Hot water preheat tank for instantaneous gas system

Supervstech said:

I also install a mixing valve when I replace a tank. Set temp high over 140F, and adjust the water temp at the mixer. Doubles effective tank capacity, and removes legionaries disease concerns.

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I didn't want to derail the diy thread but this comment got my attention.

Now that we have this place on solar, I'd love to have a hot water preheat tank ... but our system is an instantaneous gas one and it is very handy in that we can dial up a temperature and not have to worry about the cold tap for showers.

This is important here because we care for my wife's mum and being able to dial up a safe temp for her shower is one less thing we have to worry about.

I understand that a preheater needs to be high enough to prevent legionaries disease, but that will make it higher than what we need for MIL.

Can we fit a mixer valve to drop it like you have ... but then still feed it through the instantaneous gas one?

Being preheated should still save on gas ... but keeping the gas unit will retain the safety aspect.

Or is there a better way of going about it? Thanks.

The answer is maybe, but probably not (sorry can't be more definitive). The issue is many/most on-demand/instantaneous water heaters are not designed to take pre-heated water as input. They expect cold water in and run their burners full all the time water is flowing thru them, assuming it is needed to make the water hot. This is often the case with stand alone on-demand hot water heaters as well as those built into a "combi" boiler.

There are/were a few on-demand units which were designed to accept pre-heated hot water coming in, but you'd need to check your model to see if it can handle that. If yours can deal with hot water as input, then yes a mixing valve will likely be necessary, depending on how hot your the thermostat for the pre-heat tank is set to.

Just for the shower? Consider a replacement shower valve where the numeric temperature can be selected.

I recently obtained a nice 12V ball valve and could imagine using this as a bypass when the water is hot. This ball valve only takes about 9ma to run so having it on all the time would not be an issue. A check valve on the instantaneous water heater input would provide enough resistance to allow it just to bypass when the valve is open.

rogerdw said:

I didn't want to derail the diy thread but this comment got my attention.

Now that we have this place on solar, I'd love to have a hot water preheat tank ... but our system is an instantaneous gas one and it is very handy in that we can dial up a temperature and not have to worry about the cold tap for showers.

This is important here because we care for my wife's mum and being able to dial up a safe temp for her shower is one less thing we have to worry about.

I understand that a preheater needs to be high enough to prevent legionaries disease, but that will make it higher than what we need for MIL.

Can we fit a mixer valve to drop it like you have ... but then still feed it through the instantaneous gas one?

Being preheated should still save on gas ... but keeping the gas unit will retain the safety aspect.

Or is there a better way of going about it? Thanks.

Click to expand...

Legionaries is a condition from standing water. That doesn’t happen with instant tankless heaters.

With tankless hot water, I recommend setting the output temp to the full shower temp desired without any cold water mixed in.
Water heating is most efficient when heated only as high as needed. If you shower at 105F, then the tankless output temp should be 105F and you should run ONLY hot water in the shower.

Skip the instantaneous. Heat to 140, and mix to 104.

DIYrich said:

Skip the instantaneous. Heat to 140, and mix to 104.

Click to expand...

Oh, tankless has its advantages, space savings, zero energy consumption when not in use, unlimited water production, and more.
Sure, a tank heated and mixed works great, but tankless has merits of their own.

Supervstech said:

Oh, tankless has its advantages, space savings, zero energy consumption when not in use, unlimited water production, and more.
Sure, a tank heated and mixed works great, but tankless has merits of their own.

Click to expand...

HPHW much more efficient than tankless. Also easier to use on a solar setup.

Gas instantaneous water heater might be the worst type for a shower, depending on circumstances.
The temperature control is pretty crude ( its done by throttling the gas).
Any change in pressure or flow either at the up stream input or down stream output causes massive temperature swings until the slow gas throttling system catches up.

Just initially trying to adjust the shower temperature at the shower by tweaking the taps, you are fighting the control system in the heater as you adjust the flow.
It can be done, but it takes time and patience. Then someone in the house opens a tap in the kitchen, or flushes a toilet, and you hear screams from the poor bastard under the shower. Been there done that, it has been my experience with an older type of instantaneous gas water heater that fed the whole house.

