Bypass radiators too hot

mattspeke

Hi, I have 9 radiators over 2 zones. 3 bedrooms, 2 bathrooms and the hall on one zone. Lounge, kitchen and utility on the other zone. The hall and lounge radiators are the bypass radiators on each zone. The issue I have is that if I call for heat on the SRVs the lounge and hall are too hot. It’s uncomfortably hot to sit in the lounge. Ideally I would like control of the hall and lounge radiators. First I was thinking of fitting SVRs to all the rads, but I’ve read that this is not good for the boiler not having somewhere to dump heat. Next I was thinking of swapping the bypass radiators for the utility and the main bathroom, but these rads are half the size of the lounge and hall ones. Is this ok to do? I was reading about minimum heat output of boilers. My boiler is a Potterton Ultra combi 40. Will need to check manual for min output.

mattspeke

5.7kW non condensing and 6.0kW condensing. So does that mean that if I have SRVs regulating the heat to all bar one rad then that one rad will be trying to dump the remaining of the 6kW out? Possibly resulting in the return water temp being too hot? Causing the boiler to not operate efficiently and cycle on and off a lot more than it should?

GrilledCheese2

Are you sure you do not have an external bypass valve installed before the two motorised valves? I wouldn't expect a new setup for two zones to be installed without one.

A radiator cannot dump more heat than it is designed for. At 6KW output there's probably 5KW of excess heat when only one radiator is on, so your boiler will short cycle very quickly. You have 9 radiators installed in a 3 bedroom house so your heating load is probably 10KW max. Even with all the radiators running for a single zone (2nd zone off) the boiler is still going to short cycle a little bit (assuming each zone is 5KW).

mattspeke

There is a bypass valve set to 0.2 bar between the flow and return pipes. As you say, before the Y split to the 2 valves. Doesn’t look like I can post a pic to show you. The system has been installed with one bypass rad and the rest TRVs (per zone) . I have just changed the TRVs for SRVs. But now I’d like control of the temperature of the bypass rads. Is this doable?

GrilledCheese2

If a radiator circuit has an auto-bypass valve installed then there should be no need for a bypass radiator. It's one or the other, not both. Older installations used to have a bypass radiator (usually in the hall or bathroom), but in new installations the auto-bypass is used instead.

When the two motorised valves close the auto bypass valve will open so that water flows directly between the flow and return. Similarly when all the TRVs close the auto bypass will open. You should be able to install smart TRVs on all of your radiators and have temperature control of every room.

However, if your heating engineer is telling you that a bypass radiator is required for your system, then go with their advice.

mattspeke

Thanks for your reply. I saw your comments on another question and hoped that you would answer. My boiler engineer is due retirement and doesn’t know about modern control technology. I have a work colleague who is also a plumber so he’s going to have a look at the system this week. Hopefully I can control all radiators with SRVs.