Rads slow to heat up conundrum

[SimonD]

Friends of mine live in what was originally a bungalow but has been extended several times over the years. Each time the heating system has been added to. Ever since they've lived there, they've had problems with two rads in the annex never heating up. They've had several plumbers round to sort things costing them money and never resolving the problem. I kind of quite enjoy these problems so said I'd go over and have a look and also crawled into the undercroft to look at the pipework there.

 

- there's about 15 rads on the system and the two problematic rads are at one side and end of the house.

- the rest of the house gets nice and warm quickly

- 30kW heat only boiler running sealed ch system

- circulator is a DAB Evosta 40-70/130 on proportional differential pressure setting 6

- pipework under the floor in the undercroft is a terrible mess

- the two problematic rads are on a 15mm loop, that looks like it includes another two rads, probably giving a total flow and return pipe length of 24 meters for those rads

 

When I started up the boiler, I put it into service mode max and even with the whole system cold, it would cycle rapidly. When I put the system as normal it would run fine and eventually, after about an hour finally reached 70C flow and 50C return - at all times during heat up it ran at 20C differential temp.

 

I fiddled with both lock shield and trvs and the two rads eventually got hot but this was after probably 1.5 hours of running the heating on full pelt.

 

Clearly there is a flow problem here.

 

My thinking.

 

I think there's too much pressure drop along this leg of the system and some of the other pipework has long legs of 15mm suggesting the total head on the system is pretty high.

 

So questions:

- do we chuck in a more powerful circulator pump. The curve on the DAB is pretty rubbish in terms of drop in head even at normal ch circulation flow rates. So we could go for a better domestic pump, like from Grundfos or Wilo that could deal with decent head up to 7.5m, or alternatively chuck in a light commercial pump for up to 9m head?

- do we get under the undercroft and modify the pipework. There's a section of accessible pipework in the undercroft feeding two rads that do get hot. As it's so accessible my thought was this could be upsized to 22mm and we could then tee in the troublesome rads to this loop on 22mm pipe? (there are some dead legs here already suggesting that's how the old system worked before some building work was done and radiators were moved)

- they've said plummers have been round a tried to balance the system, but is it worth us trying this first?

 

Budget's limited so hesitant to just chucking in an expensive circulating pump without having a good sense of whether it might work, but is that the most sensible solution?

 

I'd appreciate any suggestions here.

Edited October 18, 2022 by SimonD

[SteamyTea]

Would a cheap, secondary pump work on the feed to the troublesome radiators.

[SimonD]

11 minutes ago, SteamyTea said:

Would a cheap, secondary pump work on the feed to the troublesome radiators.

 

It had crossed my mind but set it aside due to finding a space to fit the additional pump and then running the control wires across to the other side of the house for the boiler - there are parts of the undercroft I can't get at due to sleeper walls so couldn't just run it all under the floor.

[Temp]

1 hour ago, SimonD said:

I fiddled with both lock shield and trvs and the two rads eventually got hot but this was after probably 1.5 hours of running the heating on full pelt.

 

It sounds like the rest of the house warmed up and some TRV started to close providing more flow for the problem rads. That does sound like a balancing issue when everything is wide open. I suspect a bigger pump might not solve the issue.

 

[TonyT]

Where is the boiler in relation to the problem areas?

 

can you open the lock shield on the problem radiators more and restrict the roads closer to the boiler?

 

or as an experiment, turn a few radiators that are known to be working off to see if the problem radiators get heat.

 

[SteamyTea]

Can you get flow gauges that fit on a radiator?

[SimonD]

4 hours ago, Temp said:

That does sound like a balancing issue when everything is wide open. I suspect a bigger pump might not solve the issue.

 

2 hours ago, TonyT said:

or as an experiment, turn a few radiators that are known to be working off to see if the problem radiators get heat.

 

My friends said the previous plumbers had tried this but  it seems to be the biggest question and I'm less inclined to believe it has been done. However given how well set the flow and return temps are, somebody who knows what they're doing has had their hands on the system.

 

My biggest worry about the circulator is that a rough calc of the pressure drop across this problem loop comes to almost 1.5m head. There's another loop of similar length that feeds 2 rads which my estimate comes to about 1.1-1.2m head. Taking into consideration pressure loss through the boiler, there's not going to be a lot left for the rest of the system. the DAB circulator manages less than 6m at 0.5m3/h which is about where it needs to be flow wise I reckon.

 

2 hours ago, TonyT said:

Where is the boiler in relation to the problem areas?

 

It's the opposite end of the house, but not as far away as the upstairs rads which work fine.

 

2 hours ago, TonyT said:

can you open the lock shield on the problem radiators more and restrict the roads closer to the boiler?

 

I've open the problem rads up fully and unscrewed the trv mechanism so there's absolutely no pressure on the pins. Haven't restricted the others on the same loop yet as only found that out yesterday when crawling under the floor - before it was assumed they were connected to an entirely different loop.

 

2 hours ago, SteamyTea said:

Can you get flow gauges that fit on a radiator?

 

Not that I know of. Some of the newer pumps will display useful data, either energy consumption which can be correlated with the pump chart, or pressure and flow rates. Grundfos pumps allow connection to mobile app for balancing too.

 

[SteamyTea]

5 minutes ago, SimonD said:

3 hours ago, SteamyTea said:

Can you get flow gauges that fit on a radiator?

 

Not that I know of

Strange as you can get them on manifold systems. 

Would be a useful tool.

 

In the olden days when cars had carburettors, they could be balanced by listen to the intake noise.

Can you do something similar.

[SimonD]

20 hours ago, SteamyTea said:

Strange as you can get them on manifold systems. 

Would be a useful tool.

 

It would be so handy if it was standard practise to install some test tee in strategic places for connection into the system with a guage, wouldn't it. More common I think on commercial installations.

 

20 hours ago, SteamyTea said:

In the olden days when cars had carburettors, they could be balanced by listen to the intake noise.

Can you do something similar.

 

No, I can't. My ears are still ringing from the years of trying to do that on racing motorbikes.... manometer was always better

 

[SteamyTea]

54 minutes ago, SimonD said:

My ears are still ringing from the years of trying to do that on racing motorbikes

Should have stuck with 4-strokes.  2-strokes start off sounding broken.

[SimonD]

1 hour ago, SteamyTea said:

Should have stuck with 4-strokes.  2-strokes start off sounding broken.

 

Only ever had one 2-stroke. The others were v twin, v4, straight 4, straight triple and a few big single cylinder 4-stroke thumpers - it's the single thumpers that were the worst out of all the lot but most fun to ride and tune, except for how much they went pop!

[Nickfromwales]

My mate had an Armstrong 399, 2 stroke single, left hand kick. At 17, I weighed in at around 7 stone soaking wet. I kicked the bike, it kicked me back......ouch. Thank goodness for my Axo's or it would have broken my leg I recon