UFH using unvented cylinder

[Jim1234]

Hi,

I’m new to this forum and looking some advice for a new build installation.

I am considering plumbing my ufh flow and return into a secondary coil in my unvented cylinder to heat my house, rather than directly into the boiler flow and return.

 

The theory is that the oil boiler will heat the domestic hot water and I will extract the heat from the cylinder using a pump and blending valve.

 

I see a number of benefits in this type of system such as:

- flow temperature to ufh manifolds at the correct temperature rather than 70deg from the boiler and mixing down to 35deg.

-no short cycling of boiler 

-zones can be heated individually without boiler having to fire

-hot water can be possibly stored at a lover temperature and boosted to over 60deg once a week for legionella purposes

-storage of hot water in summer months unlike a thermal store which require higher temps to ensure hot water production all year

Rough schematic attached

 

Anyone ever done this before or be able to provide some advice.

 

Thanks

[JohnMo]

I would be concerned with continuously running out of DHW. You are basically running the cylinder as thermal store but with DHW in it instead of heating circulation water. You would have to be careful with the coil placement to get meaningful energy available for the floor and DHW.

 

New build, why an oil boiler and not a heat pump?

[Jim1234]

Hi John,

I believe on initial start up the ufh will strip all domestic hot water though when up to temperature though I expect this to be ok when running.

The boiler will be controlled through the cylinder stat to keep the domestic hot water up to temperature 24/7

As for coil placement I was thinking boiler coil at the bottom of the cylinder to help with de stratification and then a high gain could towards the top.

 

with blending on the flow only the heat required to bring return up 10deg will be extracted from the cylinder.

 

I know someday I may have to go heat pump but I don’t believe they are the answer just yet.

oil boilers are simple and work compared to heat pumps.

 

Thanks

[John Carroll]

What is the ufh demand in kw, how many loops?. Coil rating is often based on a flow temperature of 80C and maybe 30/35 LPM flowrates so ensure both coils are suitably rated.

[Jim1234]

Total ufh load 8kw across 20 loops

ufh flow 35 return 25

if the top coil was rated at 40kw based on a 80deg 

how would I work this back to know if it would work with a 60deg storage temp

 

[JohnMo]

55 minutes ago, Jim1234 said:

with blending on the flow only the heat required to bring return up 10deg will be extracted from the cylinder.

Trouble is it will not really work like that as the system is a closed loop with a blending valve. The return water from the UFH as you say will need to be heated by say 10 degs. So you start with the water going through the coil, hot water coming out say at 70 Deg say at 20L/min. This water will be mixed with cooler return water say at a 25-75 ratio. So now via the mixer, you have water returning from UFH at 35 less 10 degs, so at 25 degs, 75% of this goes in to the bottom of the mixer and has hot water added by the coil to be at 35 for the UFH.  The other 25% of the return water goes through coil. So you have a 5 L/min of 25 Deg water entering the coil to be heated back to 70 degs. So now the centre of the cylinder is cool and the stratified cylinder is no more, because that water will want to sink and cause lots of heat currents in the cylinder.

 

Not really sure it will work that well. Think you will just end up with warm DHW most the time. For the period we are heating the UVC we have only warm water, as it completely destroys stratification. You will have this continually.

 

Just do a thermal store?

[Lofty718]

This plan makes no sense, better of getting a UVC with a system boiler that runs on weather compensation

[dpmiller]

buffering through a gravity cylinder? I wouldn't, even just gravity DHW would be enough to put me off...

 I did it the other way round as a TS to give boiler stove redundancy. No legionells risk either as no stored volume. Check out for example-

 

https://copperindustries.co.uk/products/maxipod/

 

although many other manufacturers can offer the same now too. I had a bespoke one made up by World Heat

[Jim1234]

So a thermal store means I need to heat water to 70deg to get hot water plus I would have no storage.

i also struggled to find an unvented thermal store.

 

If my calcs are right at max demand the flow rate will be 14.45 litres/min.

 

with a return temperature of 25deg and flow temp out of the coil at 55deg and a mixed flow of 40deg I would need the cylinder to heat 7.2 litres/min from 25 deg to 55 deg.

 

at a 35deg mixed temp I would need 45deg from the coil at 7.2 litres/min

Edited November 25 by Jim1234

[Jim1234]

8 hours ago, Lofty718 said:

This plan makes no sense, better of getting a UVC with a system boiler that runs on weather compensation

Weather compensation on an oil boiler?

[Jim1234]

3 hours ago, dpmiller said:

buffering through a gravity cylinder? I wouldn't, even just gravity DHW would be enough to put me off...

 I did it the other way round as a TS to give boiler stove redundancy. No legionells risk either as no stored volume. Check out for example-

 

https://copperindustries.co.uk/products/maxipod/

 

although many other manufacturers can offer the same now too. I had a bespoke one made up by World Heat

The buffering would be in an unvented cylinder.

 

i looked at the maxipod but all open vent and I have no option for this in the house 

[JohnMo]

I would definitely split system. If you are fixed on oil. Get a 100-150L buffer for heating and or WC capable boiler. Then an UVC for DHW. If you wanted you get a coil in the buffer and use as cold water pre heat for the DHW cylinder.