New heating system - Hot water pump poor performance

davkav

A couple of months ago my mother had a new heating system installed.

The house is a semi-D with main bathroom and en-suite upstairs.

The original gas boiler was replaced with a Worcester Bosch combi boiler.
The old hot water cylinder was removed and the existing Stuart Turner Monsoon 1.5 bar pump re-plumbed to serve hot and cold to the showers and taps (kitchen cold tap still on mains).

Since the new heating system installation, when the shower is running the other taps off the pump will not flow. She contacted the heating system installer and stated that surely two showers should be able to work in parallel and if not they should have taken that into account when spec'ing the new system. They just fobbed her off stating that is how these systems work.

Could this be a pump performance issue or is the pump undersized for the new system?

Sleeper12

Combi should not be connected to the mains. Combi & cold water should be Pumped from the cold water tank. Maybe this is the way they did it but a 1.5 bar pump is barely good enough to run one shower. I wouldn't expect brilliant pressure from this even with only one shower.

davkav

Sleeper12 wrote: »

Combi should not be connected to the mains. Combi & cold water should be Pumped from the cold water tank. Maybe this is the way they did it but a 1.5 bar pump is barely good enough to run one shower. I wouldn't expect brilliant pressure from this even with only one shower.

Combi supplied by pump and cold water tank alright.

Reading the stuart turner website it states "Designed for installation into vented systems to pump both hot & cold water supplies, boosting both equally to an individual shower, complete bathroom, or whole house. Suitable for positive head applications only."

But I had only read over the 1.5bar pump page. Looking at the other pump webpages, the same spiel is written for each.

link

What size pump would be the norm for supply of hot and cold in a standard semi-D?

K.Flyer

It can be a minefield specifying a Combi Gas Boiler there is so much to take into consideration particularly the expectations of the home owner versus their budget.
I would like to see a picture of how the pump has been reconfigured for the new installation as a typical shower pump is not intended for that type of set up.
To answer one of your questions, I would always suggest a pump with a working pressure of 3 Bar for a Combi boiler set up utilising the cold water storage tank for the supply to the pump.

John.G

davkav wrote: »

A couple of months ago my mother had a new heating system installed.

The house is a semi-D with main bathroom and en-suite upstairs.

The original gas boiler was replaced with a Worcester Bosch combi boiler.
The old hot water cylinder was removed and the existing Stuart Turner Monsoon 1.5 bar pump re-plumbed to serve hot and cold to the showers and taps (kitchen cold tap still on mains).

Since the new heating system installation, when the shower is running the other taps off the pump will not flow. She contacted the heating system installer and stated that surely two showers should be able to work in parallel and if not they should have taken that into account when spec'ing the new system. They just fobbed her off stating that is how these systems work.

Could this be a pump performance issue or is the pump undersized for the new system?

A query first. is this a single or twin pump, (pumping cold only or hot and cold)
If the HW cylinder was permanently removed the obviously a single?.

davkav

Thanks for the replies.

I'll try and get some pictures later of the pipework. It is definitely a twin pump. As far as I know the cold is tank > pump > tap, and the hot is tank > pump > boiler > tap.

John.G

When the shower is on, are both cold and hot ensuite/bathroom taps not flowing?
What is power output (KW) of the boiler?.
Have you still a hot water cylinder supplying the other hot taps? by gravity or pumped?.

davkav

Ok a bit more investigative work done.

The boiler fitted is an Ideal Logic Combi C24ie with 24W output. The house is a standard layout 4 bed semi, with sun room extension at the back.

I was slightly wrong with the pump configuration. The pump is connected to the hot for all taps/showers, and is configured tank > boiler > pump > taps.
As for the cold output of the pump, this is only connected to the en-suite shower.

When the hot tap is on in the ensuite, and the hot tap turned on in the bathroom, the pressure drops to half in the ensuite. I suppose this is to be expected BUT the pressure is almost too low when this happens.

John.G

A photo of the pump connections would help.

You say "The pump is connected to the hot for all taps/showers, and is configured tank > boiler > pump > taps" so this implies that the supply to the "hot" side of the pump is also taken from the cold water storage tank (as is the supply to the cold side of the pump.) If so, then if both sides are supplied from one tank outlet, the pump performance may be affected as there should be two separate supplies from the tank to the pump, one to each side, (maybe there is).