A new "smart" electronically controlled instantaneous gas heater that only supplies the shower might be a very different thing, of which I have no personal experience. But I would expect large sudden step changes in up stream water pressure might effect even one of those.

For a shower, a storage water heater is probably your best bet, either heated by gas or electricity. And set the storage temperature not much hotter than the shower. Very little cold water will then be needed to lower the temperature as required, and its then very tolerant of water pressure or flow changes.
A low temperature storage will be more efficient as far as thermal insulation goes. You can also easily add extra insulation beyond what the tank manufacturer provided.

You can then add a high temperature preheater ahead of that, with a suitable mixing valve to feed the dedicated low temperature shower storage supply and the rest of the house.
Even if the low temperature storage is depleted, it offers a thermal buffer to prevent sudden step changes in water temperature that you get with any type of slowly correcting mixing valve.

It all needs to be thought out, and can be made quite thermally efficient and pleasant to use, but please first consider the victim under the shower, which might even be you.

Warpspeed said:

Gas instantaneous water heater might be the worst type for a shower, depending on circumstances.
The temperature control is pretty crude ( its done by throttling the gas).
Any change in pressure or flow either at the up stream input or down stream output causes massive temperature swings until the slow gas throttling system catches up.

Just initially trying to adjust the shower temperature at the shower by tweaking the taps, you are fighting the control system in the heater as you adjust the flow.
It can be done, but it takes time and patience. Then someone in the house opens a tap in the kitchen, or flushes a toilet, and you hear screams from the poor bastard under the shower. Been there done that, it has been my experience with an older type of instantaneous gas water heater that fed the whole house.

A new "smart" electronically controlled instantaneous gas heater that only supplies the shower might be a very different thing, of which I have no personal experience. But I would expect large sudden step changes in up stream water pressure might effect even one of those.

For a shower, a storage water heater is probably your best bet, either heated by gas or electricity. And set the storage temperature not much hotter than the shower. Very little cold water will then be needed to lower the temperature as required, and its then very tolerant of water pressure or flow changes.
A low temperature storage will be more efficient as far as thermal insulation goes. You can also easily add extra insulation beyond what the tank manufacturer provided.

You can then add a high temperature preheater ahead of that, with a suitable mixing valve to feed the dedicated low temperature shower storage supply and the rest of the house.
Even if the low temperature storage is depleted, it offers a thermal buffer to prevent sudden step changes in water temperature that you get with any type of slowly correcting mixing valve.

It all needs to be thought out, and can be made quite thermally efficient and pleasant to use, but please first consider the victim under the shower, which might even be you.

Click to expand...

This is why I usually recommend the water temp output be set to showering temp, so there is ZERO mixed cold water in the shower, netting maximum hot water flow through the pump.

Supervstech said:

This is why I usually recommend the water temp output be set to showering temp, so there is ZERO mixed cold water in the shower, netting maximum hot water flow through the pump.

Click to expand...

That is fine, if all family members can reach a consensus.
I rather prefer hotter showers in winter, and cooler in summer. But that's just me...

But have to agree, very close to desired final temperature is best.

Thanks for all the comments so far. I'm supposed to be working so I'll work through it all later tonight.

We have a Rinnai Infinity26 and I downloaded the manual and did a quick search but so far can not see a maximum supply temperature.

There's a chart that shows the max temp increase per flow rate but not a definitive maximum supply temp.

The unit does work extremely well in controlling the shower temperature even if the second shower or washing machine is being used ... so no worries there ... though one of our biggest complaints was just how much water was wasted before the outlets actually supplied the warm water ... to the extent that I've fitted a couple of instantaneous electric water heaters under the kitchen sink and the bathroom vanity to supply the small useage needs. That has saved us a lot of water and annoyance.

Plumbing and hot water was one of the few things my dad didn't teach me about and I haven't needed to get too involved otherwise.

I guess I envisaged a solar pre heater that got the water up to a safe temp (legionaires wise) ... with a mixer valve to drop it back down just lower than the lowest temp likely to be used in the shower ... and let the gas just top it up a bit.

