Connecting a boiler to a buffer with UFH

[tailsnaf]

Hi All,
Here are the images I put together of the current setup:

and proposed setup:

The area of concern is the CH tank which is used as a buffer for UFH, and heated towel rads. May be used for some rads in future, but not connecting them at the moment.

The hope is that if the UFH needs heat, the boiler will heat up the buffer quickly, so it remains in condensing mode and does not cycle. This should be the most efficient way to use the boiler, even if UFH heat requirement is low.

Issues appear to be boiler cycling with current setup, and not being able to heat the tank beyond 50c.

Note: there are no NRVs in this design, as heard they should not be needed.

Hope this helps clear up matters.
Note: all this is installed as per 'current setup'. I may add a weather compensating kit, if it could be fitted to this system, but no chance (i.e. money) for buying a new system from scratch.

 

[looneyfitter]

looking at your current setup, i cant see any reason as to why the cylinder wouldnt get above 50 deg? Have you tested the cylinder stat, boiler stat?

What temps do your primary flow and returns get to? You may have answered the above previously but i cant remember.

Edit does the current setup still only get to 50 deg with no other demand calling?

IMO both of them buffers should also be fitted with T&P relief valves.

Also i assume there is pressure vessels in the setup also?

You will need primary pumps off the ch buffer to demands. The pump at present would generally be used as a shunt pump. You also require a by- pass. I would use a 3port valve. before the ch 2 port.

Im not 100% on the CF to the blender but i would say take it from the bottom of the ch buffer.

Weather comp comes as standard with the 2010 Viessmann 200w. You could also then add 1 other WC circuit iirc

Gareth

 

[tailsnaf]

Thanks for the reply - please see my responses inline:

looking at your current setup, i cant see any reason as to why the cylinder wouldnt get above 50 deg? Have you tested the cylinder stat, boiler stat?

No problems here - checked them all with hand-held sensors, and confirmed temps.
I think the problem is the surface area of the middle coil being too small.

What temps do your primary flow and returns get to? You may have answered the above previously but i cant remember.

I believe flow was between 67 and 70 and return was between 62 and 65. So approx 5c temperature differential, which I suppose means not much heat was being transferred to the cylinder.

Edit does the current setup still only get to 50 deg with no other demand calling?

The boiler was only heating the cylinder at the time, but the UFH was running, and I didn't check what temp it achieved without the UFH on, but the 35kw output of the boiler should far exceed the max draw off of the ufh (10kw I beleive, if running at full tilt...)

IMO both of them buffers should also be fitted with T&P relief valves.

Also i assume there is pressure vessels in the setup also?

Yes, both of them have T&P relief. I was thinking that if I do insert the mixer, so the water in the CH buffer and boiler (and ufh) is all the same, then I would move the T&P off the tank, and back near the boiler.

There are pressure vessels hookedup there also, one on the boiler, one of the UFH and one on the solar.

You will need primary pumps off the ch buffer to demands. The pump at present would generally be used as a shunt pump. You also require a by- pass. I would use a 3port valve. before the ch 2 port.

The UFH manifolds have pumps built in - do you think another pump is needed also, or will the UFH pumps 'draw' the water from the buffer as needed - this was my assumption, and seemed to work during testing.

There will be a separate pump on the heated towel rails coil output.

Im not 100% on the CF to the blender but i would say take it from the bottom of the ch buffer.

I had initially thought of putting it at the bottom of the ch buffer, but then it would not mix also for the DHW tank. So I thought about putting it back at the boiler, so the blender would work whether the DHW or CH was running. I assume it would only allow water to flow between boiler flow and return if return was less than 55C, and if return was > 55C it would shut off?

Weather comp comes as standard with the 2010 Viessmann 200w. You could also then add 1 other WC circuit iirc

Weather comp sounds like a great thing to add, but I fear its too late for me at this stage... unless you know a way of fitting it onto the system as above? Is it just a fancy controller?

 

[looneyfitter]

Hello matey, not sure how you reply with the quotes inline as you have.

The temp diff accross your coil is way out. Which coil does the MF instructions say to use? You could like as been said link the top and middle coils to give more transfer but thats not how it should be done unless specified.

The mixer is in the right place. but you want it a metre or so away at least.

You will need T&P valve on BOTH the cylinders and a PRV on the boiler also.

I would have individual pumps feeding the UFH circuits and let the manifold pump serve to circulate the loops

You need a potable vessel for the HW cylinder also.

 

[tailsnaf]

Thanks mate, really appreciate the sagely advice!

I can add a pump for the UFH also, to help the flow to the manifolds. Would that pump need a bypass also, in case the manifold mixers were opening and closing a lot, maybe cutting off the flow.

I'll also add the potable vessel to the DHW - good idea on that!