A 24kw combi will supply just under 11LPM, (10.75), at 40C (from water at 8C) and if all this water is supplied from one side of the pump, no mixing, then the pump head should be 1.3 bar and even if the flowrate (at very reduced temperature) was 20 LPM the pump can deliver this at 1 bar.
Don't know if you can set the HW outlet temperature on that combi or not but if so and it was set to 60C then the pump will only have to provide 6.6 LPM from one end and 4.2LPM from the other end and should result in a pump pressure ~ 1.8 bar.

I know a 1.5bar pump is too low a head for two showers but this is purely academic as the boiler has only enough thermal power to supply one at the time.

I may be misinterpreting your description, is all your hot water now supplied from the combi or do you still have a (new) HW cylinder?.

davkav

John.G wrote: »

A photo of the pump connections would help.

You say "The pump is connected to the hot for all taps/showers, and is configured tank > boiler > pump > taps" so this implies that the supply to the "hot" side of the pump is also taken from the cold water storage tank (as is the supply to the cold side of the pump.) If so, then if both sides are supplied from one tank outlet, the pump performance may be affected as there should be two separate supplies from the tank to the pump, one to each side, (maybe there is).

A 24kw combi will supply just under 11LPM, (10.75), at 40C (from water at 8C) and if all this water is supplied from one side of the pump, no mixing, then the pump head should be 1.3 bar and even if the flowrate (at very reduced temperature) was 20 LPM the pump can deliver this at 1 bar.
Don't know if you can set the HW outlet temperature on that combi or not but if so and it was set to 60C then the pump will only have to provide 6.6 LPM from one end and 4.2LPM from the other end and should result in a pump pressure ~ 1.8 bar.

I know a 1.5bar pump is too low a head for two showers but this is purely academic as the boiler has only enough thermal power to supply one at the time.

I may be misinterpreting your description, is all your hot water now supplied from the combi or do you still have a (new) HW cylinder?.

Thanks John.

Hot water cylinder has been removed, so all hot taps are supplied by the combi boiler.

I'd imagine the pump has two separate feeds from the tank. The original cold water feed that was in place for the cold taps, and the original feed to the hot water cylinder has been repurposed as feed to boiler for hot water supply.

I'll get some pics up later when I call around to her.

To make matters worse the monsoon 1.5bar pump is now acting up, staying on, not coming on etc. I wonder is it being over worked. It's probably 15 years old at this stage.

John.G

Don't know if the installers were aware or not but the only way to get the two showers in parallel operation at ~ 40C is to fit flow restrictors in the shower heads which will then restrict the flow to ~ 5 LPM each irrespective of whether one or two showers in operation.
Obviously if the pump needs replacing then a 3 bar would be recommended.
When you call around, may be worth just removing the pump inlet strainers to prove clear, the pump itself may only require a flow switch, 3 bar replacement pumps are not cheap.

davkav

Ok so I'm over at the house now.

Configuration is as follows hot water side of pump has its own feed from tank and is pumped down to the boiler. All hot taps/showers therefore pumped.

The cold side of the pump has its own feed from the tank but only pumps to the tap and shower in the ensuite.

Photos attached. Left side of pump is hot.

John.G

That looks like 1/2" piping on the suction/discharge, 3/4" would be more suitable on the suction, discharge maybe OK if not over say 2 meters but anything more than this reduces the effective pump head, have you cleaned the strainers?.

davkav

Just cleaned the strainers there.

Hot side was fairly clean.
Cold side was nearly blocked with limescale.

Going to plumb the pump back in now so we'll see what happens. Not holding my breath though....

I had only just commented to the mother about how it doesn't make sense to have 3/4" inlet/outlet reduced to 1/2" on the pump....

John.G

You could also check combi HW temperature setting.

Should also have suggested measuring the flowrates from inlet hoses when disconnected for strainer cleaning.

davkav

Thanks for all of the help John, it is very much appreciated.

So the pump is connected back up but the pressure is no better.

There is a fault with the pump however. It is intermittently working. I can hear the relay click when the flow sensors detect flow but the pump does not turn on. A tap of a screwdriver on the side and the pump kicks in. So I would say the motor itself is worn.

When working the pressure is OK but could be alot better. I would say bumping the pump output up to 3 bar would make a drastic change.

I'm in need of a pump myself for one shower in our house, so I may take this 1.5 bar off her and strip the motor down and inspect.

I have a few questions now that I have had a good look at the setup:

• What is the ideal way to fit the pump. Before or after the combi boiler? If it should be ideally located downstream of the combi boiler, I should be able to pull another length of pipe from the combi boiler downstairs to the hot press upstairs. This length of pipe would be from the hot outlet of the boiler and feed into the pump then. The original hot water outlet that would have been used for the hot water cylinder can then be connected to outlet of pump.
• If only hot is being pumped to all taps except the ensuite, is this putting too much effort on the pump with the cold not flowing?
• What would the ideal pump size be? 3bar, 4bar?

