Hot Water Circulating Pumps - cooling my hot water?

[baxb]

Moved into a new (to me) house a few months ago, getting to grips with things, but have noticed from early on that the hot water is lukewarm after 3/4 hours regardless of how much we have used. There are only 2 of us here most of the time & we have a 170L tank, I was home alone last Friday night, got up Saturday, did a few jobs & went to shower at 10am ish & only tepid water there - no other hot water had been used up to that point.

The system was installed in 2019 & it has a Grundfoss UPS3 pump. If this is constantly pumping water around the house (4 bedrooms, 1 x ensuite, 1 x bathroom, 1 x downstairs loo, 1 x utility) is this going to cool my water that quickly? The water is on 5-6am & 5-6pm - same as our last house with 150L tank & that always left us with plenty.

I get the point of the circulator pump, but it still takes 60+ seconds for my ensuite shower to warm up & surely its better to just waste a bit of cold water than having 170L of 60 degree water cooling down to 35 degrees after 3-4 hours?

Am i missing something here? or are these pumps only really useful if you have the water on for 6+ hours per day?

[Mr Pointy]

Why don't you put the hot water on permanent run rather than just for two hours a day & then the cylinder stat will fire the boiler when the water cools. Next make sure the hot water pipes are well insulated & finally put the hot water pump on a timer so it only runs when you are likely to be at home.

[baxb]

Mr Pointy said:

Why don't you put the hot water on permanent run rather than just for two hours a day & then the cylinder stat will fire the boiler when the water cools. Next make sure the hot water pipes are well insulated & finally put the hot water pump on a timer so it only runs when you are likely to be at home.

Surely having it on 24/7 is way less efficient than just having it on 2 hours per day? We aren't using all 170L or anything like it twice per day so it should just be bringing it back up to temp? I guess what I'm trying to work out is a) is it the circulator pump that's definitely cooling down the water & b) is there any point in having one for our situation where we don't have a house full & multiple showers/baths per day?

[dozen]

I have my hot water on 24/7. The cylinder is well insulated and it means there is always hot water.
My hot return pump runs for 5 minutes every hour and, with well insulated pipes, the water is instantly hot from all the taps.

[outnumbered]

The timers on the Grundfoss pumps are pretty coarse, and I found the same issue that you have, just running for the minimum period per hour.

I'm now controlling the pump from a smart plug, and running it for 2 times 3 minutes an hour during the day is enough to allow fairly quick hot water.

Without the pump it takes 30+ seconds to get warm.

[baxb]

I previously had the hot water on for two hours a day & that was enough to mean I also always had hot water. Now I don't & it seems the circulator pump is the difference. I'm trying to work out is that the case?

[LocoBlade]

Running hot water through pipework constantly will certainly cool it down over a few hours particularly if the entire outward and return circuit isn't fully insulated, so you need to add an element of control to it to make it efficient.

The simple way to do that is on a timer to run at peak usage times in the morning / evening if you have regular usage patterns, the next step is what we've got which is a smart Grundfoss pump with sensor on the outlet pipe on the tank that's supposed to learn your hot water usage habits then ran only in those periods without the need for a manual schedule.

In reality that didn't work that well for us, so I put it back into dumb pump mode then installed a smart switch to control the pump instead, then installed some cheap Sonoff motion sensors (about £12 each) above the door in the bathrooms/toilets connected to Home Assistant, so when motion is detected in those rooms an automation triggers the pump to run for 3 minutes then stop again. That works really well because you generally go in and use the loo or get undressed first before needing hot water so its ready by the time you need it and it's not circulating water at times you don't need it. I don't have a motion sensor by the kitchen tap because it would be triggered too much from general occupancy, our kitchen is pretty close to the tank anyway so water runs hot quite quickly anyway, but you could install a Sonoff button by the sink if required to manually trigger it

If your hot tap is still taking 60s to run hot even with the pump running though, the pump and/or recirculation circuit isn't working very well. I suspect you'll probably find hot water is reaching some outlets and not others as the water is finding the path of least resistance and not pulling water round the entire circuit. If that's the case you may need to do what I had to and install some ball valves in the return circuits under the floorboards so you can balance them like you do radiators.

Edited by LocoBlade on Thursday 17th October 19:13

[Mr Pointy]

baxb said:

I previously had the hot water on for two hours a day & that was enough to mean I also always had hot water. Now I don't & it seems the circulator pump is the difference. I'm trying to work out is that the case?

You're undoubtedly losing heat somewhere so is the cylinder really well insulated? If it is you can easily tell if it's losing it in the pipework by turning the pump off & seeing if you get (delayed) hot water all day.

At the end of the day 99.9% of houses don't have recirculation so you could just decide you don't want the extra cost of topping up the hot water during the day.

Presuambly the hot water piping is done in copper as I don't think plastic pipe is allowed for recirculation systems, so pipe insulation is really important as well.

[No ideas for a name]

baxb said:

I previously had the hot water on for two hours a day & that was enough to mean I also always had hot water. Now I don't & it seems the circulator pump is the difference. I'm trying to work out is that the case?

Just try it without the circulator for a day or two and compare your result.

