Temperature problems with large commercial hot water system

[cr0ft]

Hi all. As most of you will know I've gone back on the cards now and am now fault-finding one of our commercial hot water systems in a 5,800sq.m building. It's a large building on a hospital site.

The hot water system is an old system! Heating is run from a steam feed from the main hospital and that heats the water in 2 very large calorifiers (3000 litres each). The water is heated to 65C in these and is then pulled around the system with the standard 2x pumps (one on, one standby) on the hot water return.

The hot water flow pipework is in 54mm and quickly splits off into 6x 35mm 'risers' feeding different parts of the building (6 branches really, all pipes run through between the 2 floors). The issue is that hot water return temperatures furthest from the boiler on some risers are down to 37-42C, and there is a legionella problem in the system. No shock there! When you open hot taps in the affected areas the temperature will then rise by up to 5C before dropping back as soon as the tap is closed. In the affected areas the water temperature doesn't even get up to 50C when running the hot tap.

From having inspected two risers (they are above suspended ceilings) I see no balancing/gate valves on the main return pipework. What I do find odd though is that every individual tap has it's own tee'd off hot water return pipework. The end result is there are loads of loops for the water to follow to get back to the calorifiers on each riser.

I suspect that the hot water is following the shortest routes back to the calorifiers and there are large stagnating areas where recirculation is not occurring. I don't see the point in the hot water return pipework to individual taps (they are all within 5m or so from the main pipework) and feel it complicates the system. My plan is to remove all of them and fit automatic balancing valves on the return leg of each riser in order to properly balance the system - something like the Honeywell Kombi 4.

All the schematics I see online show taps etc connected to the hot water flow pipework only, I'm assuming connecting them all to flow and return is wrong and is going to cause this issue I have surely?

Anyone else got any thoughts? Have spoken to our commercial plumber we contract in and he agrees with me but wanted to get other people's ideas before briefing management on Friday!

Thanks a lot in advance for all advice.

 

[ShaunCorbs]

So every tap is connected to the return loop ?

It sounds like maybe someone's fitted a tmv wrong or its failed

Even with it short circuiting to the first outlet you should be getting return temp higher than that 60 dc as it's doing a loop if it's insulted

 

[cr0ft]

Yes mate almost every hot tap!

The 'primary return to the calorifiers is within 5 degrees of the 'primary flow from it which hid the problem.

As you move along the 6 return loops the temperature drops the further you get from the plant room. Some loops drop more and some less but Working on the water being lazy and taking the easiest path it's taking the shortest routes back to the plant room.

 

[cr0ft]

Should say that all the tmvs get stripped and serviced regularly. Also, the hot and cold water pressure is balanced as both are fed from boosted cold water supply. Can't see how you would get flow from one side to the other in that case? None of the hot pipes on the TMVs I've felt so far feel cold at all either!

Surely a system this size with this many return connections (circa 100 with all the taps) must need balancing?

Also is this setup normal i.e. all taps connected to return directly??

 

[ShaunCorbs]

Right I get you now

Main hot 65
Src 60

Furthest outlet 40 (ish)

Yep it's taking the easiest route

Cap all tees into on the same run except one and fit a balancing valve to that with a lever valve also lever valve on before the first hot outlet

 

[ShaunCorbs]

Should say that all the tmvs get stripped and serviced regularly. Also, the hot and cold water pressure is balanced as both are fed from boosted cold water supply. Can't see how you would get flow from one side to the other in that case? None of the hot pipes on the TMVs I've felt so far feel cold at all either!

Surely a system this size with this many return connections (circa 100 with all the taps) must need balancing?

Also is this setup normal i.e. all taps connected to return directly??

You would be surprised how many hot single checks in tmv get stuck open or shut, common occurrence if there no hot water to give the check valve a poke and it opens

 

[cr0ft]

Brill. The way I see it is all the tap return connections are pointless and I will need to get rid of them all (bar the furthest one on each leg). Ultimately I will fit balancing valves to all 6 main return runs. Guessing i need a bypass of some sort too in case they all shut off??

Whole job is complicated by it being a working hospital and no isolation valves on main flow and returns at all!!

 

[cr0ft]

You would be surprised how many hot single checks in tmv get stuck open or shut, common occurrence if there no hot water to give the check valve a poke and it opens

Yeah I understand it's quite a common problem. I would look at that in more detail where it not for the fact that we have a regular service schedule for them all.

 

[ShaunCorbs]

Brill. The way I see it is all the tap return connections are pointless and I will need to get rid of them all (bar the furthest one on each leg). Ultimately I will fit balancing valves to all 6 main return runs. Guessing i need a bypass of some sort too in case they all shut off??

Whole job is complicated by it being a working hospital and no isolation valves on main flow and returns at all!!

