UFH bypass valve location?

[vmort]

Hi, I am not a plumber and am currently having an 'discussion' with one. Just after a basic answer.

New UFH and boiler (not combi) installed to a new kitchen extension in Dec. Boiler drives HW, CH and UFH independently as you'd expect. Works fine on the first two, however when the thermostat calls for heat for the UFH, boiler comes on and then constantly switches on/off/on/off (about every 5 mins) during the demand period.

Makers of the UFH (Nu Heat) advised on needing a auto bypass valve for when the pump is unable to dissipate the heat quickly enough and that will cure it. Makes sense. Plumber says there already is a bypass fitted but it is in the airing cupboard which is upstairs and about 7 metres from the manifold.

Now, I know that when ANY of the demands are made to the boiler, the heated towel rails in the two upstairs bathrooms also come on, so I understand that the water is somehow looping upstairs before or after heating the UFH, however is it right that the bypass is up there as well or should it be closer to the manifold? Nu Heat techs say that if correctly fitted, the bypass valve will stop the boiler from having to switch on and off.

Would love any independent views on this.

 

[Ryler]

Small demands will cause most boilers to cycle on and off.

 

[vmort]

Does this cause any harm to the boiler? Is it possible if I slightly turn down the temp control on the boiler it will run slightly less hot and therefore longer?

 

[gasbanni]

Something is plumbed in wrong the towel rads shouldn't get hot if there is only a demand on the uf circuit.

Presumably you have three independantly controlled zone valves.

Each circuit should only get hot when that circuits control calls for heat.

So put a demand on one circuit at a time and see what gets hot and what stays cold and check the operation of the zone valved involved.
It will be easy to see what's happening.

Regarding the cycling of the boiler before that's looked at check what's happening on each of the circuits ......Some boilers have a built in bypass so check whether your boiler needs one or has one.

 

[vmort]

The towel rads are in the system before any of the zone valves which we like! That’s actually not a problem for us and I assume is ok otherwise would have caused a problem having them run separately.

Should the bypass valve though be that far from the ufh given the boiler is about two feet above the manifold?

 

[Madrab]

Depends where the return to the boiler is from the UFH. If it's close to the boiler and the UFH return is hot enough then the boiler will cycle in no time. That being said the UFH's flow rates should be balanced properly to ensure the return temp is low enough to stop the boiler from cycling too much, it should be modulating rather than cycling .

It's critical to know how the system is working and set the system up accordingly when mixing space heating, if the UFH has just been bolted on as an extra zone and not setup properly alongside the normal CH system, then it will cause short cycling and will waste energy.

 

[vmort]

The UFH was put in when we had a new room added. So basically we kept the original CH and HW and the UFH is in the new room only. The boiler was replaced at the same time. So there are three 'zones' (CH, UFH, HW).

Spoke with the boiler manufacturer today (Worcester Greenstar Ri 24kW) and they said the cycling will not do any real damage but the heat exchanger could suffer long term. Said that when it's just the UFH on, with the floor needing such a small amount amount of power and the boiler throwing out up to 24kW, there will be too much for the pump to handle. But a correctly operating bypass valve should sort that issue out and prevent cycling.

Manufacturer of the floor (Nu Heat) said more or less the same thing and that he had not heard of a boiler cycling when the bypass valve was operating correctly. He suggested it may be a simple thing like a pressure release valve in the bypass being too tight.

Tried the heating this evening and monitored the boiler. With just the UFH on, boiler stayed on for 1 minute, off for about 3, then on for 15 mins, then off, on, off on at around 5 min intervals. Definitely a fault somewhere.

I'm afraid I have no idea about the UFH return other than the manifold and flow/return pipes for the UFH are only a metre from the boiler (boiler mounted above the manifold).

Not sure if any of this info helps anyone. Builder chasing the plumber.

 

[Madrab]

The key with correctly functioning UFH is setting the water temp drop across the mass which is balanced by the specific flow temp and flow rates.

The function of a bypass valve it to ensure there is a constant pressure and flow around the system and maintain a sufficient amount of water passing through the boiler to dissipate the heat in the boiler and protect the main HEX, typically on boiler overrun. When some or all the valves in the system close it is that operation of the valves closing that transfers the pressure to the valve allowing it to open, I wouldn't say they are installed specifically to stop a boiler from cycling.

A boiler will naturally cycle when the return temp of the system water reaches it's upper set limit and it then shuts down the burner. It will keep the pump running if there is still a call for heat and once the return temperature drops below it's set limit the burner will light back up and it will continue doing that until there is no more call for heat. The trick is to minimise that cycling process by setting the system temperatures up properly and keeping that return temp lower. A lower return temperature will also keep the boiler at it's most efficient as a condensing boiler needs a cooler return temp.

Ideally you need an engineer that is experienced in setting these systems up to optimise your installation properly.

 

[vmort]

Thanks for that info. I get the gist of what you're saying, even though i don't understand the detail. Builders are ging to send out the original plumber/heating engineer. He apparently has been putting thses systems in for over 20 years so I assume he knows what to do, although he has been fobbing me off telling me it's all fine. I'll give him one run at it and then demand another engineer. Luckily we still owe some money.