Of course if it was just me here I'd turf out the gas and go back to straight electric and use the hot and cold taps for the shower ... but I have to watch out for Gran.

Steve777 said:

There are/were a few on-demand units which were designed to accept pre-heated hot water coming in, but you'd need to check your model to see if it can handle that. If yours can deal with hot water as input, then yes a mixing valve will likely be necessary, depending on how hot your the thermostat for the pre-heat tank is set to.

Click to expand...

Thanks, I've had a good read of the specs and manual and can't find any reference to a max supply temp. I'll keep looking.

time2roll said:

Just for the shower? Consider a replacement shower valve where the numeric temperature can be selected.

Click to expand...

Good point, and yes it's really only on an extension from a bath so should be easier to do than ripping out the mixer in the shower alcove.

efficientPV said:

I recently obtained a nice 12V ball valve and could imagine using this as a bypass when the water is hot. This ball valve only takes about 9ma to run so having it on all the time would not be an issue. A check valve on the instantaneous water heater input would provide enough resistance to allow it just to bypass when the valve is open.

Click to expand...

My understanding of the different hot water systems and components is limited ... but what you suggested sounds like something I could use. Thanks for the suggestion.

Supervstech said:

Legionaries is a condition from standing water. That doesn’t happen with instant tankless heaters.

Click to expand...

Yeah I understood that ... I guess my comment was that if I didn't have to worry about Legionares, I would fit a preheat tank set to the lowest temp we need, 39C ... and then let the gas look after anything higher. But of course to stay safe, the preheated tank needs to be at least 60C / 140F.

Supervstech said:

With tankless hot water, I recommend setting the output temp to the full shower temp desired without any cold water mixed in.
Water heating is most efficient when heated only as high as needed. If you shower at 105F, then the tankless output temp should be 105F and you should run ONLY hot water in the shower.

Click to expand...

Yes that is how we do it now and is definitely convenient ... just dial in a temp and enjoy the shower.

Supervstech said:

Oh, tankless has its advantages, space savings, zero energy consumption when not in use, unlimited water production, and more.
Sure, a tank heated and mixed works great, but tankless has merits of their own.

Click to expand...

Apart from trying to reduce our gas bill and use up more of the available wasted solar, I wouldn't even consider making any changes ... the convenience is awesome.

DIYrich said:

HPHW much more efficient than tankless. Also easier to use on a solar setup.

Click to expand...

Certainly the idea is great ... and we do have regular government rebate schemes advertised here too. But what worries me about them ... is every facebook ad I see for them is followed by lots of comments bagging them and how much trouble people have with them and how they've had to pull them out after two years because they couldn't get parts etc.

I appreciate there must be good brands and bad brands, so I'd need to do some research ... but the safety and convenience of being able to dial in a temperature for our MIL means the instantaneous one is still a good choice.

Warpspeed said:

A new "smart" electronically controlled instantaneous gas heater that only supplies the shower might be a very different thing, of which I have no personal experience. But I would expect large sudden step changes in up stream water pressure might effect even one of those.

Click to expand...

I'd never even heard of them until we bought this place but it certainly works well and I would recommend them for anyone with small children or someone infirm. The only issue we've had with hot water is just how long it takes for warm water to get to the shower and the kitchen and wasting our precious rainwater ... but I don't think that is really a fault of the gas heater ... more the distance from it to the outlets.

I'm just doing my usual "Scooge" thing and thinking up ways to save on our gas bill.

Supervstech said:

This is why I usually recommend the water temp output be set to showering temp, so there is ZERO mixed cold water in the shower, netting maximum hot water flow through the pump.

Click to expand...

Yes, we definitely do that now ... and it is easy with a push button temp control for each shower. But we need 39C for MIL shower and 43-46 for everyone else ... so a preheater has to be below 39C ... so of course that raises the Legionaires issue.

Warpspeed said:

For a shower, a storage water heater is probably your best bet, either heated by gas or electricity. And set the storage temperature not much hotter than the shower. Very little cold water will then be needed to lower the temperature as required, and its then very tolerant of water pressure or flow changes.
A low temperature storage will be more efficient as far as thermal insulation goes. You can also easily add extra insulation beyond what the tank manufacturer provided.

