Maxipod Thermal Store

funkey_monkey

Hi,

We have a maxipod installed into a new build.

Overnight, the tank is dropping approx 20DegC (60degC down to approx 40degC).
Is this expected?

I was under the impression that a thermal store would keep the water warm until needed - within reason - and I would have expected overnight to be within reason.

The maxipod has connections to an oil burner and a stove with integrated back boiler.

Any ideas as to what might be leeching off the heat overnight?

John.G

funkey_monkey wrote: »

Hi,

We have a maxipod installed into a new build.

Overnight, the tank is dropping approx 20DegC (60degC down to approx 40degC).
Is this expected?

I was under the impression that a thermal store would keep the water warm until needed - within reason - and I would have expected overnight to be within reason.

The maxipod has connections to an oil burner and a stove with integrated back boiler.

Any ideas as to what might be leeching off the heat overnight?

20C loss over 8 hrs = 2.5C loss/hr. 20C loss over 10 hrs = 2C loss/hr, some manufacturers claim figures as low as 0.25C loss/hr but its probably more realistic to accept 0.5C loss/hr which in your case = 4C loss over 8 hrs or 5c loss over 10 hrs so it would appear that something is definitely leeching heat.
Have you checked that the stove circ pump is stopping on its pipe stat?, and also that the boiler circ pump is stopping when room stats/cylinder stats etc are satisfied? also check that there is no thermosyphon effect back through any of the rads etc. Feel all the pipes that you think should be cold after the overnight shutdown of stove/boiler and you may figure out something.

funkey_monkey

The stove works on a thermosyphon.
There is only a circ pump for the central heating which I can feel/hear is off. The central heating controls do not switch is back on overnight.

The thermostat on the tank is at 70degC and the thermostat for the stove boiler is set slightly higher.

I initially suspected the stove, but was unsure how to prove it and then how to resolve it.

John.G

funkey_monkey wrote: »

The stove works on a thermosyphon.
There is only a circ pump for the central heating which I can feel/hear is off. The central heating controls do not switch is back on overnight.

The thermostat on the tank is at 70degC and the thermostat for the stove boiler is set slightly higher.

I initially suspected the stove, but was unsure how to prove it and then how to resolve it.

The tank thermostat is set at 70c, I presume you are referring to the maxipod? what does this thermostat bring in/on? and ditto for the stove, what pump does this bring on? I am assuming that the CH & HW get their heat from coils immersed in the maxipod store cylinder,

Edit: I think its the HW only that gets its heat from a mains supplied coil? & the CH is from the main body of water in the store so the same question re what does the 70C tank thermostat do and is the stove thermostat a pipe stat or what?

funkey_monkey

John.G wrote: »

The tank thermostat is set at 70c, I presume you are referring to the maxipod?

Yes.

what does this thermostat bring in/on?

It sets the temp for the oil boiler. Tank temp gauge regularly goes to around 80degC.

and ditto for the stove, what pump does this bring on?

No pump for it. It is actually a pipe stat.

I am assuming that the CH & HW get their heat from coils immersed in the maxipod store cylinder,

HW comes from the coils, CH from the tank itself (judgining by the installation guide.

Edit: I think its the HW only that gets its heat from a mains supplied coil? & the CH is from the main body of water in the store so the same question re what does the 70C tank thermostat do

and is the stove thermostat a pipe stat or what?

It is a pipe stat. Sorry for the mistake.

See attached installation diagram - which looks like our setup. Note pod is on ground floor. (single story).

John.G

funkey_monkey wrote: »

Yes.



It sets the temp for the oil boiler. Tank temp gauge regularly goes to around 80degC.


No pump for it. It is actually a pipe stat.



HW comes from the coils, CH from the tank itself (judgining by the installation guide.




It is a pipe stat. Sorry for the mistake.

See attached installation diagram - which looks like our setup. Note pod is on ground floor. (single story).

Thanks, its becoming clearer now, you said in post #3 that the stove (pipe stat K, fitted on stove return) is set slightly higher than 70C but the schematic shows it set to 50C. What is the purpose of this stat?. and is it attached to the stove return as shown close to the pod itself?.

funkey_monkey

I've no idea. I was adjusting it as for some reason we could not get the dump radiators to engage when the tank temp rise too high. It was initially set to 90degC, but I think that was an error during installation.
I wonder is it the dump stat?

EDIT: Just remembered, that if I screwed it way down to approx 50degC it turned on the pump for a short period of time. But never cycled it on again.
I couldn't figure out why it would need to be set so low when the thermostat on the tank was set to 70degC.

John.G

funkey_monkey wrote: »

I've no idea. I was adjusting it as for some reason we could not get the dump radiators to engage when the tank temp rise too high. It was initially set to 90degC, but I think that was an error during installation.
I wonder is it the dump stat?

