Twin Coil Cylinders

[MortarThePoint]

Perhaps a daft question. @Nickfromwales or @TerryE you probably know the answer.

 

I understand that one way of connecting two heat sources to one cylinder is to have two coils in the cylinder itself, but can't you achieve the same thing using two non-return valves and a single coil? Example diagram below.

 

Reasons that come to mind for two coils:

• Is it because the two heat sources may need different Return temperatures to be operating most efficiently? That assumes the Return temperature is not at the temperature of the water in the cylinder, which would be the case with maximum heat transfer. Also that would only be a problem if using both heat sources at the same time.
• Is it more to do with one heat source being used to heat one part of the cylinder and the the other another part of the cylinder? 

 

 

Edited January 26, 2023 by MortarThePoint

[TerryE]

I fitted my last cylinder over 30 years ago then the Bregs were very different, so I will let Nick reply on this one.  😊 

 

[SteamyTea]

If you think of it as electronics, 2 diodes in parallel is an OR gate I think.

[MortarThePoint]

22 minutes ago, SteamyTea said:

If you think of it as electronics, 2 diodes in parallel is an OR gate I think.

OR gate but importantly not an XOR gate 😀

 

That said, if they were pressure driven pumps, rather than positive displacement driven, then one pump would bully the other closing that one's non-return valve and mean you only have the stronger pump circulating. You'd want positive displacement pumps. I think you could solve it with pressure driven pumps by using pressure regulators after the non-return valves, but you'd not start there.

[Nickfromwales]

All depends on what the heat sources are. 
Typically low grade heat gets injected much lower down the cylinder and vice versa. 
Heat sources are? 

[JohnMo]

Plumb it the same way as a two port buffer, with over sized tees to give hydraulic separation. You need these on the inlet and outlet.

 

You will need check valves as well

[SteamyTea]

28 minutes ago, Nickfromwales said:

Heat sources are? 

Solar, though not sure of PV or ST.

28 minutes ago, Nickfromwales said:

Typically low grade heat gets injected much lower down the cylinder and vice versa. 

I assumed it was for drawing convivence.

 

Would, adding a small buffer to the solar side help even out the temperature to a usable level?

Solar tends to be a variable source, where as, and ASHP, is pretty stable.

[MortarThePoint]

41 minutes ago, Nickfromwales said:

All depends on what the heat sources are. 
Typically low grade heat gets injected much lower down the cylinder and vice versa. 
Heat sources are? 

I'm trying to see if I should hedge my bets. I have been currently quoted a single coil cylinder (3.0m2) but there is a twin coil (2.2m2 + 1.0m2) option I could ask for which adds about £60. I'd rather not ask for it as I have been quoted without and I don't want to upset things, but I don't want to miss a trick.

I don't think I would be going solar thermal and PV could just use the ASHP or immersion heater I guess.

I could possibly imagine two future changes:

• Adding an oil boiler as a dual source option
• Adding a wood burner with back boiler

[MortarThePoint]

17 minutes ago, SteamyTea said:

47 minutes ago, Nickfromwales said:

Typically low grade heat gets injected much lower down the cylinder and vice versa. 

I assumed it was for drawing convivence.

 

Cylinder drawing and coil placement is just a rough sketch so don't worry about placement or indeed if I have the FLOW and RETURN connected the right way round 🙂

[MortarThePoint]

34 minutes ago, JohnMo said:

You will need check valves as well

I thought check valves and non-return valves where the same thing

[Nickfromwales]

If you go for a solid fuel appliance, then you have to allow for terminal failure scenarios, like the WBS being over loaded and the heat demand being zero, so the WBS eventually boils the water and steam becomes the killer. 
Unless you plan for a sealed and pressurised WBS with a cold mains fed quench coil, forget the setup that you show in your picture. 100% no go.

6 hours ago, MortarThePoint said:

Adding an oil boiler as a dual source option

Fit an oil hybrid ASHP from the get-go. LINK
Simples. 

[Miek]

In principal this could work well but as always its in the detail. Possible issues are - the two different  systems for heating have to have the same coolant and same pressure. So if Solar thermal then probably propylene glycol mix at upto 1.5bar roughly, if it were drainback then much lower pressure. This would have to work for the other heating source too.

 

It's likely you wouldn't run the two simultaneously , especially if it's solar input IMO, but if you did then the input with the highest flow rate requirement will dominate. Solar has very low flow rate, whereas a HP could have a much higher flow requirement. 

[MortarThePoint]

9 hours ago, Nickfromwales said:

Fit an oil hybrid ASHP from the get-go. LINK
Simples.

Interesting, I didn't know such a thing existed. I think there is only one model that is pretty large and incorporates a 17kW ASHP (15-26kW oil). This hybrid is about £3k more than the straight ASHP 17kW unit.

[Nickfromwales]

34 minutes ago, MortarThePoint said:

Interesting, I didn't know such a thing existed. I think there is only one model that is pretty large and incorporates a 17kW ASHP (15-26kW oil). This hybrid is about £3k more than the straight ASHP 17kW unit.

Yup, you’ll need a bigger buffer, agreed, but £3k now is bucketloads cheaper than all the faff and retrofitting of another device later on / more complex controls / integration etc etc.

Physical size will be smaller and neater than an oil boiler being retrospectively plonked alongside the ASHP .

You won’t really get a smaller oil boiler either way, so if you want oil then discussing size is moot ( and needs to be based on a min of 15/16kW for this exercise.  

[MortarThePoint]

2 hours ago, Nickfromwales said:

Yup, you’ll need a bigger buffer, agreed, but £3k now is bucketloads cheaper than all the faff and retrofitting of another device later on / more complex controls / integration etc etc.

Physical size will be smaller and neater than an oil boiler being retrospectively plonked alongside the ASHP .

You won’t really get a smaller oil boiler either way, so if you want oil then discussing size is moot ( and needs to be based on a min of 15/16kW for this exercise.  

 

Doesn't sound like there is any point in getting a twin coil cylinder then in my case.

 

Any future oil based changes would be grievous anyway and probably not be impossible with only a single coil.

 

Spacing heating is the lion's share of heat demand anyway and doesn't affect the cylinder.