Currently renovating old stone-built detached house, insulating walls internally with 100mm multipor.
We have 2 solar thermal panels fitted (Viessman 200), and a Morso DB15 boiler-stove heating a 300l heat store. Heat store also has 3kW immersion heater.

It's our first full winter in the house after living in a caravan-in-a cabin for most of the previous four and a half years... so even with problems this is luxury..!

We're using half the house at the moment:
- the kitchen where the boiler-stove is fitted (5.2m x 4.3m x 4.5m to the apex ceiling); this room also has a 2.6kW radiator
- the bathroom, which has a 1.1kW radiator, also the heat leak radiator for the stove and low power (290w) electric UFH
- bedroom, which has piped UFH at 150mm spacing, and is 12m2

We find that the stored heat from running the stove the previous day (and topping up with the immersion heater for a couple of hours if necessary from 5am to 7am) brings the heat store temp up to about 60c middle to top. The temp in the bottom depends on how hot it is fromn the previous evening.

This gives us enough HW for 2 good showers and some left, but is insufficient to run the CH.
We find the temp in the lower half of the heat store - where the CH water is drawn from - drops quickly as soon as we run the CH.
We can get the whole store up to 60c+ after running during the day, so that we can run the CH for an hour or so in the evening. The bathroom usually stays very warm without the radiator drawing any heat from CH and the kitchen radiator thermostat is turned right down to conserve CH supply heat, so we're only using the CH to warm the bedroom via the UFH. We find we can run the CH for an hour or so and that will bring the temp up about 1degree - usually sufficient to warm the (bare concrete!) floor - but that will drop the temp in the heat store, maybe unless we have the Morso running flat out... It's the Pure Wood model and we're still experimenting with wood supplies and getting to the point where we have sufficint well-seasoned wood stored for a whole winter.

There is never enough heat stored in the morning to be able to run the heating.
Part of the issue is that even if we get the thermal store really hot by the evening - and we have had it up to 80c a few times - it will lose that heat by the morning. Some along the copper pipe runs, some from brass fittings around the heat store.

We are meeting with the heating engineer soon to look at options. One possibility may be retro-fitting a backup LPG boiler and somehow linking it to the system: there are spare tappings in the heat store toward the top and at the bottom, although it would involve dismantling some of the finished work on the house.
Another option I'm wondering about is somehow installing an additional heat store and linking this in to the existing one, using those same tappings, to give a much larger overall volume of water, and of potential stored heat. This would have to be located in the utility room, which is on the ground floor, so below the existing heat store.

We will have to look at the feasibility of the LPG boiler with the heating engineer, but I thought I would ask here about the idea of a linked second heat store. Does anyone here know if that's feasible, and how we might control temperature and flow of water between the two?

A couple more questions related to this situation:
1. Following guidance (mistakenly) that came with the stove, we fitted a pipe thermostat to the boiler-to-heat-store output pipe rather than a flue thermostat. This is set at about 65c but it means that, if the water at the bottom of the heat store is over 60c, water continues to circulate through the stove for some time after the fire has died down, which can deplete stored heat.
A flue thermostat would be better but the heat store is on the other side of a 700mm thick wall, a metre or so (with a floor/ceiling between) above the boiler-stove. All the walls are finished (lime plastered) and all the pipe work connected so we can't run a cable from a flue thermostat to the heat store.

Does anyone know of a wireless solution, by which we could connect a flue thermostat to a wireless transmitter in the fireplace and the signal be received on the other side of the wall where the heat store is located?

Lastly, any suggestions for the best way to insulate all those brass components on the heat store and pipework?

Ian

Hi Peter,
Thanks very much for taking the time to give this detailed response - much appreciated.

Re internal insulation, it's aesthetics: I've attached a couple of pictures. We didn't want to spoil this lovely old house and 100mm multipor insulation on all the external walls, 100mm PIR insulation + glasswool on ceilings and 100mm under floors seemed a good compromise solution.

Controls on the boiler to heat store:

Pipe thermostat on the boiler output to heat store switches on boiler circulation pump, set to about 65c.