You can then add a high temperature preheater ahead of that, with a suitable mixing valve to feed the dedicated low temperature shower storage supply and the rest of the house.
Even if the low temperature storage is depleted, it offers a thermal buffer to prevent sudden step changes in water temperature that you get with any type of slowly correcting mixing valve.

It all needs to be thought out, and can be made quite thermally efficient and pleasant to use, but please first consider the victim under the shower, which might even be you.

Click to expand...

I can see I'm probably biting off more than I can chew here. And I'm a bit afraid I wouldn't win the argument at home to remove the existing gas heater without having the same functionality of dial up temperatures.

Thanks for all the suggestions ... I'll keep mulling them over and see what works out.

When I gutted my bathroom, I put in a bidet. Being an overachiever, I wanted warm water. Reducing complication, I put in one of these mixing valves to at least have the max temp be < scalding. Seems to work reasonably well, isn't hard to install and set, and is easy to adjust.


You can put one in the MIL shower so that the other heads get max temp, but hers will be maxed by the valve.

plympton said:

When I gutted my bathroom, I put in a bidet. Being an overachiever, I wanted warm water. Reducing complication, I put in one of these mixing valves to at least have the max temp be < scalding. Seems to work reasonably well, isn't hard to install and set, and is easy to adjust.
View attachment 213725

https://www.amazon.com/gp/product/B00I2VW1VK/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&th=1

You can put one in the MIL shower so that the other heads get max temp, but hers will be maxed by the valve.

Click to expand...

I set those on tank water heaters, so output temp is easily adjusted.

Supervstech said:

This is why I usually recommend the water temp output be set to showering temp, so there is ZERO mixed cold water in the shower, netting maximum hot water flow through the pump.

Click to expand...

Exactly this

I have a Takagi Tankless and yes, it functions best when more water flows through it.

If you raise the temp and have to mix in cold then the heater has very little water flow and fluctuates temp.

It's also worth noting that many shower valves have an anti scald bleed that let's cold in no matter what. I recommend closing this bleed when using a tankless for the reason listed above.

I have the Australian version of that, "adjustable tempering valve" Model TVA50HF fitted to my bath.
https://www.rmc.com.au/wp-content/uploads/2020/10/RMC_HeatGuard_Installation_Manual_1020.pdf
Its a great way fill a bath, but it does have some limitations for other purposes.

When you first open the tap, the water at the mixing valve is stone cold.
It senses that and goes to "full hot".
Within a few seconds the water coming out of the tap goes from stone cold, to full 100% hot.
It then gradually recovers to the set temperature over a few more seconds.
Fine, if you are expecting that, but may pose a scalding hazard to kiddies or the elderly.

Not really a problem, but something to be aware of, if you are using the output of the mixing valve directly for hand washing (or a shower).
It goes from stone cold to scalding hot, then settles down.

No problem filling a bath, or filling a hot water storage tank.
Just one small factoid to add to your research in designing the perfect hot water system !

Warpspeed said:

I have the Australian version of that, "adjustable tempering valve" Model TVA50HF fitted to my bath.
https://www.rmc.com.au/wp-content/uploads/2020/10/RMC_HeatGuard_Installation_Manual_1020.pdf
Its a great way fill a bath, but it does have some limitations for other purposes.

When you first open the tap, the water at the mixing valve is stone cold.
It senses that and goes to "full hot".
Within a few seconds the water coming out of the tap goes from stone cold, to full 100% hot.
It then gradually recovers to the set temperature over a few more seconds.
Fine, if you are expecting that, but may pose a scalding hazard to kiddies or the elderly.

Not really a problem, but something to be aware of, if you are using the output of the mixing valve directly for hand washing (or a shower).
It goes from stone cold to scalding hot, then settles down.

No problem filling a bath, or filling a hot water storage tank.
Just one small factoid to add to your research in designing the perfect hot water system !

Click to expand...

Why do you have the tank temp set so high?