Secondary return cools cylinder in 2 hours

[Ben Adams]

I'm a plumbing novice so first of all I apologise if I'm not explaining this situation correctly. I've recently renovated a house for my family to live in and we're having an issue with water not staying hot when the secondary return circuit is on. From a starting point of fully heated the cylinder is usually cold again within two hours of running the secondary return. When the secondary return pump is switched off the water will stay hot in the cylinder for maybe 8 hours, depending on usage. The plumber who installed the system (who has now left the job and won't return) never insulated a single pipe on the secondary return, in fact he didn't lag a single pipe in the entire house (hot water and heating). Is it possible the secondary return could cool the cylinder that quickly due to heat loss through the pipes? The boiler and cylinder are fitted in an uninsulated loft and so there is exposed pipe work up there before the pipes feed down one floor into the main envelope of the house. I've done some searching on the internet and it looks like most pipes around the cylinder and boiler are usually insulated and so I'm wondering if I should get up there and start lagging the pipe work myself, as my bills are excruciating at the moment. One other potential cause a friend suggested was that there may be issues with how a non-return valve is set up, and so cold water could be unintentionally entering the cylinder as a result. I'm really at a loss, so any suggestions would be much appreciated. I can post pictures of the setup if necessary! Thanks

 

[polesapart]

Secondary returns demand high quality insulation.

CBA to check regs but surface area/heat loss of an extended pipe run make it a no-brainer.

 

[Bod ...]

Can you send a photo of the Cylinder and its pipework

 

[oldbutnotdead]

Re the back check (non return) valve - - it should be in the cold water feed to the cylinder before the return T. Is this gravity or unvented by the way- if it's unvented then you are limited to what you can DIY.
Pipe lagging- yes do it, the thicker the better in the loft. Def do the dhw feed and return everywhere and the heating pipes if they're in ground floor (cold) void. In the first floor (warm) void lag the heating pipes if you can.
You might want to look at frost protection on the boiler if it's in the loft....

 

[Ben Adams]

Thank you all very much for the responses. So it sounds like I at least need to get up there and lag what I can in terms of pipework. The problem I have though is that I can only now lag pipework in the loft as the rest of the house is sealed up (and in fact decorated!). Basically the pipes run down the house from the loft through battened out cavities either side of the chimney breasts. Although as my gas bills are excruciating it may payoff in the log run to ruin the decorating and open up voids again!

I've uploaded some photos here if that helps shed some light at all:

https://imgur.com/a/U5KNNmK

And I hadn't even thought of frost protection for the boiler if it's in the loft. I will have to look into this. Unsurprisingly the plumber didn't mention this either!

Many thanks

Ben

 

[Harry Bloomfield]

And I hadn't even thought of frost protection for the boiler if it's in the loft.

Most modern boilers have built-in frost protection.

Regarding the secondary pump, do you run it 24/7? Do you need to run it constantly? You could save a lot of money, by running it on a time-clock, so it is only running when essential. It looks, in the photo, as if it plugs into a 13amp socket, in which case you could run it via a plug-in time-clock.

 

[Ben Adams]

Hi Harry, thanks for responding. So we did indeed run the pump via a smart plug, allowing us to switch it on during peak times of the day. But if that peak time of the day was say 2 hours long (morning showers and breakfast time) we would end up with a cold tank by the end of the period. Handy to know that most modern boilers have built-in frost protection. I will have a google and check ours. This is another slight issue we have, he installed a Wiessmann Vitodens 200-W which by all accounts is a great boiler but he never really showed us how to set it up properly. It's a DHW priority boiler apparently, so it also means that when we are heating the cylinder (which we've had to do a lot historically until we turned the pump off completely) we have no central heating.

Many thanks

Ben

 

[Ben Adams]

Re the back check (non return) valve - - it should be in the cold water feed to the cylinder before the return T. Is this gravity or unvented by the way- if it's unvented then you are limited to what you can DIY.
Pipe lagging- yes do it, the thicker the better in the loft. Def do the dhw feed and return everywhere and the heating pipes if they're in ground floor (cold) void. In the first floor (warm) void lag the heating pipes if you can.
You might want to look at frost protection on the boiler if it's in the loft....

Thanks for this oldbutnotdead. I will try and get back up into the loft tonight when I finish work and check the non-return valve.

Ben

 

[Harry Bloomfield]

It's a DHW priority boiler apparently, so it also means that when we are heating the cylinder (which we've had to do a lot historically until we turned the pump off completely) we have no central heating.

The pipes should have been insulated, but at this time of year, the heat lost from the circulating water will not be wasted, it will be going into your home. In summer it will be wasted.

It is normal now for a boiler to serve either the CH or the HW, one at a time, because the boiler flow temperatures needed for each are different - much higher for stored HW. My system also works like that, but in practice it is not a problem - my house fabric stores and gives up enough heat whilst the boiler is busy serving the HW.

 

[Justin Passing]

Apart from several tonnes of insulation,

You could also use
A thermostat on the pipe so it only runs when the water is cool enough, or
proximity sensors in the rooms where the taps are that you care about, to run the 2ndry pump (It probably only takes a matter of seconds to circulate the water?)

By all means get the good insulation (thick walled) for most of the pipes. but if you have an old sleeping bag or carpet that could be used too.

Hot tap water pipes in houses usually aren't insulated if between floors, or habitable spaces, though it's advised.

Heat loss/m from a 20mm Cu pipe with 10mm insulation and Δt 60° is around 30W but with lotsofinsulation (silly, like 80mm) it goes down to 7 or so. Heck of a lot, in the loft especially, compared with say a 5W led!
WIth no insulation, the multipllier depends on the air flow (hence the carpet laid on top, etc. for fittings etc which are hard to insulate.)

Iirc it's inversely prop to the log of the thickness. So loss drops by 35% each time you double the insln..
The thin stuff is only usable for frost protection.

 

[Ben Adams]

Thanks Harry and Justin, appreciate the input. I have a plumber coming over to take a look tomorrow, kinda dreading what he's going to make of it all. Hopefully we can get pipes insulated in the loft at least.

Oldbutnotdead, I couldn't see a back check valve on the cold water feed, could you see one in the pictures at all? Is it the part with the blue lever on the pipe labelled 'mains' (in the picture labelled 'LHS cylinder connections'?

Many thanks

Ben

 

[oldbutnotdead]

Thanks Harry and Justin, appreciate the input. I have a plumber coming over to take a look tomorrow, kinda dreading what he's going to make of it all. Hopefully we can get pipes insulated in the loft at least.

Oldbutnotdead, I couldn't see a back check valve on the cold water feed, could you see one in the pictures at all? Is it the part with the blue lever on the pipe labelled 'mains' (in the picture labelled 'LHS cylinder connections'?

Many thanks

Ben

Cant see one in the cold feed but there appears to be one in the pic RHS Cylinder connections (in the pipe going into a boss marked .... Return). Which won't achieve the desired result on its own.....

 

[Ben Adams]

Cant see one in the cold feed but there appears to be one in the pic RHS Cylinder connections (in the pipe going into a boss marked .... Return). Which won't achieve the desired result on its own.....

Ah that's the secondary return pipe you can see there. So it should also have one on the cold water feed as well as the secondary return there? Thanks. Ben

 

[oldbutnotdead]

That check valve will prevent water running up the return to the tap (which may affect delivery temperature if the return is going into the cool end of the cylinder).
The check valve in the cold feed is to prevent hot water being circulated back up the cold pipework. There's a very similar thread to this one around a week ago- have a read, tis interesting.