R. Cott. 28

[Tim Lamb]

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.

Decisions needed.

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 DHW.

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?

[Dave Liquorice]

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 store.

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 the top 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 CH one.

[Tim Lamb]

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 and thermostat.

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:-(

>

[Dave Liquorice]

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.

thumping

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.

[Tim Lamb]

Yes.

Umm.. The thermal store header tank is only marginally above the store. I thought pumping over issues might necessitate pressurising the boiler circuit?

I'll pick this up again this evening. Morning spent on SWMBO tax return!

>

[Dave Liquorice]

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?

[Tim Lamb]

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.

>

[Dave Liquorice]

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.

[Tim Lamb]

It is a Gledhill boilermate BMBP 185

The heating coil is roughly the bottom 30% with the rest DHW.

>

[John Rumm]

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 situations.

Anything gravity fed will be poor in this circumstance[1], 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?

[1] 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.

[John Rumm]

ok, some of what I just posted elsewhere is probably not relevant then ;-)

[Tim Lamb]

In message , John Rumm writes

Snip

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 buffer store!

[Dave Liquorice]

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 connections.

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 differently to:

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 store has recovered.

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.

[Tim Lamb]

Maybe:-) I think I am going to have a very close look at boiler modulation performance before going there.

Might upset the boss if a low priority heating demand upsets her morning bath!

>

[John Rumm]

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)

[John Rumm]

Yup DHW only basically...

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.

[Tim Lamb]

snip

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

7.30am!

I can find space for a buffer store on the ground floor but pressurised operation is not very practicable and make-up/expansion arrangements inconvenient.

Something to check before purchase.

[Tim Lamb]

OK John. I'll have a look at Veissman (stainless heat exchanger) system boilers and come back if I need further explanations.

>