EDIT: Just remembered, that if I screwed it way down to approx 50degC it turned on the pump for a short period of time. But never cycled it on again.
I couldn't figure out why it would need to be set so low when the thermostat on the tank was set to 70degC.

Yes that is a "dump" stat which is wired to one of the zone motorized valves and the CH circ pump, so in this schematic at least it is utilizing a existing zone to protect against stove boiler overheat, the reason its fitted on the return I guess is because the coolest cylinder water is here, If it was fitted on the flow then it could pick up 70C + temperatures and activate the dumping, I agree that it should be set at ~ 70C+.

I would also think that its very likely that you are getting reverse thermosyphoning due to both the pod and the boiler being on the same level but maybe the bottom of the pod is higher than the top of the boiler(s)?.

I asked before but are the flow&return pipes cool/cold by feeling them as close as possible to the stove and indeed the oil boiler as well, early the next morning?


Just got the second attachment from Maxipod.

funkey_monkey

Okay - I thought that is where you would ask me to check.

We are using the low flow (B in the diagram) for the stove.

In the event that this is the problem, what would the remedial action required to prevent the reverse thermosyphon?

The bottom of the tank is 6" higher than the top of the stove.

John.G

funkey_monkey wrote: »

Okay - I thought that is where you would ask me to check.

We are using the low flow (B in the diagram) for the stove.

In the event that this is the problem, what would the remedial action required to prevent the reverse thermosyphon?

The bottom of the tank is 6" higher than the top of the stove.

You seem to have done everything by the book so I think the first thing to establish is; if in fact you have got reverse thermosyphon in either or both the stove and boiler.

funkey_monkey

In relation the oil boiler - where do I take the measure from for its height? Is it also from top of the oil boiler in relation to bottom of the pod or to the level of the return pipe into the pod?

John.G

funkey_monkey wrote: »

In relation the oil boiler - where do I take the measure from for its height? Is it also from top of the oil boiler in relation to bottom of the pod or to the level of the return pipe into the pod?

I would think so, is the oil boiler not more or less on the same level as the stove?.
Just as a matter of interest what is the rated stove output (to water).

funkey_monkey

The oil burner is on the same level, but is a taller item.

It is a 5KW Stove with a 2KW indirect boiler.

John.G

funkey_monkey wrote: »

The oil burner is on the same level, but is a taller item.

It is a 5KW Stove with a 2KW indirect boiler.

Maybe the oil boiler is the culprit, also whenever you decide to carry out your testing, wind up the stove pipe stat to max to ensure that its not energising and dumping heat during the night, and don,t forget to reset it back down afterwards although I don't think the maxipod would have much problem in absorbing 2 kw even if the dump stat was never enabled, most stove overheats occur with power failures so that dump stat setup isn't of much use anyhow, I have seen one system where a dedicated dump radiator was fed from a normally closed motorized valve only which failed open on power failure.
I assume your stove pipe stat is wired correctly, your testing of it seems to verify that it is.

funkey_monkey

Just checked this out for myself. The pipes at the stove actually feel warmer than at the tank.

I've just checked and the stove is using the high flow - not the low flow. Is there any performance difference which would make high flow and a valve a better installation than a low flow and no valve? The diagram doesn't say that it will definitely eradicate back flow.

In regards to the oil burner as there is a pump on it, then I'd expect that to act like a non return valve?

John.G

funkey_monkey wrote: »

Just checked this out for myself. The pipes at the stove actually feel warmer than at the tank.

I've just checked and the stove is using the high flow - not the low flow. Is there any performance difference which would make high flow and a valve a better installation than a low flow and no valve? The diagram doesn't say that it will definitely eradicate back flow.

In regards to the oil burner as there is a pump on it, then I'd expect that to act like a non return valve?

Last things first, the circ pump will virtually have no effect.

The low flow installation (but must go UP first then DOWN) is a recommended "cure" for cylinder and stove at the same level.

Performance: In any thermosyphon system, the circulating force in Meters/Meter height difference is the difference in densities between the Flow & Return divided by the mean density, to put this in context if the flow temperature is say 55C & the return 40C then that circulating force per meter is less than 0.1 INCHES, even if the flow temp was 95C & the return 15C (stone cold cylinder) that circulating force (per meter height) is still only 0.4 INCHES so dropping to the low inlet may have a effect, I'm not fully sure how much as I have seen in many many old houses where the direct cylinder was mounted adjacent to the fire (with back boiler) where in some cases one would have to give the cylinder a belt of the fist to get the circulation "going" when you heard the odd bang from the back boiler overheating, but once this was done there was excellent circulation and bags of hot water.
Even if the performance is not affected by the change to low level then you must remember that you are now effectively "whole tank" heating instead of getting useful hot water at the top first but I would certainly try this modification as the losses at the moment far outweigh any perceived themosyphon (loss) effect. In any event you will probably have the cylinder pretty hot anyway before lighting the stove so maybe no big deal.