The circulator might be doing thing that look 'odd' to the user.. It most likely de-stratifies the tank. So, whereas before you had the top half hot which the taps then use, but now it is feeding back that hot back (maybe) to the bottom of the tank. You might have a fully mixed tank of luke warm water.
Depends where the return feed enters the system.

IANAP

[baxb]

Thanks all,

The tank is in the garage, a modern Joule one, so I guess well insulated, the pipework is copper & what's visible is insulated, there's about a 50ft run from the tank to En Suite shower so a reasonable length to loose heat in.

The pump is not on its own plug, but wired into the the boiler power but easy enough to disconnect, I'll try that for a day or so & see how it goes. Alternatively it would be easy enough to put it on a smart plug & set up a routine for morning & evening use.

[Familymad]

We have a gold pump doing just this. I re wired it so we can tap on an app and fire it up as it was just bleeding all our hot water out.

[caziques]

Circulating hot water systems are the work of the devil, always energy hungry.

[Sheepshanks]

baxb said:

Thanks all,

The tank is in the garage, a modern Joule one, so I guess well insulated, the pipework is copper & what's visible is insulated, there's about a 50ft run from the tank to En Suite shower so a reasonable length to loose heat in.

Was the “60+” seconds a figure of speech or have you timed it?

Our house isn’t big, but I’d estimate it’s 50ft of pipe run from the top of the tank to the downstairs shower room shower head. No circ pump, and, once out of the tank cupboard, plumbed in plastic. It’s a rainfall head, so there’s a fair bit of water coming through. I haven’t timed it, because it’s never been an issue, but it takes a few seconds for the shower to get hot.

[PRO5T]

Locoblade that’s a cracking solution, well done.

Ours takes on average two mins for the en suite shower to heat up-I know as it’s the same time as the electric toothbrush to buzz off!

I’ve thought about upgrading to a circulation pump but I think it’ll be simpler to install a second cylinder at the other side of the house with new boiler feeds when I rebuild the master suite.

[PRO5T]

Locoblade that’s a cracking solution, well done.

Ours takes on average two mins for the en suite shower to heat up-I know as it’s the same time as the electric toothbrush to buzz off!

I’ve thought about upgrading to a circulation pump but I think it’ll be simpler to install a second cylinder at the other side of the house with new boiler feeds when I rebuild the master suite.

[eliot]

caziques said:

Circulating hot water systems are the work of the devil, always energy hungry.

Had one for 12 years and it’s the best feature I’ve ever had in a house.
However, I have full home automation as well. Every room has a hard wired PIR sensor for the house alarm, the alarm panel has a link into the home automation.
I have events setup for rooms that lead to hot water (bedrooms, kitchen) that runs the recirc pump for one minute and not to run it again until 20 minutes has elapsed.

I can certainly see how not having such good control would result in your tank going cold - because you hot water pipework just turns into a radiator.

Of course having motion detection in all rooms has many other benefits, the obvious one is lights that follow you round the house, but less obvious is automatically setting the house alarm when motion hasn’t been detected for a certain period and the last door movement was the front door.

Then if the HA knows if the house is unoccupied, it also knows that after two hours there shouldn’t be any water flow - so if there is it will send me a message alerting to a potential leak

[baxb]

Sheepshanks said:

Was the “60+” seconds a figure of speech or have you timed it?

Our house isn’t big, but I’d estimate it’s 50ft of pipe run from the top of the tank to the downstairs shower room shower head. No circ pump, and, once out of the tank cupboard, plumbed in plastic. It’s a rainfall head, so there’s a fair bit of water coming through. I haven’t timed it, because it’s never been an issue, but it takes a few seconds for the shower to get hot.

Not timed it, but based on 2 min electric toothbrush cycle it's at least halfway through before anything warm comes out. I'll try a few things (including disconnecting it) based on the feedback above & report back.
Thanks all.

[PhilboSE]

The correct name for a recirculating system is a secondary loop. Basically heat loss is inevitable so all you can do is insulate the system as much as possible and have large bore pipes (my primaries are 35mm, with 22mm loops to rooms and 15mm feeds to outlets). Then you need a control system, we have a timer for the pump on the secondary loop but there’s always someone at home so it basically runs 06:00-23:00. A motion based system just wouldn’t work for us, the pipe runs are so long.

[Sheepshanks]

PRO5T said:

Ours takes on average two mins for the en suite shower to heat up-I know as it’s the same time as the electric toothbrush to buzz off!

How long does it take for hot water to appear from the sink hot tap?

[ATG]

Tanks are thermally efficient because of their shape. They have a small surface area relative to their volume. Pipes are the exact opposite. The radiator in your car is trying to get heat out of the water and into the air. What's it made of? Lots of tiddly little pipes.

So ... as the man says above ... if you are going to circulate domestic hot water, large bore pipes to improve that surface area to volume ratio and then loads and loads of insulation. That insulation is all you've got to compensate for the crappy geometry.

I grew up in a house that had a sort of hot water inner ring road; a loop of really large bore pipe from the top of the hot water tank in the basement up to the top floor and then back down to the bottom of the tank. It was lagged in fibreglass, wrapped in pvc and tied off with string. Taps were fed off spurs. No pump, the system worked by convection, which works well when the loop is narrow and tall. It certainly held heat overnight without any problem.

Edited by ATG on Friday 18th October 09:04