If on a bank off the same line yes (remove the tees as someone will have a fit with the amount of dead legs 😀

And no as they will never shut just want to regulate flow and start putting lever valves in

As soon as it tees off a main line lever valve after balancing valve lever valve (labelled and red handles normally)

 

[ShaunCorbs]

Freezer kit is handy

 

[ShaunCorbs]

First tap off each riser gets to correct temp ?

 

[snowhead]

I only work on Commercial buildings.

The only time I see no secondary circ is where there is a local Electric hot water heater and the flow to the tap is a couple of meters max.

All the return legs I see have individual balancing valves close to the tap.

Be careful removing returns and leaving 100 x 5 mtr deadlegs;

HSE guidance max 2mtrs
http://www.hse.gov.uk/legionnaires/assets/docs/hot-cold-legionella-checks.pdf

CIBSE Module 87: Ensuring safe and robust hot water flow and return pipework
DHWS deadlegs should not exceed a volume of 0.5 litres – so, for example, a 20mm cross-linked polyethylene (PEX-c) pipe (internal 15.5mm) would have a maximum allowable deadleg length of 2.6m. This requirement is driven by the need to reduce legionella bacteria, but also reduces the risk of stagnation, ensuring better water quality.

Faced with your issue I'd;

Drain the system and fit Iso valves to both flow and return on the 6 risers, then you can work on each one after individually.

Then I'd find the hottest tap returns , most likely the closest to the Cal /pump and fit balancing valves in those first and throttle them down.
You'll get the fastest improvement in temperatures rather than having to do all 100 at once.

Then work around the system finding the next hottest returns.

 

[cr0ft]

Thanks a lot. Good to get more experienced engineers opinions. I was hoping to avoid having to fit loads of automatic balancing valves on the individual tap returns due to the cost of the valves but if that's the best option then we will most likely have to go with it. System needs to meet HSE guidance and deliver water at 55C to TMVs within 1 minute.

 

[cr0ft]

Any ideas how long I could be looking at to drain and refill a system of this size?? Appreciate it's not an exact science but I'm thinking more than a day with fitting the isolation valves too!

 

[ShaunCorbs]

Any ideas how long I could be looking at to drain and refill a system of this size?? Appreciate it's not an exact science but I'm thinking more than a day with fitting the isolation valves too!

Yea you could end up a day draining esp if you have to drain any cold water tanks

If you can best to freeze main legs and fit lever valves

 

[ShaunCorbs]

Thanks a lot. Good to get more experienced engineers opinions. I was hoping to avoid having to fit loads of automatic balancing valves on the individual tap returns due to the cost of the valves but if that's the best option then we will most likely have to go with it. System needs to meet HSE guidance and deliver water at 55C to TMVs within 1 minute.

How many groups of sinks you got ?

 

[cr0ft]

About 20. I'm very limited on what I can do with pipework to try and keep costs down. What are you thinking??

I don't think mgmt will swallow buying 100 odd automatic control valves plus labour but will see what they say. May have to settle for pipe runs of up to 5m in 15 to taps to be honest. In healthcare the target is 55C at outlet or TMV within 1 minute which will be achievable with this if I can sort main flow/return temps.

 

[cr0ft]

First tap off each riser gets to correct temp ?

Yes lovely and warm. They get progressively colder the further along each riser you go.

 

[cr0ft]

Out of curiosity how will I fit lever isolation valves to 54mm copper if there's no give in the pipe? I can get a pipe freezer no probs and leave hot water off for a day (and overnight to cool).

As you can guess, this is a step up for me, it's like re-learning plumbing all over again tbh!!

 

[ShaunCorbs]

About 20. I'm very limited on what I can do with pipework to try and keep costs down. What are you thinking??

I don't think mgmt will swallow buying 100 odd automatic control valves plus labour but will see what they say. May have to settle for pipe runs of up to 5m in 15 to taps to be honest. In healthcare the target is 55C at outlet or TMV within 1 minute which will be achievable with this if I can sort main flow/return temps.

Each group do you think they will all have hot water under 30 secs if yes you only need 20 balancing valves

 

[ShaunCorbs]

Out of curiosity how will I fit lever isolation valves to 54mm copper if there's no give in the pipe? I can get a pipe freezer no probs and leave hot water off for a day (and overnight to cool).

As you can guess, this is a step up for me, it's like re-learning plumbing all over again tbh!!

Press fit or compression and slip coupling

 

[cr0ft]

Sorry see what you mean. Each group will be up to 55C in under a minute without the little returns to them.

 

[cr0ft]

Compression slip coupling will work a treat, plenty of pipe length to play with. Will hire a pipe freezer in and probably arrange to do it overnight on OT. Simplest way!

 

[ShaunCorbs]

Go for that you can do one group at a time to spread the hit if they want

Would do a test one first to check

 

[cr0ft]

Yup will recommend fitting isolation valves to all risers in one shift first then and may as well fit a balancing valve to returns on each at the same time.

Then I can work on a section at a time which will make it a lot more realistic.