Esse load valve on the heat store: return to the stove mixes hot water coming from boiler with gradually increasing amounts of cooler water from the heat store. Gate only opens when temperature at the valve is 61c or more, ensuring that water returning to the stove is hot.

The store is stratified, as you describe, with DHW at the top and CH lower: heating feed is about 3/5 of the height of the store, with DHW feed above that. Heating return is about 1/4 of the way up and the solar coil connections are below that. CH flow (out) doesn't have an additional mixer valve.

Heat loads:
(a) The part of the house being used now
Bathroom, about 9m2: 1.1kW
Bedroom, 12m2, UFH at 150mm spacing: approx 1kW at 45c (?)
Kitchen (where boiler-stove is): 2.6kW
-----------------------------
(b) Not yet in use:
Sitting room: 2kW
Main bedroom: 1.3kW
Study, 12m2, UFH as above: approx 1kW
Utility room: 1kW

Total = 10kW approx.

Ian

https://drive.google.com/file/d/1uMQMX_SM-QIXB4nz3lFMkxSC_dJWgLd3/view?usp=sharing

Nice looking building. It's not a water mill is it? The arrangement on the back looks unusual.

Nice building - I can see why you want form over function regarding external building.

For the heating
For the heat loads was it the size of the rads. you have quoted or the heat loss through the fabric and ventilation?

Posted By: IanCDThe store is stratified, as you describe, with DHW at the top and CH lower: heating feed is about 3/5 of the height of the store, with DHW feed above that. Heating return is about 1/4 of the way up and the solar coil connections are below that. CH flow (out) doesn't have an additional mixer valve.

By stratification I mean the temperature gradient of the water in the store. Hot at the top, cold at the bottom and an undisturbed temperature gradient between the two.
If you have 70 deg. top to bottom and you take out CH water from 3/5s up and return the water at say 40 deg into 1/4 way up then you will be mixing 40deg with 70deg making say 55deg. so your water at 1/4 is now 55deg. If the water came in at the bottom of the tank then you would still have 70deg at the 1/4 level and 40deg at the bottom.This 40deg band would then move up the store as more return water came in and you would maintain your 70deg output for longer.

Based on the figures of where the connections are on the store you have about 120lts for DHW and 100lts for CH. 120lts for DHW should be OK - providing it does not get mixed by the force of the CH return mixing up the water, but 100lts for CH might just about warm the rads.

So the stove should provide stored DHW but for CH you will need the stove to be running. You should try to get the store temp. a bit higher and have a mixer valve on the DHW t bring it down to safe temps. as this makes the supply of hot go further. The same applies to the CH part with a mixer for the UFH temp.

For times when the stove is not running e.g. in the morning, when you have flu, or away for the weekend or just can't be bothered to light the stove - then you will need an alternative to the wood stove - enter the LPG boiler. This can be plumbed into the store either direct or through a coil (if you have a spare coil in the store) and probably best if plumbed into the middle of the store roughly where the CH in/out is, although with a 300 ltr store it probably doesn't make much difference.

The electric immersion heater should be just above the CH output because you don't want to be heating the CH water with the electric immersion heater.

I am presuming
1 the DHW is indirect via a coil in the store
2 the stove is a direct connection to the store
3 the CH is a direct connection to the store
4 the solar is indirect via a coil
5 there are suffucent connection options on the store to enable the connection you want

Plate heat exchangers (PHX) are bad news for heat stores. For a heat store to work efficiently you need an undisturbed heat gradient from to bottom and gentle flow rates. A PHX needs a high flow rate on the primary in order to get the heat output to the secondary. This will create disturbance within the heat store and mix up the hot at the top with the cold at the bottom. In your case you will be taking hot water from the top of the store removing some heat through the PHX and then shoving still hot water into the store just below the CH return to mix with the cold area that has never been heated by the immersion heater (which is above this point?) The result of this will be to mix up your hot water to a not very useful cool/warm temperature.

