I see the plumbers are active so....
I need a plan for piping and operating the cottage heating/DHW system!
Progress so far... gas supply and meter installed. Largest thermal store
I can fit in the space, installed. Upstairs and downstairs underfloor
heating pipework laid and coupled to separate manifolds.
Buffer tank? It has been suggested that running the boiler to satisfy a
thermostat calling for heat for say a bathroom circuit will lead to
boiler cycling and inefficient operation. How does one include this in
the control system and how large does it need to be.
The thermal store is probably undersized based on the usual
shower/bathroom number calculations but space limited. With the expected
slow response from underfloor heating, it seems best to prioritise the
Chalet bungalow with limited headroom. Pressurised system? System
boiler? Serious lack of knowledge area:-)
Apart from picking the brains in here, is there a useful on line source?
On Thu, 11 Jan 2018 10:04:22 +0000, Tim Lamb wrote:
Doesn't the thermal store act as a "buffer tank"? The boiler being
controlled by the temperature of the store not by room stats.
A zone, be that DHW or space heating, takes heat from the store. If
the store has enough heat that's all that happens, if it doesn't and
it cools below a given point the boiler is fired up to reheat the
The store doesn't have to supply the total heat demand in one hit,
once it cools below it's "reheat" point the boiler will be thumping
heat back into it. We have a 300 l store and 30 kW boiler that
reheats at about 2 C/minute, with say a heating zone demanding heat,
Remember the boiler will be taking water from near the bottom of the
store that'll be at 30 to 40 C and returning it to near the top at at
a round 80 C.
Our store is about 6' tall and 18" ish dia. The DHW coil is across
half, the CH circuits across the lower half. Solar coil, right in the
bottom 8" or so, wood burner from near the top to just above the
solar coil, oil burner top to just above the solar coil (IIRC, but
not grossly wrong).
Stratification in the store means that the DWH coil is always in the
hottest water at the top of the store and the CH can't cool that
significantly without the store getting below the reheat point. The
sensor for the reheat is between the lower DHW level and the upper
Umm.. I don't know:-) I assume the boiler supplies heat to the thermal
store until the DHW stat. is satisfied. The thermal store water is not
pressurised and simply transfers heat to the DHW demand coil when that
is fed with mains cold water.
There are pumps supplied with each manifold but I assume they only
circulate water round the associated floor heating circuits.
I imagine the boiler pump (or main circulator) would feed either the DHW
top up coil or the heating circuits which could include a buffer store
Ah! I don't have a separate coil for CH.
The system suppliers are not very forthcoming and seem to assume you
will use professional installers:-(
On Thu, 11 Jan 2018 11:49:24 +0000, Tim Lamb wrote:
You don't have any stored DHW so no DHW stat. To control the DHW
temperature there should be thermostatic mixer valve on the thermal
stores DHW coil output to stop 80+ C water being deliverd to the
taps. DHW is heated on demand by passing through the coil in the
(upper part of the) thermal store.
Possibly, no pratical experiennce of wet underfloor heating.
Neither have I. The water in the store is the same water circulated
around the CH zones, the wood burner and oil burner loops. The DHW
passes through a coil immersed in that water to pick up heat. The
solar thermal has a coil because that loop is heavyly dosed with
glycol to stop it freezing at the collector.
Who might not understand it either... ours didn't.
Umm.. The thermal store header tank is only marginally above the store.
I thought pumping over issues might necessitate pressurising the boiler
I'll pick this up again this evening. Morning spent on SWMBO tax return!
On Thu, 11 Jan 2018 13:20:19 +0000, Tim Lamb wrote:
The header tank on our store is integral with the store... ie plonked
right on top of it and covered by the same foam insulation. I think
there is a spurt of pump over when one of the circulators starts but
I've not investigated in detail. I doubt there is much if any
insulation between the top of the store and bottom of header tank.
Maybe a bit of wood for strength?
Mine is slightly further up the roof slope. Prolly 500mm water head
diff. However, there is a more fundamental difficulty, the boilermate I
have, has a finned coil fed from the boiler. I imagine there may still
be way of utilising the energy stored in the lower part of the tank but
I am struggling to see how.
On Thu, 11 Jan 2018 15:57:41 +0000, Tim Lamb wrote:
I think I need to know what you have, link? Seems rather odd not to
have access to all the heat in a store. Where is this finned coil
positioned? If right at the bottom it might be intended for solar
thermal, finned to get maximum heat transfer is a short a coil as
possible so it stays close to the bottom of the store thus in coolest
water whci also aids heat transfer.
On Thu, 11 Jan 2018 20:38:26 +0000, Tim Lamb wrote:
Ah two coils, one from the boiler, one for heating on demand DHW,
both coils immersed in the same, isolated, body of water.
I now see the problem, you don't have access to the body of water
with all the heat. Short of utilising the drain and feed/expansion
Of course the primary (boiler) coil will work to remove heat from the
store if the water flowing through it is cooler than the store. So
there might be scope to design a control system that will take heat
from the store but with only 60 l ish at the bottom of the store it's
probably not worth it for something complicated.
