Gravity fed low hot water pressure - pump?

Millstone · 2026-02-01T11:22:47+0000

I have very poor hot water pressure upstairs but decent cold water pressure from the mains. Takes ages to run bath.

My system is gravity fed. The header tank in roof space does not have much height above upstairs bathroom and no room to increase that. The boiler is a very old Glowworm, working fine. The hot water tank is in the cellar.

Plumber suggested switching to a combi boiler. However I have now had several quotes for this and it is so expensive - some quotes north of £6k... need to put in new gas pipe to boiler seems to be causing the extra cost. Cost of boiler would dwarf any energy savings.

I am wondering if a simple pump could not solve the pressure problem? But one plumber thought I'd "run out" of hot water. I'm not sure I understand this. I'd be running the same amount of water for bath as usual, just a bit faster... why would that exhaust supply stored in cylinder? And then, would that not just mean a bigger cylinder or switch boiler on longer in advance?

I have no plumbing knowledge or DIY skills... just wondering what is feasible.

Harry Bloomfield · 2026-02-01T11:38:45+0000

Millstone said:
My system is gravity fed. The header tank in roof space does not have much height above upstairs bathroom and no room to increase that. The boiler is a very old Glowworm, working fine. The hot water tank is in the cellar.

The slow flow, is due to the length of pipe, bends etc.. The water is having to travel all the way down to your cylinder, in the basement, then back up again to the bathroom. Hopefully, it will be piped in 22mm, or 1" pipe, all the way.

oldbuffer · 2026-02-01T12:38:56+0000

1. A combi boiler is likely to be even slower filling a bath. OK for a single shower, but hot flow rates are usually around 12 litres per minute.
2. The head (pressure) of hot water will be around 0.15 bar given that tap is likely to be 1.5 metres below base of cold water storage cistern (CWSC).
3. Given, as Harry Bloomfield has explained, the resistance to flow likely to be encountered with water going to basement and then back up again around 2 storeys is the most likely explantion for poor flow to the bath.
4. As he has also stated, the pipework should be 22mm down from CWSC to hot water cylinder (HWC) and up from HWC to taps. It shpuld also be 22mm from CWS to bath cold.
5. You could consider a pump to increase flow rate. A single pump on the hot should suffice, provided you have separate hot and cold taps on the bath. Probably a "whole house" pump would be best. My preference is for the Stuart Turner "Monsoon" range. Expensive to buy initially, but you can get spares for them. Have a look at there web site to see recommended installation schemes and sizes of hot and cold storage required. You would need to know the capacity (in litres) of your CWSC and HWC.
6. Another alternative, but more expensive, would be to replace the current HWC with an unvented HWC. This would give you mains pressure hot and cold at every outlet. However:
6.1 Installation would cause some disruption.
6.2 You would need adequate incoming pressure (around 2.0 bar dynamic) and flow rate (around 20 litres / minute) to make it feasible.
6.3 If you relevant pipework isn't 22mm you would probably not achieve the best results.

Millstone · 2026-02-01T12:44:54+0000

Thanks, all really helpful. I always thought it odd that the cylinder was in basement. Perhaps they were trying to save space. But I was told the issue was lack of drop between header and tap.
I was told combi boiler would give me equivalent hot water pressure to cold water coming in - which would be much higher than the current feeble trickle...is that not correct?
Suspect pipes may not be 22mm all the way - can't see them all, some behind pannels.

denso13 · 2026-02-01T13:32:47+0000

Millstone said:
But I was told the issue was lack of drop between header and tap.

That's correct, it doesn't matter where the cylinder is, within reason.

Millstone said:
was told combi boiler would give me equivalent hot water pressure to cold water coming in - which would be much higher than the current feeble trickle...is that not correct?

Same pressure but the flow rate is a different thing. Measure how long it takes to fill one litre of water in a bucket, which would give a better measurement against a combi.

Harry Bloomfield · 2026-02-01T14:30:26+0000

Millstone said:
Thanks, all really helpful. I always thought it odd that the cylinder was in basement. Perhaps they were trying to save space. But I was told the issue was lack of drop between header and tap.

My tank is in the loft, directly above the cylinder, which is adjacent to the bathroom, and all plumbed in 22mm. Cold water pressure is around 2bar. We have good hot flow at the bath, a little faster than the cold tap.

The shorter, the fewer the bends, and the larger the pipe, the faster the flow. In your case, can the cylinder be moved to somewhere nearer the bath?

Millstone said:
I was told combi boiler would give me equivalent hot water pressure to cold water coming in - which would be much higher than the current feeble trickle...is that not correct?

