Hot Water Tank flow return differential.

[Fly100]

Hello,

 Need pointing in the right direction, my DHW tank is very close to the pump and boiler, however  when the water is on most of the flow goes to the tank and the rest to the heating. The DHW tank is robbing the heating system as the water flow is takes the easiest path.

Ive fitted a gate valve to slow the flow, but wonder what sort of flow / return differential i should be aiming for on the tank flows and returns.Is it the standard 20c like a boiler or is there another figure to use. 

Id like to ref a figure so I can than manage my system from that point, Im trying to achieve a decent balance of Heating and hot water by restricting the DHW tank flow.

 

Thanks,

FLY

[PeterW]

Sounds like the mid position valve is failing. What sort of boiler is it ..?

[SteamyTea]

Ideally, to get maximum power transfer, the return temperature is half the difference between the store temperature and the input temperature.

This gets confusing when the boiler is in condensing mode though, that often sets a minimum temperature.

 

That all sounds a bit confused.

Say your input temperature is 70⁰C, the store is at 40⁰C. Half that is 15⁰C, so return should be at 55⁰C.

But as the store warms up, say to 50⁰C, the return should be 60⁰C.

 

Edited November 4, 2020 by SteamyTea

[Fly100]

Worcester Oil Boiler, there are no valves in the boiler. Pipe run has valves ( 2 port ) on both the Heating Circuit and the DHW Circuit.

[PeterW]

Sounds like this is wired as S Plan with 2 port valves. Does the tank have a stat on it ..?? And is the DHW valve closing properly ..? You can check this by turning off DHW on the programmer and just running the heating circuit and seeing if you get flow after the valve and the pipe getting hot. 

[SteamyTea]

8 minutes ago, PeterW said:

S Plan

Any chance of starting a new thread about this. I see it mentioned, along with other plans, but don't understand what it means, and too lazy to Google it.

[Fly100]

Everything is working ok, its just the resistance of the Heating circuit is higher than the DHW. Hence the water is flying through the tank coil and not going to the Rads / Underfloor.

[ProDave]

But this will only happen until the tank reaches it's set temperature.  Is it really a problem if the heating goes off or down a little while the tank heats up (this is normal with a heat pump for comparison)

[Fly100]

On 04/11/2020 at 13:18, ProDave said:

But this will only happen until the tank reaches it's set temperature.  Is it really a problem if the heating goes off or down a little while the tank heats up (this is normal with a heat pump for comparison)

 

But its robbing 15C of flow temp from the underfloor heating. Sending cooler water into the UFH system that's counter productive.

[Gus Potter]

But how much cooler is the water going to the UF, after all the blending / mixer valve  is meant to reduce the temperature of the water so it does not harm the flooring or give you sweaty feet.

[Fly100]

52 minutes ago, Gus Potter said:

But how much cooler is the water going to the UF, after all the blending / mixer valve  is meant to reduce the temperature of the water so it does not harm the flooring or give you sweaty feet.

 UFH set at 45c, with the hot water on its measuring 32c ish.

[Gus Potter]

Does not look to drastic to me, see @ProDave etc above, also Peter re sticky port valve etc, Steamy tea.

 

 

[PeterW]

2 hours ago, Fly100 said:

 

But its robbing 15C of flow temp from the underfloor heating. Sending cooler water into the UFH system that's counter productive.

OK but is this only happening when both DHW and UFH are calling for heat ..  and is there any bypass on the UHF manifold ..? If it is hitting the hot side of the mixer and being blended down then it is not surprising the flow is less. Also try turning up the flow rate on the UFH and turning down the flow rate from the boiler and see what happens. 

[Fly100]

On 04/11/2020 at 11:16, PeterW said:

Sounds like this is wired as S Plan with 2 port valves. Does the tank have a stat on it ..?? And is the DHW valve closing properly ..? You can check this by turning off DHW on the programmer and just running the heating circuit and seeing if you get flow after the valve and the pipe getting hot. 

 

Yup checked that, valve is closing properly, im sure this is just water taking easiest route .Hence the gate valve to add some resistance and slow it down a little.

[Nickfromwales]

7 hours ago, Fly100 said:

 

Yup checked that, valve is closing properly, im sure this is just water taking easiest route .Hence the gate valve to add some resistance and slow it down a little.

Just crack the valve shut at then open it to be set at 40% open. Test the system and move in 5% increments until happy. Simples.

Flow dynamics are completely academic as the system will always see different flow rates to the various heating at various times. Put the calculator down, and go for Neanderthal maths   

Open it 100%, and count the turns to shut it. From that you can work out how to ascertain open %.

Edited November 6, 2020 by Nickfromwales

[PeterW]

If that is the case then change the programme and put the DHW on earlier / later than the heating ..?

[Temp]

Are we sure the DHW tank isn't being used as a buffer for the UFH? I think some systems put hot flow into the tank and take it out to send to the UFH via the same pipe when the boiler is off.