Connecting a boiler to a buffer with UFH

19 Jul 2010
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United Kingdom
Hi All,

I was wondering what the best way to connect an oil boiler to a 300L buffer tank would be.

What I have:
A 300L tank (with solar) for DHW - the boiler can connect to a coil on this (I think this part is ok)

A second tank for CH - this supplies UFH and towel rads.
Currently I have the boiler connecting to the bottom coil, and the UFH connected to the primary flow and return.

My concern is that the boiler cannot 'efficiently' heat the tank via the coil - the flow and return seem quite hot, and the tank takes a long time to heat.
Would it be better to have both the boiler and UFH connecting to the primary tank inputs and leave the coil 'spare' for future use.

Would there be any problems with doing this?

The idea would be to have the boiler and ufh return at the bottom of the tank, and the ufh flow out the top connection, and boiler flow via the top-side connection.

Another mostly unrelated question - the upstairs and downstairs UFH have a circulating pump built in to the manifold - but do I need another pump back at the tank itself to push the water to the manifolds, or will the pump in the manifold automatically 'draw' the water it needs?

Towel Rads
I intend to have the towel rads on the same circuit as the under-floor, but would have a pump for them back at the tank. Do I need any non-return-valve, or motorized valve with these, or do the pumps generally have them built in. i.e. if the pump is off, will it stop the flow in that circuit? I want to prevent accidental flow around circuits that are supposed to be off...

I really appreciate any and all answers to these questions. Unfortunately things are not going so well on this project, and sound advice would be a huge help!
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What type of boiler is it?

You need a twin coil cylinder for solar- the upper one is heated by the boiler and the lower one by solar.

If you heat a buffer vessel [primary store] or combination cylinder for CH and DHW, the buffer has to be kept very hot in order to get a satisfactory DHW performance. However UFH requires a much lower flow temperature, and a high store temperature is therefore wasteful. This is particularly so if it is a condensing boiler, in which case the UFH should be fed directly from the boiler, preferably with weather compensation on the boiler.

Can you post a schematic please?
I always like to connect the boiler up direct as it prevents the boiler cycling and also keeps it in condensing mode, but these connections need to be carefully positioned for correct flow and return temp!
Sorry, I don't have a schematic to hand - but here is a quick outline

I have 2 separate 300L tanks
One for DHW
One for CH

The DHW uses solar and boiler, and as suggested, bottom coil is solar, top coil is boiler, and primary flow and return are for hot water.

The second tank, the CH tank, is the one I have the question about -

I have UFH coming from this tank, and it is currently connected from the primary tank outputs - i.e. it takes the hot water of out of top of the tank, and returns cold back in the bottom. it is a closed system.

The boiler is connected to a loop on the bottom of the CH tank - but I was wondering if it would be better to do:
Not use a coil at all for the boiler, and instead send the hot water IN what's known as the secondary-return (top connection), and take the cold water out the bottom of the tank (the same point the UFH return connects to).

The idea is to:
1) prevent cycling as the boiler directly heats all the water in the tank
2) as John67 says, it should keep it in condensing mode, as the return water should be cold.
3) water in the UFH and boiler is the same water (no coils are used at all)

John67: Can you elaborate on the how the position the connections - the tank only has two direct connections - its a standard 300L, dual coil cylinder.

Long story short - will direct connection of ufh and boiler to the tank be more efficient/recommended?
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taking water directly from the bottom of the cylinder is not a good idea as it would be too cold to return to the boiler, another way to do it which would be perfect for you is to use a thermostatic mixing valve so you can get the required return temp. DPS at has a good diagram of this method under GX heatbanks.
Hi John,

Thanks for that - very imformative, and looks like it could be done with a mixing valve.

The diagram is interesting - it seems to suggest the UFH takes the water off quite low on the tank - and the boiler inputs at the top of the tank.

As for the mixer - what temperature should be returned to a condensing boiler for optimum operation?

