Converting single to two pipe system

[Kurt Stone]

Hi All

I have a single pipe central heating system which was extended 20 years ago when we had an extension. The heating upstairs has never been great particularly in the extended part with only a few of the original radiators getting very hot. We replaced the downstairs radiators with UFH which I thought would improve the heating upstairs but it seems to have had little effect. Because of this I am looking at changing the upstairs to a 2-pipe system and have a number of questions.

• The existing pipe work is copper but I am looking at using plastic for ease. I presume there is no problem with mixing the two?
• To reduce number of connections I am hoping to use 22mm pipe rolls. How flexible are these for feeding under the floor and do they straighten easily or do I need pipe clips?
• Do I need to use swept tees at the radiator tails?
• I assume the feed to the radiator should be on the TRV side?
• I am splitting the feed to the radiators to reduce the length as currently it goes right round the house on a bit of a circuitous route. I will end up with 3 circuits and 4 different routes for the returns so there will be a number of tees on the supply and returns. Is there any issue with having a number of T’s.
• I have one radiator downstairs which will be very difficult to convert. Is there any issue with leaving this as is?

Any advice welcome. Thanks

 

[George Kaplan]

Hi Kurt, I am not an expert so this is all to be taken with a pinch of salt. I am not a plumber but I am an engineer with a lot of experience with hydraulics so I am not 100% uninformed about the general principles.

I think you are OK with plastic, I have seen loads of properties with a mixture. I have just removed my single pipe system and replaced it with a two pipe one but in my case the pipes were vulnerable to rodents so I have done mine in copper which is a much bigger job than plastic.

The "official" recommendation from the pipe manufacturers is to support plastic pipe every 500mm. Its a recommendation not a rule. Use your common sense for your particular situation. Dont think swept tees will make a difference, the pressure acts in all directions. The key issue if the pipe size which you seem to have covered.

It is recommended that the TRV is on the flow side.

Not sure about your last TT question. It depends on how you have designed the pipes.

Not sure, why cant you expend the existing tails to where you can connect to the main pipes?

 

[Robooo]

You might need to move the pump from the return pipe to the flow pipe right after the 'cold feed' and 'vent pipe'. If you have a 'cold water storage tank' and a 'feed and expansion tank' (aka F&E tank). You need to do this so the pump is not drawing air into the system. In order to keep the air out you may think about an air separator etc as well.
Good luck!

 

[Last Plumber]

If your heating is old enough to be a 1 pipe system, I presume you have a cylinder and gravity hot water? As above feed, vent and pump configurations are very important. If you get it wrong you'll wish you hadn't bothered.

Plastic is OK. I personally don't use it but most seem to do. The size is important as the wall thickness on plastic reduces the bore so the capacity for volume is less. That matters where output is concerned. Calculations are needed to tell you pipe size unless you're experienced enough.

If you are going to tee many circuits together, include a method of balancing them.

Re- last rad:
I haven't seen your job but in my opinion mixing one and two pipe is not good. Mainly because you are in effect leaving a fully open bypass on a two pipe system. They both have different flow and return temperature differentials.

Going off what you're asking and without putting a damper on it or sounding offensive, I think you would be better asking a heating engineer to convert it for you. You could spend a lot of time, effort and money only to find it went wrong and then you will struggle to find a Plumber who will be willing to take on the work to put it right.

 

[Chuck]

I second @Last Plumber's suggestion that you get an experienced heating engineer to look at the problem. There are several 'big' questions that need to be addressed first. For example, are you are looking for a relatively minor fix to the existing system or do you want to future-proof by upgrading to low-temperature emitters, thermal stores, solar panels, etc. Could the issue be addressed by upgrading the insulation? Do you have the budget to spend now in order to cut future consumption?

The right solution for you might be as cheap and simple as balancing the existing system, maybe installing a separate pump to boost the one-pipe system. Or it might be costly a full refit to current standards.

 

[Kurt Stone]

Hi Kurt, I am not an expert so this is all to be taken with a pinch of salt. I am not a plumber but I am an engineer with a lot of experience with hydraulics so I am not 100% uninformed about the general principles.

I think you are OK with plastic, I have seen loads of properties with a mixture. I have just removed my single pipe system and replaced it with a two pipe one but in my case the pipes were vulnerable to rodents so I have done mine in copper which is a much bigger job than plastic.

The "official" recommendation from the pipe manufacturers is to support plastic pipe every 500mm. Its a recommendation not a rule. Use your common sense for your particular situation. Dont think swept tees will make a difference, the pressure acts in all directions. The key issue if the pipe size which you seem to have covered.

It is recommended that the TRV is on the flow side.

Not sure about your last TT question. It depends on how you have designed the pipes.

Not sure, why cant you expend the existing tails to where you can connect to the main pipes?

Thanks George. just got back from holiday so hoping to start this at the weekend. I have now read that the bi directional TRV can be on either side but best practice to put on the flow side.

 

[Kurt Stone]

You might need to move the pump from the return pipe to the flow pipe right after the 'cold feed' and 'vent pipe'. If you have a 'cold water storage tank' and a 'feed and expansion tank' (aka F&E tank). You need to do this so the pump is not drawing air into the system. In order to keep the air out you may think about an air separator etc as well.
Good luck!

The pump is already situated on the on the flow pipe.

 

[Kurt Stone]

Update on the conversion. Work completed now. Took a bit longer than I thought as some of the existing pipes took a different route to what I anticipated. Lots of floorboard lifting and one leak on a soldered joint when refilling but all done. All tested and works a treat as radiators getting much hotter far more quickly than previously. Really pleased with though not the best week to be turning the heating on!! Thanks for your help.

 

[Last Plumber]

Thanks for letting us know.