Thanks for sanity checking the setup.

I don't know what the manufacturer recommends, the 'plumber' swapped the dual solar cylinder I had bought for a triple coil (and I let him as I liked the idea of a spare coil to connect to something, like outdoor jacuzzi maybe as alternative dump load), but no manuals came with it, though it is 300L like the dual I had.
I'd say I could have been had!

 

[looneyfitter]

No problem mate. The manifold will return the water back fine. HOwever for piece of mind you could fit a bye-pass after the pump to the return if you wished.

Link the 2 flows of the top and middle together, along with the returns and see what results you get. You could do this in plastic if you wanted for now

Do you know what flow rate your getting through the coil? I wonder if you could feed a length of sparks fish wire down the flow so we could measure how long the coil is.?>

 

[tailsnaf]

Won't linking top and middle coil only heat the top of the tank then, making the buffer effectively 150L instead of 300L ?

I'll try the fish-wire, I have some lying around. Any idea what a 'good' length coil would be, assuming it is an 1nch diameter?

 

[looneyfitter]

Won't linking top and middle coil only heat the top of the tank then, making the buffer effectively 150L instead of 300L ?

I'll try the fish-wire, I have some lying around. Any idea what a 'good' length coil would be, assuming it is an 1nch diameter?

You using the bottom for solar arnt you? Not sure on a good lenth to be honest. Im sure there is calcs out there. you could also gove gledhill a call, sure they would give some info

 

[looneyfitter]

can i ask what software you used for your diagram mate? You wouldnt have a spare copy would you?

 

[tailsnaf]

Bottom is currently connected to solar, but chances of the solar ever doing CH heating (when needed, i.e. in winter) are low, so would probably get away with connecting it to the middle coil...

As for software, I spent a while searching for a good program, but in the end used microsoft powerpoint. Kinda rough to get started, but just copy-pasted shapes time and again.

I can send you the source file as a powerpoint, so you could copy-paste the symbols I used, if that helps at all?

 

[Agile]

I realy dont see the point of the CH store.

The reality is that the power output of solar is so small compared with CH needs. Thats apart from the fact that heating is needed in the dark winter when there is no solar gain.

Two solar panels might get about 3 kW in the height of summer but thats when you dont need CH. Most homes need at least 10kW of heating power.

The other is that a 300 li store, even at 80° would only power CH for a very small time, perhaps just 15 minutes.

Tony

 

[tailsnaf]

Hi Agile,

Can you elaborate on that further - you are saying that 80C in a 300L CH tank would only last 15 minutes?

Take the solar out of the equation for now, and just assume the boiler is the only source - are you saying the UFH would draw off that much heat that fast that the boiler would be on again in 15 mins after heating the tank up?

If that is the case, then in the dead of winter, I'd expect the boiler to be on 50% of a given day - say 15mins on to heat up tank, then off 15 mins.

On the surface that sounds like a extremely high usage - would it not?

Even if the boiler does cycle off and on like this, maybe it is still better than the case of without, where the UFH can't take the continuous output of the boiler, and the boiler cycles on and off in 1 minute bursts.

The CH buffer is intended (hopefully) to reduce the short cycles - by having a high output for a short amount of time, and then drawn off as needed. I had hoped maybe heat for 10 mins and draw off for an hour, or so.

Are there ways of calculating the amount of time it should take to deplete 300L at 80C into a UFH system of say 180 sqm? (6Kwatt peak floor output)

 

[tailsnaf]

I just tried to do these calculations myself, and I'm not a physicist so I probably made a big mistake here somewhere....

Tank starts at 80C
Working temp down to 40C
-> 80-40 - 40C temperature differential of 'effectice heat'

Specific heat capacity of water: 4.2 KJ/Kg.K
300L of water = 300Kg
= 4.2 x 40K x 300Kg = 50400 Kjoules of energy stored in the tank

50400 / 3600 = 14 Kwatt-hours of energy.

If the UFH system is designed to use 6K-watts, then this suggests 2.3 hours before the tank reaches 40C

So if the boiler can heat it up in 15-30 mins (guess), it will last 2 hours before needing to cycle again.

Does this sound even remotely right?

 

[looneyfitter]

Agile can you elaborate more as to why you think the Ch buffer will run out of puff mate? With the boiler running, i cant see one reason why it wont cope.

At the end of the day, its basically just a bigger Balanced Header. As per the solar coil to the buffer. Even if it only gave a 5deg rise then surely that will be better than nothing. ( i just want to point out i know nothing about solar) Thats my net mission

Tailsnaf that would be fantastic mate. Where can i pick up a copy of powerpoint? Send to [email protected]

Good man

Gareth

 

[tailsnaf]

No problem mate, just sent on the email.
Let me know if you don't receive it.