I heard hot water returning is bad, I was not aware the water could be too cold also! A built-in mixer in a boiler would be a nice feature ;)

Return temperature will be in your boiler manual but most return 55 deg thats 20 deg below your flow temp, thats assuming its a condensing boiler but if not it should be 10 deg temperature difference

The ufh flow normally comes of that low because at that level it will be the correct temp, don't bother about that though because the mixing valve in your ufh manifold takes care of that.

good luck
Hi John,

one final question - assuming I don't go for the mixer valve and stick with the current hookup (boiler connected via a coil) - the boiler seems to run for about 30 seconds, and then stop for a minute or two, and then starts again.

It is due to the boiler cutting out due to the thermostat on the boiler itself - i.e. I guess the pump cannot get the heat out of the boiler fast enough.

The pump is connected via 1" pipes, running approx 12-15 meters to the tank. My guess is the coil in the tank is too short for the water to transfer it's heat to the tank, and is returning quite hot - which of course the boiler heats up quickly.

The tank is also not heating beyond 50 degrees.

The plumber has also connected the return on the upper connection of the coil and the flow on the lower - which I think would make matters worse, and the return will be warmer.

Any suggestions for fixing this - is the mixer valve the way to go to solve all these problems?
Nothings ever straight forward is it.

Does your ufh have a thermostatic mixing valve, I am only asking because i cant see any other reason for the temp to be set so low.

The boiler is cycling because the coil cant transfer the heat fast enough

If the temperature is left so low then you might find it difficult to obtain the correct return temp. If you were to connect the boiler direct through the mixing valve set the cylinder stat just below boiler operating stat it would work well.

You have a point about the flow been connected to the bottom of the coil, this is a popular way to connect up to dhw cylinders to give you more hot water storage an help prevent legionella. having said that I think that if you changed it around you wouldn't notice much of a difference.

Hope this helps
If you have engaged a competent plumber then he should be telling you how it should be done!

Why do you feel its necessary to ask here when you already have a plumber fitting it?

Hopefully he is qualified for unvented cylinders as it sounds as if some of the safety features/interlocks may be missing.

Why do you feel its necessary to ask here when you already have a plumber fitting it?

Plumbers do drains and unblock stuffed toilets and such like....
Complex heating installations such as the OP's requires skilled, experienced and clever operatives.
There like hens teeth these days.
Exactly, I would say he needs a competent heating engineer but the OP has already employed a plumber.
agile he is asking these question because some plumbers don't have a clue what is being discussed. do you
Hi Guys -
you hit the nail on the head - I do have a plumber (who seemed to know what he was at - he talked the talk when I first employed him to fit out the house).

The more research I did about the heating system, and started to learn the complexities of them, the more I discovered he was a sink and drains man who just sounded like he really knew what was going on, but clearly he doesn't.

If I had my time around again, I'd have gotten someone who really knew what they were on about - but as one poster put it - they are like hen's teeth.

So my only resort now is to 'design' the system myself and get him to fit it.

On specifics - the UFH does have it's own mixing valve. I would like the CH tank to be hotter than 50C - but with the current setup it's not getting enough heat transfer.
Last night I took a thermocouple and probed the flow and return temps at the boiler - looks like 70C flow and 65C return! so clearly the heat is not getting transfered out via the coil.

Luckily the cylinder is a triple coil (only needed a dual) - so I can get the boiler to run through 2 of the coils to ensure better heat transfer - or else I will go with John67's suggestion of using the direct connections and a mixing value to set the ideal return temp of 55C

I think the mixing valve idea is really nice - I like the sound of the boiler always running at it's most efficient.

I heard that there can be issues with running a boiler through the direct connections as it can cause turbulence and effect the stratification of water temp in the cylinder - but I guess that's not an issue here as I want the entire tank hot, not just the top-half - because it is a buffer.

I really appreciate all the comments and help here.

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