John.G

You can't install the pump after the combi as the head loss through its heat exchanger might cause the pump to cavitate and pump even less.
Using only one end of the pump I was told doesn't damage the other side (if brass) but I wouldn't be too happy doing it as the seal might fail due to the heat generated.
A 3 bar pump IMO should be quite adequate at it will have a 2bar head pumping 20 LPM vs the 1.5bar's 1bar head at 20 LPM.

I would first measure the pump suction line flow rates and even if adequate at say 20 LPM I would still replace the 1/2" with 3/4" to the pump suctions.

K.Flyer

davkav wrote: »

Thanks for all of the help John, it is very much appreciated.

So the pump is connected back up but the pressure is no better.

There is a fault with the pump however. It is intermittently working. I can hear the relay click when the flow sensors detect flow but the pump does not turn on. A tap of a screwdriver on the side and the pump kicks in. So I would say the motor itself is worn.

When working the pressure is OK but could be alot better. I would say bumping the pump output up to 3 bar would make a drastic change.

I'm in need of a pump myself for one shower in our house, so I may take this 1.5 bar off her and strip the motor down and inspect.

I have a few questions now that I have had a good look at the setup:

• What is the ideal way to fit the pump. Before or after the combi boiler? If it should be ideally located downstream of the combi boiler, I should be able to pull another length of pipe from the combi boiler downstairs to the hot press upstairs. This length of pipe would be from the hot outlet of the boiler and feed into the pump then. The original hot water outlet that would have been used for the hot water cylinder can then be connected to outlet of pump.
• If only hot is being pumped to all taps except the ensuite, is this putting too much effort on the pump with the cold not flowing?
• What would the ideal pump size be? 3bar, 4bar?

The entire configuration is wrong and the 1/2" pipework is doing you no favours at all.
1.5 bar pump for what you are doing is woefully inadequate especially with the state of that pipework.
If the pump is not starting, but there is a faint click, change the PCB on it. The relay is going on it. Change the capacitor as well while your at it.
If you want to do it right I would suggest installing a pump similar to the walrus TQ 400 or 800, preferably with a speed controller, plenty of controlled water volume enough to do what you want.

John.G

Wonder how did that same pump perform with the HW cylinder?, the two differences that I see is the added head loss through the combi heat exchanger and there was more balance between the hot and cold volumes with the hot end supplying ~ 60% & the cold end 40%.

K.Flyer

John.G wrote: »

Wonder how did that same pump perform with the HW cylinder?, the two differences that I see is the added head loss through the combi heat exchanger and there was more balance between the hot and cold volumes with the hot end supplying ~ 60% & the cold end 40%.

The pump would have run far more efficiently in its original set up being directly fed with 3/4" pipework and the pipework from the pump also being 3/4 almost to the taps or shower valves.
Now that it's been fed by 1/2" and what it appears to be 1/2" from the pump to the boiler and then probably back to the hot press to the original hot water connection the pump is struggling to deliver any decent flow rate after that keeping in mind the loss of internal diameter of pex.
It all needs to be pulled out and redone properly otherwise it's always going to be disappointing.

davkav

Thanks guys.

All the copper pipes in the photo are 3/4" with the exception of the CH fill being 1/2".

There is no hot water from boiler back to hot press.
The combi boiler is downstairs in the utility press with the washing machine. So I reckon the hot outlet from the combi boiler is tapped into the hot at the kitchen sink...

From the Ideal Logic manual, the Domestic Hot Water (DHW) inlet and outlet are 15mm.

I'm curious to what the ideal configuration should be now.


The tank feed to the hot side of the pump I can plumb as 3/4" as can be seen in the photo.
If it makes sense I can pull a length of 3/4" from outlet of pump to DHW inlet of boiler.
If I am going to that hassle should I pull a second length of 3/4" and connect to DHW outlet and into the old 3/4 hot water feed in the hot press. I'll disconnect and blank wherever the installer has connected the DHW outlet downstairs.
As the cold side pump is supplying only the shower in the ensuite and its 1/2" inside the wall, is there any point in replacing the 1/2" inlet and outlet to the cold side of the pump?

John.G

"The tank feed to the hot side of the pump I can plumb as 3/4" as can be seen in the photo.
If it makes sense I can pull a length of 3/4" from outlet of pump to DHW inlet of boiler."

