as you said, and you can specify 6kw heaters with the bespoke service, I did originally, (or buy them yourself of course) however solar diverters on the market will not run a 6kw element, as the resistance is too low, they will only run a 3kw (or sometimes a 3.6kw), so if your plan hinges on solar heating, id suggest perhaps 1x 3kw and 1x 6kw, but now in 4 hours of cheap rate you can only do 36kwh of heating.
This will dramatically reduce your heating from gas and will likely cover all but your coldest 2-3 months I'd wager, so probably not as big a downer as you might think.

The pumped loop, for me, I dont think it's necessary, when I changed to the tank I timed the furthest away hot tap, up around 4M and around 8M away horizontally and it took 11 seconds for hot water to come through.
I then changed the hot water piping from the tank to the mixer down from 22mm pipe to 15mm pipe and that dropped to around 7 seconds (that is on full flow though, takes longer on less flow).
Hot water circulating constantly at decent heat can be problematic in terms of legionella,  especially for dead legs or nearly dead legs, like the bathroom that isn't much used, imo it's safer to let those pipes be cool when they are not being used... plus it saves you a pump.

In terms of being a novice, honestly, me too, it's been a big learning curve all of this stuff, and I only know what I know, and picked up some bits and bobs from various jobs (like legionella from time working in hospitals), so if it seems like I'm trying to preach to you or similar, I truly apologise, I'm just trying to give you my experience. 

Anything mechanical will use energy, and anything mechanical will eventually break down, and there is always that principle of KISS, keep it simple stupid.

With that in mind, I'd say that if you spec a custom build with two coils, you negate the need for two plate heat exchangers and two pumps, which I'd imagine will end up around cost neutral (additional cost of custom tank) when you add it all together, but will be a simpler system.

If you then have the cold and hot inlets/outlets plumbed to your system boiler, you have your heating circuit without a 3rd coil.

On the flow detector, I haven't got mine to work.
It's essentially a paddle in the water flow, but the truth is that most of the time running a tap doesn't actually use very much water,  most tap hoses have an inner diameter of no more than 8mm, so trying to detect that flow in a 22mm pipe didn't work, and in a 15mm pipe didn't really work either. If I could convince you to remove the loop, then there is no need for the sensor either. But obviously it's your system and you need to decide how important instant hot water is.

It's reassuring that you found the same 45min *facts* about the 500l tank on searching, I have to admit it caused me to falter, but I eventually pressed on.

I couldn't find any real calculation for taking radiators of various sizes and including piping to get them from say 15C to say 55C and hold them there for hours in any kind of quantifiable way.

It's not really that cold just now, but the wife has had the heating on for the last few days for between 3 and 5 hours each night and around an hour each morning... roughly.
So I can emphatically and categorically rule out the 45Mins, but I do need destratification as I'm drawing heat out from one side of the tank, and unfortunately the myenergi add on board that does that, is still on back order for another couple of weeks (so far).
At one point a few nights ago, the tank was down to 58C in its mid point after the heating had been on 4 hours, from a starting point of 78C iirc, I told eddi to put on the 3kw element on the other side of the tank (I'm still messing about with what element to prioritise and getting the heating cut off at the right temperature) and within 5 mins I seen the tank go down another 5C to 53C, which essentially meant that the heater was causing some of the water to move about showing the central heating side of the tank was actually colder than it appeared.
For that reason I can't really do full facts about temperature until I'm destratifying while central heating is running as that's the only way I can see an accurate temperature of the whole tank.

Finally a couple of notes.
The telford cylinders come with extra bits.
They come with an inlet set, which is basically a pressure regulator which is set at 3 bar, so you don't need to buy that.
They also come with an equivalent expansion vessel, so 50L for 500l tank.
They also come with 1 motorised valve , a stat and a tundish (think simple overflow indicator)

That's got to be the longest post I've ever done.
I really hope it makes sense, and helps a bit.

Freepost

Wow, that's a lot of information …..  it's going to take me a while to digest it but thank you and I most definitely do not think " it seems like I'm trying to preach to you" ... I'm immensely grateful for all information together with the time that you and other contributors give in providing such information, no matter if the information agrees or disagrees with what I’m trying to achieve.

Once again, thank you and I’ll get back to you later.

F.