It is best to separate the CH /space heating and the DHW. The problem is how to do this economically. From an earlier post you are already thinking of getting a second heat store, perhaps use this as the opportunity for separating the DHW from the CH.

Your DHW will have 4 sources of heat input, solar, wood stove, LPG and electric immersion. Do you need all of them. DHW tanks can have an internal pipe heat exchanger which gets away from the issues of PHXs. With this sort of tank you would have a coil for the solar input at the bottom, a coil for the DHW out at the top and the stove and LPG direct.
Do the sums on the cost of heating water via LPG or E7 and you might find that it is not worth the hassle to put LPG to heat the DHW, (especially if you are considering PV in the (near) future) in which case I would suggest a twin coil DHW tank, one for solar and one for the stove, a normal immersion heater and the DHW without any heat exchanger – which gives a better DHW flow. And of course your existing store serves the CH only with inputs from the LPG and wood stove.

It is useful to have a temperature gauge on the heat store, and I would locate the gauge readout close to the wood stove. With the gauge about 1/3 up on the store you will be able to see the temperature and so know when to stop feeding the stove so as not to overheat the store. (My heat store (2000lts) has 5 gauges top to bottom to guide the loading of the boiler with a separate 300ltr DHW tank driven from the heat store. This serves 2 houses)

Control of the LPG boiler I would expect to be driven by the temperature of the store with the thermostat somewhere towards the top with the flow and return also in the upper half. The wood stove will serve the whole store. In this way the LPG only comes on when there is insufficient heat in the store and then only heats the top of the store. If you have a separate DHW tank with LPG supply then you will need a diverter valve so that either DHW or CH can call for LPG heat but the DHW will get priority.

Just a point on Plate Heat Exchangers. We have one attached to our 350l heatbank for the DHW and I haven't found any problems with it ruining stratification. I have actually deliberately tried to get it to do so on hot summer days when our solar PV has heated the top third of the tank to 80C and has shut off dut to the immersion heater thermostat. I ran the hot tap in the hopes of mixing up the water to get more out of the solar PV, but it didn't work. The bottom two thirds of the tank remained at the same temperature. In the end I fitted a second immersion heater in the bottom third of the tank, which is activated once the top immersion hits 80C.

What is great about the Plate Heat Exchanger is that it's very efficient and it can be set to a specific temperature, which is very useful when your tank is bubbling away at 80C but the water at the tap is a nice 45C (or whatever temperature you want to set it to).

Posted By: Pile-o-StoneJust a point on Plate Heat Exchangers. We have one attached to our 350l heatbank for the DHW and I haven't found any problems with it ruining stratification.

If your heat bank is say 80deg at the top and 40deg at the bottom and your PHX has a temperature drop of say 15 deg then you will be putting water at 65deg into the 40 deg water at the bottom. Given that PHX usually run at a fairly high flow rate this will cause the 65deg and the 40deg water to mix, raising the temperature at the base of the heat bank. The more the PHX runs the more the mixing will occur and so the heat bank will, over time, have a more even heat top to bottom. this will make it harder to get the required output temperature on the secondary.
Have you measured the temperature at the bottom of the heat bank with the PHX running to see if there is any temperature rise there

Posted By: goodevansNot necessarily Peter, - the water being heated will have come from the outside mains - perhaps 11 to 20 centigrade - If the heat exchanger is efficient the top of the tank water should be cooled to nearer say 25 centigrade after it has passed through the heat exchanger then placed into the bottom of the tank.

Except that most PHX are designed to have a 15 -20 deg drop from input to output otherwise you can't get the rated output.

Certainly the Gledhill adjusts the PHE pump flowrate if the DHW demand is slow (i.e. almost closed tap) and it controls the produced DHW temp to a setpoint, which is 60°C IIRC.

Posted By: Pile-o-StoneI tried to google the temperature drop across a heat exchanger, but couldn't find anything. Do you have a link with details of the 15C to 20C drop you're quoting? If that temperature drop is accurate, then I guess the water pump attached to the heat exchanger is pumping HW through the HX more than twice as fast as the mains water is coming in. We have pretty fierce mains water pressure, but the pump is on the lowest of the three speed settings.

