Best Replacement for Boiler+Solar combo?

mcorner · June 23

I have a evacuated tube dhwh system with an approximately 110 gal. storage tank and a propane boiler. This is a single tank system and is 15 years old. The solar portion seems to work well and our boiler rarely fires in the summer. Unfortunately the boiler is on its last legs and our plumbers have told us to replace it. The repairs have also gotten expensive.

We have lots of pv and want to stop burning things so I was thinking we could install a standard 55 or 80 gallon hphw (rheem etc) and hook the outlet of the solar storage to the inlet of that. The plumbers don't have much experience with solar thermal, but this seems like the right approach? Is there a better alternative? With incentives etc, this is likely $2k-$3k.

This is in the US in MA.

Thanks!!

Jamie Hall · June 23

You need to do a very careful load calculation for your design week — not day — in the winter. You also need to do an equally careful calculation of your actual ability to capture energy with the PV system and your solar hot water system for the same conditions.

Then — and only then — can you figure out first, whether you can indeed capture the energy you need, and, just as important, how much storage (both thermal and electrical) you are going to need.

Then, having done that, you can decide whether the project is feasible at all, and what you need to do to accomplish it.

If you really do have lots of PV, say 20 to 30 kilowatts or so, it may be feasible for a smaller house. But do the numbers first — and be sure to take into account the low source temperature performance of any heat pump based device.

Hot_water_fan · June 23

This is for domestic hot water if I’m understanding correctly?

If so, this is easy. Install a HPWH. They use little electricity that you might see no reason to keep the solar thermal. Or you can pair the two.

Jamie Hall · June 23

Hmm. You may be right, @Hot_water_fan , I took @mcornerto mean both hot water and space heating since they mentioned a boiler.

Can you clarify, @mcorner ? Similar load calculations will apply, but the loads will, of course, be much smaller if it's just domestic hot water that's involved.

DCContrarian · June 23

My summer house is not far from you.

If you search in the "Free Stuff" section of Craigslist you can often find panels like that being given away.

With the advances in photovoltaic over the past decade or so, thermal solar just doesn't make sense any more.

Ditch the panels and go electric. You might be able to reuse the mounts.

mcorner · June 24

Just hot water, yes. We have 25kw of solar PV and are maxed out there.

Since the thermal system is still working and we have this huge storage tank, it seems like we should continue to take advantage of it.

The best option seems like routing the output of the existing storage tank into a heat pump hot water heater. I assume there is would work fine? But you are right, the hphw by itself may be just fine, I am just having a hard time ripping out the solar hot water since it is working.

The other options would be a new boiler ($$$) or an on demand system downstream of the storage tank. But both of those are propane and I am trying to give those up.

DCContrarian · June 24

The maximum for residential net metering in Massachusetts is 60kW, you're well below the max.

mcorner · June 24

For full net metering it is 25kw and even that hasn't quite take effect yet. And at the moment we have excess solar.

Regardless, I don't see how this affects the decision about how to replace our boiler.

The question here is: can I hook our storage tank up to a hphw? Or is there some other method that is typically used?

Jamie Hall · June 24

That is a big PV array — 100 amperes or so. Is this a residential applicstion? Do you actually use that much power, or close to it? Are you on a full net metering connection?

I would skip the heat pump part of the deal entirely, and with that much power available when the sun is shining I'd go with a plain vanilla resistance water heater. Much less maintenance — and much less cost. And, yes, I would hook it up to the output of the solar tank. But — I think I would also have an aquastat on the solar storage tank and a small recirculating pump from the output of the electric heater to the solar storage. That way, if the solar storage cooled off to below 120 or so (the minimum safe holding temperature for hot water) the electric heater and the pump would kick on to bring the solar storage up to the desired holding temperature — say 140 or so — from the electric tank.

If you do not have a grid connection, though… keep the propane. It will work off a standby generator. The electric water heaters — any flavour, heat pump or not — won't.

DCContrarian · June 24

https://forum.heatinghelp.com/discussion/comment/1804678#Comment_1804678

If you have surplus electrical capacity then using it is free.

Just a clarification: usually when people say "boiler" they mean a device primarily for space heating. Some boilers have a provision for also heating domestic hot water. Do you have a boiler currently that is used only for heating domestic hot water?

Larry Weingarten · June 24

Hi, Do you know what fraction of your domestic hot water is done with solar thermal now? Knowing that and knowing your hot water use would let you/us know how much heating needs to be done with the backup heater. If it's a small amount, just a plain electric tank-type heater makes sense. Heat pump units don't really appreciate being fed with warmed or hot water as it reduces their COP.