Not necessarily... Hot from a combi, is dependent on the Kw capacity of the combi, and the pressure/flow of the mains - which ever is the worst of the two.

Millstone · 2026-02-01T14:37:28+0000

There's not masses of space to move cylinder to but I wondered if it could just about fit into the loft. Could there be issues with weight? I wondered if that might have motivated the choice of cellar. Presumably a cylinder full of water is heavy.
Is it in fact unusual to have a cylinder in cellar?
One of the engineers did measure the flow rate (I think that's what it was) from upstairs tap using some little device that lets water out at a set speed. He agreed it was rubbish - can't remember the reading he gave. Cold water was ok.

fixitflav · 2026-02-01T14:53:22+0000

Millstone said:
But one plumber thought I'd "run out" of hot water. I'm not sure I understand this. I'd be running the same amount of water for bath as usual, just a bit faster... why would that exhaust supply stored in cylinder?

It wouldn't. If the pump fills the bath faster than the cold feed to the header tank, the header tank (not the HW cylinder) will draw down somewhat. But that's unlikely, and in any case the header tank is likely to hold enough to fill the bath, some of which will be cold water. The header tank then refills when the bath is full and you turn the taps off.

fixitflav · 2026-02-01T15:04:58+0000

Millstone said:
Is it in fact unusual to have a cylinder in cellar?

Well, yes, because it's unusual to have a cellar at all.

Millstone said:
There's not masses of space to move cylinder to but I wondered if it could just about fit into the loft.

I suppose it could be done, but a fairly big job. Fitting a pump might be an easier option.
My HW cylinder is in the airing cupboard on the1st floor, and there's about 3m vertical between the bath taps and the header tank water level. Flow is fine. If your level difference is similar the low flow is likely to be due to pipe restriction.

Edit - can you go halfway and put the cylinder on the 1st floor? And of course wherever it goes the pipes to the primary coil will also need modifying

Harry Bloomfield · 2026-02-01T15:05:08+0000

Millstone said:
There's not masses of space to move cylinder to but I wondered if it could just about fit into the loft. Could there be issues with weight? I wondered if that might have motivated the choice of cellar. Presumably a cylinder full of water is heavy.
Is it in fact unusual to have a cylinder in cellar?

It is a rather unusual place to put it. Pipe lengths are not only a concern for reduction of flow, but in the case of hot water - the wastage, and time it can take before the hot appears at the taps.

Moving it to the loft, subject again to pipe lengths, might be a good idea, if you can ensure adequate support of it. The tank, would have to be raised up, above the cylinder, probably on stilt supports, and both tank and cylinder over a structure, such as a wall.

fixitflav · 2026-02-01T15:14:04+0000

Harry Bloomfield said:
in the case of hot water - the wastage, and time it can take before the hot appears at the taps.

Good point!

Harry Bloomfield said:
The tank would have to be raised up, above the cylinder,

OP says there's not much scope for that

Harry Bloomfield · 2026-02-01T15:33:44+0000

fixitflav said:
OP says there's not much scope for that

His loft, almost has to be as big as the house. Maybe the OP could explain?

fixitflav · 2026-02-01T15:39:14+0000

Harry Bloomfield said:
His loft, almost has to be as big as the house. Maybe the OP could explain?

I'd guess it's height he meant, but over to the OP!

Madrab · 2026-02-01T15:39:33+0000

In a gravity system the head of HW pressure and in correlation, the level of flow, would be determined by the height of the Cold Water Cistern (CWSC) above the HW outlet that's being used, minus the resistance of the pipework (bends/length). The fact that the cylinder is below in the cellar wouldn't alter that.

The risk of running out of actual HW from the cylinder is the same whether it was pumped or not. The other factor would be that the pump could draw the HW faster than the CWSC could replace it but that would be an on site estimation as to the supply from the CWSC (normally 22mm) against the flow rate from the pump and the pump sized accordingly.

Is the whole bathroom fed from the CWSC in the attic, hot and cold?

denso13 · 2026-02-01T16:34:38+0000

Millstone said:
One of the engineers did measure the flow rate (I think that's what it was) from upstairs tap using some little device that lets water out at a set speed. He agreed it was rubbish - can't remember the reading he gave.

That's the important bit, without it you are just guessing at what improvements may, or may not, help.

diynot

If you need to find a tradesperson to get your job done, please try our local search below, or if you are doing it yourself you can find suppliers local to you.

Select the supplier or trade you require, enter your location to begin your search.

Are you a trade or supplier? You can create your listing free at DIYnot Local

Gravity fed low hot water pressure - pump?

Similar threads