I cannot, both from a legal and performance point of view recommend the fitting of even a swing check NRV in the return to the stove BUT if you are losing heat through the oil fired boiler then you can install a NRV (in the boiler) as it is fitted with both a thermostat and a Hi Limit stat in the event of the NRV sticking closed or whatever.

funkey_monkey

Thanks for the detailed reply.
From your post, as there is no monitoring on the stove circuit, a NRV is not an ideal solution as crud can build up over time and hold the valve closed creating a pressure backup, is this correct?
The oil boiler can have an NRV as its circuit is monitored and can accommodate this fault.

Can a loop be added to the stove return and keep the high flow in place, is is lowering it the only solution?
Are there any maths required for the loop or is it just to make it as deep as possible? Does the width of the channel in the loop matter?

John.G

funkey_monkey wrote: »

Thanks for the detailed reply.
From your post, as there is no monitoring on the stove circuit, a NRV is not an ideal solution as crud can build up over time and hold the valve closed creating a pressure backup, is this correct?
The oil boiler can have an NRV as its circuit is monitored and can accommodate this fault.

Can a loop be added to the stove return and keep the high flow in place, is is lowering it the only solution?
Are there any maths required for the loop or is it just to make it as deep as possible? Does the width of the channel in the loop matter?

Yes, correct above re NRVs, safety in stove's case, no safety issues in boiler case as two stats and a PRV (pressure relief valve fitted).
It is also considered good practice to fit a PRV in the stove as it gives protection in the event of the Vent pipe freezing in the attic space.

Regarding the loop, you already have the UP leg, keep the down leg as practically close to it as possible as hot water doesn't like trying to "rise" horizontally and ensure both legs are very well insulated.
If you have a problem it should be relatively easy to undo.

The only maths that I know is that the circulating height is the distance between the mid points of both the cylinder and the stove flow and returns.
I don't really know if the loop has any beneficial effect in say changing the circulating height, I wouldn't have thought so as its just a loop but if you google it it may tell you. I will have a look myself later.

funkey_monkey

I'll not be telling the plumber what to do, but I just want to understand what the craic is with it all and the possible solutions.

None of the pipe work in the cupboard where the tank is are lagged.

John.G

funkey_monkey wrote: »

I'll not be telling the plumber what to do, but I just want to understand what the craic is with it all and the possible solutions.

None of the pipe work in the cupboard where the tank is are lagged.

Well you can politely point out to him that connecting to the low level with a loop is the manufacturers recommendation. The pipes should be lagged but especially the stove's as the temperature gradient is essential to proper circulation.

funkey_monkey

It looks like the water to the stove is not circulating. On the return pipe, it is warm until it reaches the downwards bend. It then goes cold.

funkey_monkey

Just checked again this morning and the pod dropped from 65degC to 40degC overnight. The boiler flow pipe on the pod is hot exiting the tank, but there are two 90deg bends, after which is is cold. The top exit to the mixer for hot out is hot too. Stove flow and heating flow are warm to touch, heating return and stove return are cold.
I checked the CH controls and all are off.

I'll try tonight just using the oil burner to see what happens overnight.

Dtp1979

Not to be staring the obvious, and not sure if it was mentioned yet, but is there a small leak underground on a dhw pipe?

John.G

Yes, might be worth shutting off the domestic cold water supply to the pod overnight and see if temp still falls.

funkey_monkey

Dhw?

Dtp1979

funkey_monkey wrote: »

Dhw?

Domestic hot water.

funkey_monkey

If there was a leak in the DHW would it not have manifested itself somehow by now?

Dtp1979

funkey_monkey wrote: »

If there was a leak in the DHW would it not have manifested itself somehow by now?

If it’s under concrete then no. I rarely come across visible leaks underground.

funkey_monkey

Yes, its concrete floor.
One final thing I can try is that there is a valve on the stove return. Would there be a danger in turning it off at night with the tank at around 70degC to see if the reverse thermosyphon would be prevented?

Dtp1979

funkey_monkey wrote: »

Yes, its concrete floor.
One final thing I can try is that there is a valve on the stove return. Would there be a danger in turning it off at night with the tank at around 70degC to see if the reverse thermosyphon would be prevented?

If it’s on the gravity circuit then it’s absolutely a no no and shouldn’t he there. Do not turn it off

funkey_monkey

Dtp1979 wrote: »

If it’s on the gravity circuit then it’s absolutely a no no and shouldn’t he there. Do not turn it off

Yes, on gravity circuit return to stove.