Would certainly pumb in 3/4" on the inlet to the pump, both ends.
What length of 1/2 ins piping is there roughly from the the hot side of the pump to the boiler inlet??

davkav

John.G wrote: »

Would certainly pumb in 3/4" on the inlet to the pump, both ends.
What length of 1/2 ins piping is there roughly from the the hot side of the pump to the boiler inlet??

That can be done pretty easily too I suppose. The attic isn't floored so I could run a new 3/4" feed to the cold side pretty easily.

I'd say the run of 1/2" from pump to boiler is about 6m.

K.Flyer

If the hot water out from the boiler is tapped into the hot water feed to the kitchen tap then that pipe usually runs back to the hot press in 1/2" and from there in 3/4' back to bathrooms.
So you do have two long 6m runs of 1/2" from the pump plus the 1/2" feed.
Not really that good for showers, especially on a 1.5 bar pump.

davkav

K.Flyer wrote: »

If the hot water out from the boiler is tapped into the hot water feed to the kitchen tap then that pipe usually runs back to the hot press in 1/2" and from there in 3/4' back to bathrooms.
So you do have two long 6m runs of 1/2" from the pump plus the 1/2" feed.
Not really that good for showers, especially on a 1.5 bar pump.

Just video called the mother there and sure enough there is 1/2" copper running from the DHW outlet of the boiler down behind the washing machine....

So it looks like there is a good bit of work to do.
Plumb 3/4" to inlet of pump hot and cold side
Plumb 3/4" from pump to boiler and then boiler back to hot press and use original 3/4" feed to hot taps that was used for the HW cylinder.
Fit larger pump.

John.G

Changing from 1/2" to 3/4" will effectively add another 0.5bar to the pump at 15 LPM and bearing in mind that that you can only run one shower at 11LPM or two at ~ 6LPM you may well find that the existing pump is adequate.

davkav

John.G wrote: »

Wonder how did that same pump perform with the HW cylinder?, the two differences that I see is the added head loss through the combi heat exchanger and there was more balance between the hot and cold volumes with the hot end supplying ~ 60% & the cold end 40%.

I forgot to add, in the old configuration the pump was only used for the ensuite shower so probably worked pretty well. The new boiler installers repurposed it to pump the hot water supply to the new boiler. I suspect they pulled a fast one and charged for a new pump anyway....

John.G

I'd say you will see a marked improvement if you replace ~ 12M of 1/2" with 3/4", have you any idea of the present flowrate from the ensuite shower and/or the bathroom shower at normal showering temperature?.

Asdfgh2020

John.G wrote: »

I'd say you will see a marked improvement if you replace ~ 12M of 1/2" with 3/4", have you any idea of the present flowrate from the ensuite shower and/or the bathroom shower at normal showering temperature?.

I’ve read through this thread and I’m a bit confused about combi boilers in general as a result. I posted a query a couple of weeks back about upgrading to a combi boiler (oil type-as no gas close to me despite living in an urban area GNI never ran gas into the estate) the one or two responses I got put me off combis completely as they were saying that the feed for the hot water to the combi boiler has to come directly off the mains supply and if the pressure is low ( which it is in my case) then the combi would struggle to ‘drive’ the water around the house. Here I’m reading that the combi can be fed from a tank and then the hot water can be pressure boosted with a pump on the ‘up steam’ side of heat exchanger and if you spec the pump correctly and have the correct sized pipe work they will work well...or will they...?

John.G

Yes, they will, they are NOT supposed to be mains supplied. One problem I see is that they have to oversized to provide adequate hot water, you will need a 36kw combi to provide 15 LPM of HW at 40C but your CH demand may only be 15kw and far lower when the house heats up so the boiler will cycle very frequently, oil fired boilers spend their whole life cycling anyway so probably not a huge problem but you should be aware of it at any rate.

K.Flyer

John.G wrote: »

Yes, they will, they are NOT supposed to be mains supplied. One problem I see is that they have to oversized to provide adequate hot water, you will need a 36kw combi to provide 15 LPM of HW at 40C but your CH demand may only be 15kw and far lower when the house heats up so the boiler will cycle very frequently, oil fired boilers spend their whole life cycling anyway so probably not a huge problem but you should be aware of it at any rate.

The typical combi boiler has two separate temp controls, one for hot water and the other for heating. You can balance the central heating system and set it up to try keep it as close to or in condensing mode.
When a hot tap is opened it triggers a flow switch inside the boiler and diverts the heating water to a built in plate exchanger and also ramps up the burner to produce the desired hot water temperature. This prioritises over the central heating mode. When the h.w. demand is finished the boiler then reverts back to the lower burner settings for central heating.