A question. Your existing solar tank is about 110 gallons. Is this a glass-lined tank? If so, and if it hasn't been maintained, you may be looking at a replacement tank soon, and they aren't cheap. That cost could affect your decisions moving forward.

Another question. Do you have an instant hot water recirculation system? These can waste more heat than a heat pump can generate, so that dynamic needs to be addressed if you do have the recirc system.

Yours, Larry

mcorner · June 24

A few photos of the system: https://photos.app.goo.gl/2nfaT785yjfbc1749

There is no recirculation system for the house. It is a buderus boiler that is only used for this hot water system, the house has a propane furnace for heat. The house is on grid.

The solar tank came from stiebel eltron and is 15 years old. I would guess no maintenance at all has been done.

Unfortunately I don't know how much is provided by the solar vs propane boiler. I do know that during the summer the boiler rarely if ever fires. During the winter, typically just once a day in the morning. It is a large house with six people.

Another option would be to replace the solar tank with one that has a built in electric backup. Stiebel eltron seems to make them. Unfortunately I am not sure any plumbers will have any experience with these. The bacteria question is somewhat concerning...

DCContrarian · June 24

https://forum.heatinghelp.com/discussion/comment/1804680#Comment_1804680

This is the way to go. Make the tank big enough that it can provide all of your hot water even when there's no sunshine.

If you don't want a second tank you could do electric tankless, but that's going to require a big electrical capacity.

Larry Weingarten · June 24

Hi, I agree with Jamie Hall and DCContrarian. If you have a big tank that is electrically heated at the top, but solar heater from the bottom, then the top will only use power when there is insufficient sun, and only as much as is needed. Convective currents from the solar heated portion of the tank keep the top warm or hot. The same thing can be done with two tanks, but it often is done with a pump and controls. Simpler is better.

Is there a way to find out what your present tank is made from (glass-lined steel?), and let that guide us in knowing what maintenance is needed? Then, can an electric element be added in the upper half of the tank?? 🤔

Yours, Larry

mcorner · June 24

The current tank is an Stiebel Eltron SBB 400 plus. It does not have an electric element, but they do sell the same tank as SB 400 E which has an electric element. So I could just replace the tank with the electric one and have one tank.

But..that is resistive electric. What is more efficient, this or just unhooking the entire solar water system and using a standard, HPHW heater. I imagine it is hard to say without data, but HPHW are cheap, subsidized and everyone sells them. I hate to give up on a working solar hot water system, but maybe it is just a dinosaur!

mcorner · June 24

Here is one way to think about it. Take the energy label from an 80gal hot water heater:

HPHW 1400kWH/yr

Resistive: 3500 kWH

If the solar hot water is nearly free (ignoring pump costs), and 50% of the heat comes from solar:

HPHW: 1400 kWH/yr

50% Solar + 50% Resistive backup: 0+1750 kWh = 1750kWh.

So honestly with all of the complexity of the solar system, ripping all of it out and using a HPHW looks like a no brainer which is kind of….sad.

DCContrarian · June 24

https://forum.heatinghelp.com/discussion/comment/1804707#Comment_1804707

It depends entirely upon what fraction of your annual water heating comes from solar. The more it provides, the more the resistive tank makes sense.

Put another way, if your solar setup isn't doing much anyway, it's not worth keeping! And if it is, it is.

hot_rod · June 24

How many evac tube? Even a small array will provide 30- 40% of your DHW, called the solar fraction. I'd run it until something expensive breaks, the tubes, pump station, controller, etc.

I've run dozens of solar simulations for jobs across the country and SDHW does a great job.

Two tank solar thermal are very common, let the sun contribute whatever it can. Even on overcast days there is solar radiation available.

I'd put a mix valve on the outlet of the HPWH as the solar could contribute 160F on a sunny warm day.

mcorner · June 24

We have 50 tubes. Higher up in the thread there is a link to pictures.

I am getting a plumber to come in a few weeks to give me his thoughts.

hot_rod · June 25

50 tubes, yikes! Does it have a dump zone? keep an eye on glycol ph. Those tubes can stagnate well over 400°F.

mcorner · June 25

Yes, it has a radiator dump zone with overheat protection from the controller. I don't think it has ever come on though.

hot_rod · June 25

50 tubes, you could run a small absorption cooler!

Find an old Servel to play around with.

mcorner · June 25

Unfortunately there appears to be a leak in the coil of the storage/indirect tank as I am getting high pressure in the boiler loop. So…the boiler is bad, the tank is bad. Not looking promising.

psb75 · June 25

If the 50 tube solar is working…keep it. Over heating is a reality even with a +100 gal. tank. Make sure the circulation, controller and fluid are all good. There should be provisions made for overheat i.e. summer vacation when no one is using hot water! etc.

Best Replacement for Boiler+Solar combo?

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