Add underfloor to S-plan - S-plan+? closely spaced tees? new boiler?

[MarkWillGreen]

Hi All,

Looking for a bit of advice on the best way to pipe this.

- 24kW Worcester regular boiler in kitchen, approx. 9 years old - unvented tank, zone vavles, pump, expansion is on first floor in airing cupboard

- Currently piped as S-plan.

- Hive controls.

- Would like to fit 6-zone underfloor kit on ground floor. Kit has stainless manifold/Grundfos pump/blending valve etc.

- Controls/wiring centre (to replace hive and control everything) to be Reliance (wiring centres/wired thermostats etc.)

- Ground floor ceiling is down. 28mm primarys are on show, some of the joints look like they've been bleeding for a while, and have been chopped and linked in plastic temporarily to put in new steels within the first floor, they will need a bit of work to take them through the steels (route them to the centre of the steel where the penetrations are). Also a couple of "bypass" rads tee'd in which aren't really bypass as they have TRVs (no automatic bypass). Basically the primaries need a bit of work whatever happens for reasons not to do with best system design.

Would you:

Minimum work -

• Re-route the 28mm primaries through the steels,
• Add automatic bypass
• Extend primaries in 22mm to underfloor manifold/pump pack behind zone valve

Basically add UFH as another zone - S-plan+

Disadvantage - intreraction between the pumps? Is this a real issue?

Medium work -

• Redo the 28mm pipework as a primary circuit with circulator pump - pump probably located in back of top kitchen cupboard.
• Have the old S-plan draw off a set of closely spaced tees
• Have the UFH draw off a second set of closely spaced tees downstream of old S-plan. No need for UFH zone valve.

Disadvantage - Expansion should be before the primary circulator pump rather than secondary circulator and not much room in kitchen cabinet for same. More work than above. 3 pumps rather than 2 (additional circulator on primary circuit).

Even more work

- As above but instead of adding a pump in series to the existing boiler, replace with a system boiler in a slightly better location (already piped for gas, just need to drill for flue), and upgrade to more sophisticated controls (weather comp).

Disadvantage - cost.

---

I don't have a wall for a low-loss header or anything like that, but assume I don't need it as it's a pretty simple system.

Am I worrying about nothing with the rad/cyl pump interacting with the ufh pump?

Thanks for your time.

 

[ShaunCorbs]

If it was me I would size the system to match the ufh spec (also your ready for an very efficient ashp later down the road etc)

So heating system design at dt5 and ufh spec temperature probably 40dc

You will need hot water priority/ 4 pipe system boiler with weather comp (so it runs at ufh spec for heating then when the cylinder calls it ramps the flow upto 65-70dc

 

[MarkWillGreen]

Thanks for your input Shaun.

Rads and UFH are sized for 55 degree supply, dT 10 on the rads, and dT 5 on the UFH. Can't go less on the UFH due to the size of the sliding doors, unless we add supplemental rads.

If I stay in the house for 5 years - am I going to see a pay back on dropping say £3-4k to put in a new weather comp Viesssman boiler with HW priority?

We never have to boost the hot water as we have an Immersun electric system working off photoelectric.

 

[ShaunCorbs]

What’s the floor covering as 55dc is on the high side of ufh and would do dt 5 on the rads then you won’t need any separation

 

[MarkWillGreen]

Porcelain tile on the UFH.

The rads will work at dT 5 assuming the flow is at 55. I think probably less as I did the heatloss around the time we had minus 5 weather last year and there was no back on our house, so I pesimistically sized them based on -5 outside temp, or maybe even lower.

Presumably the issue then is sizing and controling the circulator to get enough flow thru the rads. Looks like it would need around 4m3/h which is out of range of the current Grundfos Alpha 2 but I've got a DAB Evosta kicking around that would replace it.

Approx. 2m/s in 28mm copper, which isn't the end of the world.

I'd be a bit nervous running the underfloor without a pump or blending valve. Thoughts?

 

[ShaunCorbs]

2ms is way too high and will cause noise / other issues ideall max 1ms

What head do you need

Is the ufh designed as 55 is well too high on tiles

 

[MarkWillGreen]

Ufh is designed for up to 120W/m @55 degrees, 16mm pipes on 150mm centres - approx. 30 degree surface temp.

There’s 24sq.m of glass on the back of the worst room, so the heat loss is relatively high for the size of the space. That’s to cater for -5 degrees outside though, so no need to run at that temp most of the year.

Not calculated the head yet - as depends on pipework from boiler to some extent.

Existing pump is Alpha2 25-60 on auto, I suspect it’s oversized, decent head but wouldn’t have the flow for 5 degree dT

 

[ShaunCorbs]

Is it already in the ufh ? Do it 100mm centres

 

[MarkWillGreen]

UFH is already in. I've had a look at it again with a more realistic outside temperature and more accurate U-values, and the worst room could be run at a MWT of 40 with the existing pipe spacings.

With the revised heat loss:

dT 5 gives a peak flow of 2.58cu.m/hr / velocity of 1.45m/s
dT 6 gives a peak flow of 2.15cu.m/hr/ velocity of 1.22m/s
dT 7 gives a peak flow of 1.84cu.m/hr/ velocity of 1.04m/s

 

[MarkWillGreen]

Worcester don’t quote a pressure loss at those flow rates - the chart maxes out at around 3.5m head loss at 1.3cu.m/hr.

Doesn’t look like a good boiler to design at dT 5 with.

 

[John.g]

Head is proportional to flow squared.
dT 5C at 2.58m3/hr = 3.5*(2.58/1.3)^2, 13.78M head not a chance
dT 7C at 1.84m3/hr = 3.5*(1.84/1.3)^2, M, 7.0M, no good either?.

 

[MarkWillGreen]

The current plan is to have the underfloor running off closely-spaced tees in the primary return, with a normally open zone valve (closed by DHW/CH calling) in the airing cupboard acting as a bypass across the existing S-plan valves (central heating / DHW).

This gets around the dT for the CH/UFH being mismatched and the issue with the high resistance Worcester heat exchanger.

UFH calls for heat, DHW/CH not calling

S-plan zone valves closed, zone valve bypass open, boiler fires, existing pump moves water around the primaries, UFH pump draws water from the primary on the return side at approx. boiler flow temp, UFH blends down to <=45.

UFH calls for heat, DHW/CH calling

S-plan zone valves open, zone valve bypass closed, boiler fires, existing pump moves water around DHW/CH and back into the primary return, UFH pump draws water from the primary on the return side at CH/DHW return temp depending on load.

 

[John.g]

What is planned boiler flow temp and what is the rad(s) dT and max rad heating demand, and same for the UFH, flow/return temps and max demand?, where are the UFH Tees on the return?, are they after all the rad returns or in between?

 

[ShaunCorbs]

That means your flow temp will need to be at 70/75 for a 55 (flow) return feed to the ufh

That’s a crap system design

 

[MarkWillGreen]

UFH only needs 42.5 degrees flow. Rest of the system (in theory) works at dT 10 or just over so boiler flow of 52.5-55.

Not sure how to do any better than that without completely re-piping everything and/or changing the boiler?

If it's a bit marginal in winter, the boiler has some headroom.

 

[John.g]

If you run the rads with flow/returns of 55C/45C then you have a T30 rad with their outputs reduced to 51.5% of their T50 rating.

 

[John.g]

If you run the rads with flow/returns of 55C/45C then you have a T30 rad with their outputs reduced to 51.5% of their T50 rating.

Anyway, you can always increase the boiler flowtemp as required in winter so a good idea with the UFH installation without major surjery.