I found a spec sheet for various PHX at
https://www.aosmithinternational.com/wp-content/uploads/Spec-sheet_PHE_UK.pdf
This quotes a primary temperature of 80/60 with a secondary temperature of 10/60. The flow rates depended upon the size of the PHX but the domestic size would be 2.15m3/h for the primary and 0.86m3/h for the secondary. Now this would be to get rated output, but a typical 1/2" tap runs at about 14 lts/ min (if I remember correctly) which is about 0.86m3/h So this PHX run as per the spec would support a 1/2" tap at full flow continuously.

But the above PHX puts water back into the store at 60 deg. whih will b mixing with what ever water temperaure happens to be there. Now if you are not demanding the outflow (e.g. tap not fully open) then your smart gizmo pumping the primary can reduce the flow which could reduce the return temperature.

Posted By: Pile-o-StoneIs it not the case that to raise mains water temperature by 48 degrees, it would reduce the temperature of the feed by at least 48 degrees (with a little extra for inefficiencies)? So to use your initial example, 80C from the top of the tank would be reduced to 32 degrees when it is fed to the bottom of the tank, which would cool the 40C water that was stored there.

It can't work like that if you want a high output from a small unit. Temperature transfer is dependent on many things amongst them the temperature difference. The greater the difference the faster the heat transfer. The temperature coming from the PHX is nothing like as cool as the mains water going in, if it was there would be very little heat transfer towards the end of the PHX. which is why the primary flow has a designed temp. of 80/60. Think CH radiators, these are designed with a temp. drop of 20 deg across the rad and the tables for output will quote kW output at (typically) 80/60 or 70/50 and if you step away from this you loose output. (which is why you can't run rads at UFH temps without dramatically increasing the size)

They fit PHX to heat stores because either you can't retro fit a coil to the store or you need a PHX to get the rated output. (rated output is one thing actuality may be different) DHW coils are not simply copper coils but are usually ribbed tubing to create turbulence to get better heat transfer. (and they are not cheap)

All PHXs run counterflow, they would be too inefficient otherwise. The amount of mixing you would get will depend upon the flow rate of the primary pump (which is driven by the required temp. vs. the achieved temp - correct?) and the return temp vs. the temp at the bottom of the tank. If you only draw a small amount of water at a time then mixing will be minimal, if you draw a large bath mixing could be significant. If in the summer when the top of the store is 80 and the bottom not heated then putting 50 deg water (assumed same 30 deg drop) would be detrimental to the overall usability of the heat contained within the store.

Heat stores supplying CH have mixer valves between the flow and return to reduce the amount of hot returned to minimize the problem of mixing the store to an overall warm

Posted By: djhThe temperature of the water being returned into the bottom of the tank is just under 30°C

Well this is good news - it means that the boiler supply will always allow full condensing.

With a stores with eternal plate exchangers like the Gledhill I see the following disadvantages.

1) The bottom of the store may not be the coolest - therefore some extra, useless, heat loss from the tank in the summer months (but cost of that loss vastly reduced if solar pv used).

2) a complicated system means more parts to fail. But the tank itself is therefore very simple and all the other bits sit outside the tank and can be replaced.

Strangely the Gledhills come in at prices less than those of a simple finned coil store. Other than disadvantage 1 above, is there really a problem of the bottom of the tank being warm providing it is cool enough to allow the boiler to reach full condensing mode. Not perfect stratification but good enough.

Posted By: djhI don't know. My system is fully electric and I try to get the whole tank as hot as possible when the energy is free.

I'm the same and now I have the second immersion attached to my iBoost, I should be able to get two thirds of the tank toasty hot. I do look at the bottom third of the tank now with regret, though I think 233l of water at 80C should be enough for most of my HW needs this summer :).

Started a new thread, for a new question... includes an update on the outcome of our situation here..
http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16018&page=1#Item_0
(sorry, can't see how to get hyperlinks working just now)
Ian