The open vented set up has a divertor valve on the boiler flow that
feeds the boiler output to the store or to the heating. It ought to
be possible by arranging the plumbing and control system slightly
1) Store cold, no heating deamnd
Boiler on, pump on, all flow through coil.
2) Store hot, no heating demand
Boiler off, pump off, no flow.
3) Store hot, heating demand
Boiler off, pump on, flow through coil (1st) and heating loop
(2nd), bypass the boiler.
4) Store cold, heating demand
Boiler on, pump on, choose:
a) all flow through coil
Rapid reheat to maintain availabilty of DHW
No heating until store hot
b) flow through coil (1st) then heating loop (2nd)
Reheats store quickly, poor heating until
One would have to experiment with the hystersis on the store sensor
to get decent burns of the boiler when there is heating demand that
is more than matched by boiler output meaning the system oscillates
between 3) and 4).
If the boiler needs pump run on after fireing take that into account
if the system could bypass the boiler.
Generally the boiler controls will prioritise DHW - i.e. it will drive
the DHW zone until its satisfied, before reverting to the heating. (it
may have a timer to ensure that it will run the heating for a reasonable
amount of time in the face of an incessant DHW requirement).
If the boiler supports split temperature operation (good) then it will
never run the DHW and CH at the same time, since it would typically run
a lower flow temp for the CH than for the DHW.
OK. The operation Dave suggested would slow the recovery for DHW but
might only be a problem when the family are visiting. For a retired
couple and visitors there is no dash to get showered and off to work by
I can find space for a buffer store on the ground floor but pressurised
operation is not very practicable and make-up/expansion arrangements
This store, is it intended to run the CH and the DHW?
If the store is running the CH, then this is a non issue. If you are
running (small) CH zones in isolation, then going for a boiler with a
good modulation range will help it load match under a wider variety of
Anything gravity fed will be poor in this circumstance, with the
possible exception of a gravity fed heat bank where its heat is used but
not its water pressure (i.e. it can feed a plate heat exchanger or
internal coil for mains pressure DHW).
Do you have decent cold mains flow rate and pressure?
 We had exactly that when we moved in here. DHW header tank was about
5' off the floor of the upstairs living space. That meant even lifting
the shower head too high while rinsing the bath was enough to stop the
flow of water.
I spent some time looking at alternatives (keep in mind I did not have
UFH, and the heatloss rate is far from modern standards), and concluded
that splitting the CH into two zones each with its own stat, and adding
weather compensation would probably get the heating about as efficient
as it was realistically possible to do. That just left DHW to worry
about, and it came down to heat bank or unvented cylinder. Although I
quite liked the idea of the heat bank, I could not do it for anything
close to the price of a decent unvented cylinder.
Went for a system boiler since that would integrate with all the
controls I wanted, and allow split temperature operation (i.e. running
low weather compensated flow temps for the heating when required, but
switching up to high flow temps for cylinder recharging). (I did look at
doing it with a combi - having it heat a cylinder like a system boiler,
but then also using its internal DHW for a kitchen tap to eliminate the
longer dead leg out there, but I could not find a solution that quite
did what I wanted). With hindsight the dead leg to the kitchen seems far
less noticeable now there is copious hot water flow and pressure.
There is a reasonable amount in the wiki.
OK John. I'll have a closer look through the wiki over the weekend and
come back when I have some specific queries.
Currently I am not very clear on the system boiler functions!
We have a Veissman Vitodens100 (Combi) professionally fitted in a granny
annexe. Fitted by their trained plumbers, these come with a long
guarantee so the manufacturer is under consideration. For under floor
and 24 hour occupation, I doubt the value of weather compensation:-)
For a pressurised system with around 800m of 16mm pipe, I wondered
whether the boiler expansion vessel would be adequate and whether a back
up fitted in the buffer tank would be of value?
Oh! Scrub that! I started off trying to avoid safety discharge systems.
Back to trying to find a way to use the boiler mate heating coil as a
On 12/01/2018 09:50, Tim Lamb wrote:
You can think of it being a bit like a combi with the DHW capability
removed. i.e. it has a pump, expansion vessel, and most of the controls
required all in one box.
Some also go a step further and have a more programmed understanding of
things like UFH, and can directly control blending valves. ISTR mine
(Vaillant) even has programs for drying your UFH floor screed, by
ramping up temperatures over a few weeks.
Some directly support multiple zones as well.
The 24 hour bit would add to the argument for it. However its probably
less relevant for UFH.
800m of pipe is probably significantly less volume of water than a
conventional rad system...
I guesstimate my system has between 150 and 200L of primary water (21
rads), and that is fine on the internal expansion vessel in the boiler.
With a sealed primary (as would be used by most system boilers, all the
discharge system is built in. You normally (on a wall mounted boiler)
take a pressure relief pipe through the wall and down to low level. If
the system does vent, then it will only typically sump a few litres of
water (being sealed - there is no continuous replenishment of water - so
the damage that can be done is far less)
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