Boiler overheating with only UFH on

[Peppertack]

Good afternoon

I have a Vokera Mynute 25 HE which runs very well. HW, CH, and UFH all run no problem. Pressure is good cold and hot. Problem is, if only the UFH is on, it makes a demand for heat and gets it but the boiler not only delivers to the UFH set temp at 45 but gets a little higher, shuts off and the temp continues to rise.
I have the boiler set at 56 but with only the UFH on, after a demand cycle, the temp can raise to 92- 93 and of course it will shut down around this. The boiler itself shuts down the flame around 58 but then the temp rises very quickly after that.

This could be a result of the water recirculating in the boiler and rising too quickly but why?

Any ideas would be very gratefully received.

 

[Riley]

Welcome along. Can we get some pics of your ufh set up

 

[Peppertack]

Thank you very much for that.

Please find attached a pic. The UFH serves a small kitchen and a hall.

Kind regards P

 

[oz-plumber]

Not enough flow through boiler when only UFH on by itself.

Your UFH will probably only be doing 4 l/m through that setup and with the temp valve from the boiler only letting very small amount of water through it, the boiler flow may only be 1/2 of that.

You may want to change that valve to a 3 way valve so that the boiler will be getting a decent flow through it when the temp valve is closed.

 

[Peppertack]

That sounds very reasonable Oz - thank you. The UFH will be extended once we build the extension but in the mean time I think a 3 way would work. Thanks for your time.

 

[Howsie]

The UFH is plumbed wrong.

 

[Peppertack]

Thanks for the reply Howsie. I understood the flow from the boiler is coming in at the bottom with return above is that right? Thanks P

 

[oz-plumber]

Actually,

Howsie is correct.

Fundamental design fault in manifold set-up.

The pump will pump most, if not all the water, straight back to the boiler, by-passing the UFH circuits, even when the temp control valve is closed.
The return should be connected to the opposite end of the flow, on other side of the pump.

That set-up is basically a pumped by-pass system

 

[Peppertack]

Thank you for that, Oz. That is odd as the manifold (Uponor TM Manifold) was supplied to the plumber like that as I understood.

 

[oz-plumber]

I have looked at both images, and if that is the way it is supplied, the system hasn't been balanced through the lockshield on the return side of the system.

That system is designed to be plumbed off a LLH.

I would say that the manifold set-up in your system is not applicable to the system you have installed.
Minor tweaks and changes could make it suitable, but in the current installation situation, it will not work as it is designed to work.

 

[Peppertack]

Thank you for that, Oz. We certainly don't have a low loss header installed anywhere on the system. Putting one in will be a bit of a challenge. I cant see how we can balance on the return. I am reasonably clued up but perhaps a bit hazy this morning.

 

[oz-plumber]

Simple solution,

Connect the return to the cap at the end of the flow manifold, after the pump and drain connection, at the end of the flow manifold.
It won't totally alleviate your boiler problems, but will make your UFH system operate better.

I would still prefer the pump being the first connection off the flow pipe for mixing, but I haven't progressed from the days of making my own mixing headers out of 'off the shelf items' from heating suppliers over here.

 

[Peppertack]

Thank you Oz. Unfortunately the plumber who installed is now retired but I will point this out to whoever I get in to look at it and extend. I understand your way. I do like the idea of making with your own bits. There are very many ready made installs which can make some things easier I guess but doesn't leave a lot of room to improvise. Thank you again - Kind regards P

 

[Howsie]

I can't see the point of the TRV in that picture.

 

[Peppertack]

Thanks for that, Howsie. I have been told to run the boiler and HW to 60 to keep it clean but the ufh in a screed should not be more than 45. I have the TRV set at 45 which in turn is connected to another temp after the pump. Do you think that is odd?

 

[Howsie]

The TRV will shut the flow down and then the boiler has nowhere to put the heat. It's wrong in my book. The TRV should blend the water, but it's doing nothing in your picture.

 

[SimonG]

Same as the old thermostatic heads on cylinder pipework with the phial taped to cylinder. Prefer the blending option.

Mustnt be an auto bypass on the boiler.

 

[Peppertack]

Thanks for that, Howsie. That is exactly what is happening. I'll put all this to the new plumber who will hopefully put it right. Kind regards p

 

[Peppertack]

Thanks Simon. You say there shouldn't be an auto bypass?

 

[Howsie]

Thanks for that, Howsie. That is exactly what is happening. I'll put all this to the new plumber who will hopefully put it right. Kind regards p

Where are you based?

 

[Peppertack]

We are in Salisbury.

What I am confused about is I understand Uponor have supplied the manifold 'as is' from the TRV counter clockwise to lockshield. All they ask for is the number of zones and hey presto there it is. If I remove the TRV I get 60 degrees from the boiler meeting the loop return at the actuators. I suppose that will mix it before the pump pushes it through the top meters. Not enough hair to pull out.

 

[Howsie]

Do you have a bypass fitted?

 

[Peppertack]

I can't see one anywhere outside of the boiler but can see where we could install quite easily. Everything is quite tight in our setup.

I did e-mail Uponor as well just to make sure the manifold wasn't installed in anyway that they wouldn't advise and they've come back with exactly the same suggestion - we need an ABV. I have booked in another service and I'll ask that the internal bypass and HEx is checked. In the meantime I'll flush the two loops and clean the top meters. Chilly floor but I would rather that then a melted ali Hex.